Sciencemadness Discussion Board

Crucibles and Metalcasting

Cyrus - 7-8-2004 at 21:28

Sand casting molten metal is a great way to make stuff, including scientific equipment, think pure silver hydrazine reaction vessel, pure Al tubes, homemade everything...

However, I need a crucible.

I am trying to make a crucible, and don't want to use steel because the iron will oxidize away until it fails. When crucibles fail, bad things happen.

I need some mad scientist ways to make a crucible!:D

Don't tell me use iron (fails) or just fireclay (too weak) or porcelain (too easy;)) or a teacup:P (too small). I will test these out, and report on my results though...

The crucible must be about 0.5 L, and it must withstand thermal shock, molten metals, and somewhere around 1000 deg. C.

Many crucibles are fireclay/graphite, SiC, MgO, Al2O3, silicates, or a mixture of the above. How can these be made in the lab?

I have several ideas-

1 a compound that decomposes at high temp to form a crucible material ie MgCO3 ought to eventually turn into MgO. The problem I see is that the MgO would probably be a powder, not a solid shape.:(

2 a mixture of chemicals that can be ignited/reacted in a form/etc..., and the residue would be a crucible. I have the crazy idea of taking sugar, filling a mold with it, sealing the mold except for a few small holes, heating, and taking out a perfectly formed pure carbon crucible...:cool: not likely though :(

[Edited on 8-8-2004 by Cyrus]

IvX - 8-8-2004 at 01:28

Why not steel?It would work fine for one time and doesnt cost very much.

Your sugar idea is good and all except the sugar would just melt and fall through the holes.Dip it in sulphuric acid maybe.

I'm watching animal face of now and here top make the (alluminium)skeleton they make it first in styrofoam then put it in a sand box.The sand box completely covers the styrofoam model except for a small hole with a pipe in it.They then pour the molten AL in(using a steel label btw).It does'nt give a very precise output though but you might want to try it anyway :)

Cyrus - 8-8-2004 at 12:49

I have tried steel- it oxidizes away to nothing. That is why I don't want to use it. In a well tuned propane furnace this oxidizing is pretty slow, but in a charcoal furnace, steel is lunch! (in mine at least)

However, I am in the process of making another steel crucible:P from a 1 lb propane tank, if anyone else does this, beware! It looks like the top half and the bottom half are just brazed together in the middle! If you cut the very top off, and use the rest, there is a high probability that the bottom may drop out!:( To prevent oxidation of the steel, I want to coat it with a protective layer, the best options I can think of are fireplace cement and fireclay. Any ideas?
More details to follow....

Edit, the sugar idea was kind of a joke;), I have tried it, you just get hardened bubbles of carbon usually, but if the holes were at the top, it couldn't leak through them. Another variation would be to make a crucible, soak it in boiling sugar soln. for a while, and then heat that up.

The sulfuric acid is supposed to make a solid chunk of graphite? I have tried it
before and it only makes a loosely held mush of amorphous carbon. :(

[Edited on 8-8-2004 by Cyrus]

Crucible making guide

Democritus of Abdera - 8-8-2004 at 15:42

Saerynide - 8-8-2004 at 21:46

Cyrus: H2SO4 and sugar doesnt' make a solid block of carbon. It makes this giant bubbly mass that looks extremely disgusting.

Cyrus - 9-8-2004 at 08:21

I know it turns into mush. I have done it. That's why I was questioning Ivx's idea.

However, flinn scientific (which has been wrong before) says about this reaction, "Your students will be amazed as they watch a yellow solid-liquid mixture turn brown, then black, expand out the top of the beaker- and solidify. "

I wasn't amazed when I did this reaction, and my product didn't "solidify"! Saerynide, if there was some way to reduce the size or amount of bubbles, might the carbon turn into a solid mass? Is the carbon produced amorphous or graphite? Maybe some other reaction could be used to produce the solid graphite, such as heating pitch. I read of a method using pitch or tar to make a graphite foam block!

Democritus, umm, nice link, but I'm not about to buy a book for this, and he suggests using clay, which I am already trying. If you have this book and know the type/composition of clay and firing conditions he uses, that would be very useful! Thanks.

Saerynide - 9-8-2004 at 09:58

When I meant its "not solid" I meant that it was a block which was full of holes, like very holey styrofoam. I wasn't meaning "mushy slush". I thought you were asking for a rock-solid-hole-free block of graphite. Sorry for the confusion.

To make a hard blob, you need more H2SO4 than sugar (someone posted a good amount a while ago), or else the H2SO4 would hydrate too quickly.

But I dont think this would make a very suitable crucible though. Its a bit too much on the porous side.

Cm'on Cyrus!

Democritus of Abdera - 9-8-2004 at 10:30

If you think that you're going to mix a little acid and sugar together on a substrate and get a graphite crucible, think again. It's not going to happen.

A graphite crucible about the size of a mans fist sells for about a hundred bucks, the reason for that is that graphite is a relatively rare material and difficult to work with, a fact hidden by the preponderance of "lead" pencils. Because of the high price of graphite items proffessional research has been going on for several lifetimes, with an expenditure that must reach figures in the hundreds of millions of dollars in today's money.

If you really want to go into it, feel free, but I can imagine that most ideas that you could imagine have been thought of and most techniques that you could try at home have been tried.

Graphite occurs as dispersed geologic deposits(bedded shale or small flakes dispersed in bulky ores) and the processes required to produce graphite from soft carbon in the laboratory are just slightly less arduous than those required to produce diamonds.

Pencils have become cheap because instead of pressing the graphite powder together under monstrous hydraulic presses to form, they simply mix it with glues and lightly press it together.

large graphite objects must be pressed with tremendous force and a "large" graphite object is still pretty small if you need the graphite to be of even consistency throughout.

Crucibles are extremely pricey, turning up your nose at a 10 dollar instruction manual on how to churn them out yourself by the dozens yourself is silly, because the technology has evolved over the last millenium or so and for you to stumble across the best and cheapest way to make good crucibles throught trial and error seems to be........ foolish at best.

And someone already took the weeks or months required to gather the info and perfect a process suitable for the DIY'er.

Perhaps if you wish to perform experiments, you might find it profitible to know what is currently already known and being done today and start there, instead of from scratch.

There is little to be said for a man who spends his time re-inventing the wheel.

It profits neither him personally, nor humanity as a whole.

Besides, wasn't there something you wanted the crucible for? Wouldn't your time be better spent doing whatever that is?

BromicAcid - 9-8-2004 at 14:18

In my general chemistry class that I took when I started college we made alumina crucibles. We took aluminum oxide, a tiny amount of water and sodium silicate and mixed to a paste then pressed it into a form in the shape of a crucible imbedded in a larger brick of what I thought was plaster.

After a few days we broke them out of the plaster and dehydrated them in an oven, when it was all over we had a tiny alumina crucible.

Or take a vessel made of aluminum in the shape of the crucible you want and put it into the bottom of a larger container that is sealed air tight, pass nickel carbonyl fumes into it (denser then air so they'll sink into the vessel) and make sure the whole thing is heated from the bottom, ta dah, dissolve the aluminum when you're done and a nickel crucible you have [only 15% serious ;)]

chemoleo - 9-8-2004 at 14:22

That's interesting, your description of alumina crucibles. Do you have a more detailed description? I.e. proportions of oxide, silicate and h2O? How resistent is it to cracks (thermal stress), and temperature?

BromicAcid - 9-8-2004 at 14:31

Well it was over three years ago but I remember the paste was relatively thick and that everyone who made their crucible walls thick didn't get them back after they were fired. I have no clue on the proportions.

They were for all intents honest to goodness alumina crucibles, good for many high heat applictions, the ones that actually made it held up nicely for the advanced chem students I hear and the one that I got when I was in advanced chem served me well.

Twospoons - 9-8-2004 at 15:08

What you describe is similar in many respects to glaze formulations. Essentially you combine a refractory (alumina, silica, zirconia) with a flux ( generally an alkali oxide), the proportions determining the sintering and melting points. The sodium silicate would provide both silica and sodium oxide to your formula, as well as acting as a plasticiser for the wet mix.

Must dig out my ceramic materials books and see if I can find something more useful :)

Cyrus - 9-8-2004 at 16:46



I only have sodium metasilicate though, so the results may be different.

The Nickel Carbonyl idea just scares me...:o

-start rant-

Democritus, did you write or market that booklet?:P There is no need to defend it so much! Why pay 10 bucks to get a booklet if with a little trial and error (which I and most people on this forum enjoy) I can get the same results, have much more fun, learn lots about how different crucibles are made, gain pottery skill:P, do it my own way, and save ten dollars;)??!.

About the acid and sugar, as I have said before, it is a joke! I repeat-


I was only about 30 percent serious! Are you going to try to flame BromicAcid for suggesting a farfetched Ni crucible method? If I may quote you, "C'mon"

After mentioning it, I just wanted to inquire more about the reaction, and see if it just MIGHT be possible. I take it that this inquiry deserves criticism from you for some reason.

"There is little to be said for a man who spends his time re-inventing the wheel.
It profits neither him personally, nor humanity as a whole. "

Hmm, that includes everyone in this forum who has synthesized a chemical that they could buy easily. This is not the site for you. Sorry. We spend hours on end doing ancient reactions when we could just buy the products. Why? IT'S FUN!! I'm not making a crucible to benefit humanity, I'm doing it because I want to!

If you are out to benefit humanity (which is not a bad thing IMO), unplug your computer and do something else. This isn't the site!

"Besides, wasn't there something you wanted the crucible for? Wouldn't your time be better spent doing whatever that is?"

Why don't I just buy a crucible then. In fact why don't I just buy whatever I was going to make (let's say a lathe), and why don't (to save time and benefit humanity) I just buy whatever I was going too make with the lathe (lets say a rocket nozzle), Hey! WHY DON'T I JUST BUY AN ESTES ROCKET???

The fun is in the journey, not in the destination!

-end rant-

This may be spoonfeeding:), but twospoons, what happens when alumina, and sodium oxide are heated together, what I mean is what is binding the crucible together? Is the Al2O3 just there for bulk and heat resistance , and the particles are bound together by a sodium oxide, or sodium silicate glue? Since the melting point of sodium silicate is 1132 deg. C (from google) , what happens above these temperatures? I guess the crucible would soften. I am having a hard time finding out what reactions actually happen when things are fired.

Democritus of Abdera - 9-8-2004 at 17:30

Originally posted by Cyrus

......The fun is in the journey, not in the destination! ........ a little trial and error..... (which I and most people on this forum enjoy) ...... I can get the same results, have much more fun, learn lots....... ..gain skill...... do it my own way......

twospoons, what happens when alumina, and sodium oxide are heated together?

TSK! TSK! TSK! .... flip-flopping like John Kerry naked on a greased trampoline! :P

BromicAcid - 9-8-2004 at 19:02

I was only about 30 percent serious! Are you going to try to flameBromicAcid for suggesting a farfetched Ni crucible method? If I mayquote you,"C'mon"

Actually, the idea was mostly Theoretic's. ;)

Twospoons - 9-8-2004 at 19:48

I'm not too hot on the actual chemistry of these sintering type things, but I believe the soda acts as a flux, so the melting point of the mixture ends up lower than its individual components - like tin + lead makes solder with a melt point lower than both.

Also found one reference to graphite crucibles suggesting a 50/50 mix of graphite and kaolin, mixed wet and rammed into a mold. Same site said "No experimental results yet" so I guess its up to you to test it!

Cyrus - 10-8-2004 at 21:37

Ok, several things...

Democritus, lets stop the arguing now, if you want I can continue to wrangle with you till judgement day by u2u, but I doubt it will help anyone. I don't want to clutter up this board any more with my and your off topic arguments. Thanks.

Back on topic

I "found out" what happens when clays are fired (according to a ceramics guy who I talked to for a while earlier today). The answer, drumroll please, is LOST WATER! First, the water that was just hangin' around (think wet clay) evaporates, and then the water which was bonded chemically to any of the molecules in the clay leaves. (As heat is increased the clay goes through a cristoballite sp? stage at about 1200 deg. F, and then at about 2300 deg. F, the clay is "fully fired" (what this means he did not explain) and then as the clay cools down, it goes through those same stages again, except the water does not come back!

Huh? Fired clay is the same as unfired clay except it doesn't have any water?
I was asking the guy a lot of questions, so I didn't want to press the issue too much, that is all of the information I got, correct or not.

Anyways, instead of porcelain, which I wanted to get for the crucible, he recommended raku clay- the stuff that is dipped in water while still very very hot, it ought not to crack from thermal shock if it can take that! If anyone is interested the type I got was Coleman Raku clay. Supposedly, porcelain (which I think is a synonym for pure kaolin) is so pure that it doesn't resist thermal shock as well as clays that have fireclay added. Fireclay is a type of clay that is added to other clays to increase thermal shock resistance, but is not good on its own, and the type I got already had some mixed in.

I also got some alumina for the alumina crucible, (it was labeled alumina oxide, is this just another name for alumina, a mistake, or a different compound? )
some bentonite for greensand, and potassium dichromate- the guy didn't know what it was for, so I got it for free!:D

I love pottery stores- have I mentioned that before? Yes I have. :)

Next I am going to do some tests!

Twospoons - 11-8-2004 at 14:15

Today I have my book with me "Pottery Materials" by John Colbeck, ISBN 0 7134 46951.

With regards to firing, most clays become irreversibly hardened around 650 C (red heat), as the chemically bonded water is lost. From yellow through white heat the fluxes in the clay become active, melting and bonding the individual clay particles, and reducing the porosity of the clay body. Once the clay is completely non-porous it is said to be vitrified.

For your crucible you'll want a high firing clay, generally refered to as 'stoneware'. These clays achieve vitrification in the 1200C - 1300 C range.

Improved thermal shock resistance can be achieved by adding grog (ground up pre-fired fireclay) to the raw clay. This will also reduce shrinkage of the clay as it dries, and improve the strength at high temperatures. Grog is typically added in the 10%-30% range (by volume), though you can add more if you wish.

'Raku clay' is just that - stoneware with lots of grog added. Though the raku firing I've done did not involve tossing hot pots into water - we used sawdust. Be warned - raku firing does not produce vitrified clay, the firing cycle is too short.

To get a nice stong crucible I suggest you find a friendly potter to fire the thing properly, or join a pottery class. There are two temperatures at which a decent soak period, and good temperature control are needed - around 100C where the free water is driven off, and around 900C where carbonaceous material is burned out. For this you really need a properly controlled kiln.

Alumina and aluminium oxide are the same thing:)

I love pottery stores too - so many fabulous chemicals...

Cyrus - 12-8-2004 at 09:50

Hmm, charcoal doesn't really have a temperature gauge. :(

I was planning to wait till the clay was dry, and then fire it as hot as I possible could for about 2 hours, this might be a bad idea, it sounds like the clay has to be heated up to 100 deg, held there for a while, then to 900 deg, held there for a while, and then to full firing temperature.

So the clay will be ruined if I just roast the poor thing after maybe drying it in the oven for a while? Too bad. I'm still going to try it though, I have plenty of clay.:P

Tomorrow I am going to attempt to make a crucible on a potter's wheel, today make some plaster crucible molds. (I made the plaster myself btw, guess how;))

I had another crazy idea for a crucible, this one is about 64.3% serious.

Take an aluminum casting in the shape of the crucible desired, and anodize the heck out of it. Tada, now it's a pure Al2O3 crucible!:D

The problems I forsee are that after a while the anodizing might slow down because the oxide layer is so thick, and after a while, the Al layer will become so thin that some parts may become disconnected from the flow of electricity.

And it might take a while:P

Here's a pic. to illustrate.

IvX - 12-8-2004 at 21:37

Theres no pic dude.

About your AL2O3 idea you could try burning thermite in something like a crucible.Sand shaped roughly for instance.The metal should form a layer and the AL2O3 on top of it.


Democritus of Abdera - 13-8-2004 at 06:20

Originally posted by Cyrus
Hmm, charcoal doesn't really have a temperature gauge. :(

actually charcoal fired kilns and ovens do, and they are cheap like dirt (but only single use) They are called cones, but are actually tiny pyramids and you can formulate your own or buy them ready made.

learn more here.... :cool:

Democritus of Abdera - 13-8-2004 at 06:30

Originally posted by Cyrus
I had another crazy idea for a crucible, this one is about 64.3% serious.

Take an aluminum casting in the shape of the crucible desired, and anodize the heck out of it. Tada, now it's a pure Al2O3 crucible!:D

The problems I forsee are that after a while the anodizing might slow down because the oxide layer is so thick, and after a while, the Al layer will become so thin that some parts may become disconnected from the flow of electricity.

And it might take a while:P

This idea might present a problem, in that you might require a Dr. Frankenstein-like battery of transformer to get the required voltage to pass into your crucible through the Aluminum Oxide layer, because if I read the attatched table correctly, alumina is quite resistant to the flow of electrons.

(I had previously researched the techniques and materials for anodizing aluminum and noticed that while thinner coating could be done at home even slightly thicker ones needed extremely higher soak times and power requirements)

Cyrus - 13-8-2004 at 06:36

Thanks, Democritus. Interesting...

Here is the pic, it didn't load last time and I was much too busy to fix it...

anodizing.bmp - 938kB


Democritus of Abdera - 13-8-2004 at 08:23

I took a look at that picture and while I think it might be practically unfeasible, I do have one adjustment to offer to the thought experiment.

I feel it's greatest value lies in the exercise of the mind all concepts give rather than the invention of a new process.


In the 2nd picture you posted, it illustrates anodising the bottom of a half-sphere and dissolving the aluminum out, to leave an alumina shell. This would require a fantastic amount of chemicals and time.

I would suggest foregoing the chemical process at that point and turning to the thermal, the melting point of aluminum oxide is a robust 2072°C while the melting temperature of aluminum metal is a mere 660.32°C (give or take :D)

prolonged heating the half-sphere with a propane flame would allow the aluminum metal to be poured out of the half-shell like an egg slipping out of a shell. :cool:

However; I do not believe this to be readily practicable. The forming of the aluminum oxide half-sphere depends on an aluminum oxide crystal Al2 O3 , occupying the same space and configuration as the closest packed aluminum metal structure.

And that would defy the accepted "laws" of physics.

Since alumina occupies a much greater space than aluminum metal, I postulate that anything beyond a layer measured in microns would begin to deform and lose cohesion and structural integrity.

Cyrus - 13-8-2004 at 15:34

Aw, those stupid laws. :D

You are probably right, but there is one way to find out, which is currently beyond my means. :(

Anyways, melting the metal ought sounds very problematic - when it expands, wouldn't it crack the Al2O3 outer layer instead of smoothly popping out?

Democritus of Abdera - 13-8-2004 at 16:44

Originally posted by Cyrus
, melting the metal ought sounds very problematic - when it expands, wouldn't it crack the Al2O3 outer layer instead of smoothly popping out?

Perhaps my egg and shell analogy was a poor choice, I didn't mean for you to think of it as removing a boiled egg from it's shell, but as easy as pouring a raw one out of it's half-shell.

I don't know that you've ever worked with aluminum metal, but it's really soft and weak, and gets very much softer and weaker the hotter it gets, to break up large cast aluminum objects to fit into the crucible, it's as simple as heating the object on a BBQ grill top, and within a couple minutes it turns as crumbly as hell, simply slapping it with a wooden spoon is enough to disintigrate it into small pieces.

When it finally gets to melting temperature, it turns into a liquid almost as thin as water, and we're talking about temperature's reachable on your stovetop!

In fact, you really don't need a crucible to start casting aluminum (pure), a cast iron skillet and a gas range will do it (but careful of the fumes) :o

However, alumina is as hard as granite even at elevated temperatures. It's often used as ball material in a ball mill. (also because it's cheap and relatively inert)

axehandle - 13-8-2004 at 17:01


Pure Al, when molten, has a consistence not unlike molten sugar in air: Very high surface tension, but once that's broken it pours like a drunk crack whore. It's the alloying materials that makes it castable -- it flows much more easily. "Normal" Al contains a hell of a lot Mg, and possibly other stuff as well, Mn comes to mind. So normal Al/Mg+X alloys cast easily as goblin pie, but pure Al doesn't. You're very unlikely to find pure Al though, I had to ask specifically for it.

Still, you're correct, Al alloys get brittle while hot: I remember hitting a bar of Al with a mallet and the former broke. This property can be used (I have 2nd hand impression of it) to break up large pieces of it in a furnace.

IvX - 14-8-2004 at 00:47

In some places they make allamunium statues with clay.A little complicated but it seems to work over charcoal.

What they do is make a wax mold cover it in clay, fire it up then melt/drain the Al of.AFter that they put in the Al cover the top with clay and stick it in the fire.Takes time but since the clay hardens well before the Al will melt you get your orignal wax object.

Re: Crucibles

JohnWW - 14-8-2004 at 02:12

While graphite can withstand very high temperatures, its one fault is that, being pure carbon, it is liable to burn in the presence of oxygen.

Platinum is, of course, the best material for crucibles, especially for high-precision gravimetric chemical analyses - but it is horrendously expensive.

I have, in the past, used small crucibles made out of fused zirconia, ZrO2, for gravimetric analyses involving high-temperature heating or pyrolysis of precipitates or filtrates. Besides this, and alumina mentioned above, I am sure that other fused refractory metal oxides could be used, e.g. TiO2, where (cheaper) porcelain or borosilicate glass (pyrex) is unsatisfactory for some reason. However, such materials are attacked by fused alkalis.

John W.

Democritus of Abdera - 14-8-2004 at 07:05

Originally posted by JohnWW
While graphite can withstand very high temperatures, its one fault is that, being pure carbon, it is liable to burn in the presence of oxygen.
John W.

If ya got oxidizing conditions that high in the furnace body, your running too lean on the fuel, charcoal fuel doesn't allow that.

If you are oxidising that high in the furnace you may have more serious problems (all your molten metals forming oxides and re-solidifying as crap), in the case of many metals you may notice oxidising conditions manifesting as tremendous gouts of flame and spark belching from the flue!:D

unionised - 17-8-2004 at 08:54

"Platinum is, of course, the best material for crucibles, especially for high-precision gravimetric chemical analyses - but it is horrendously expensive. "
Erm, not really. Except in the case of analytical chemistry where you can be sure that there are no materials present that damage Pt.
It doesn't like molten lead, or carbon containing material (so be careful to use an oxidising flame when you use platinum ware) or a lot of other things. It's also rather heavy and, because it's expensive, it gets used in rather thin walled crucibles which are rather weak.

Results of research and testing

Cyrus - 17-8-2004 at 15:37

Ok, there's a lot of info. I discovered!


BromicAcid's method of alumina/sodium silicate has potential! (well duh!, he said it worked. :)) But remember I am using sodium metasilicate, and the crystals are slightly wet, so I can't powder them in the mortar and pestle, so I mixed them as they are, about the size of NaOH granules from red devil:(, with very fine Al2O3 powder in the following ratios;

1 part by weight Al2O3 to 1 part by weight Na2O.SiO2 to enough water to make it easily workable.

3 to 1, enough water. 2 samples.

5 to 1, enough water.

All of these were dried, the higher the amount of sodium silicate, the more quickly they dried/hardened. The mix is very workable when wet, and fragile when dried. I took one of the 3 to 1 batches (just a couple of mushy half dried flakes loosely stuck together, about 3 g), and heated it in the flame torch (this was right after mixing it, very little time to dry) First it bubbled a little because of residual water, (the bubbles were left in the mass at the end, make sure it is dried before doing this) then hardened, after heating until the edges glowed bright yellow for about 2- 3 minutes, I let it cool, and it was very hard! I threw it on the concrete, and pieces would come off, I think because I only loosely stuck them together. The mass was too hard to break apart by hand.
Then I tried scraping it along concrete, and it scraped the concrete, leaving scratches = very good. :D

The other batches are now drying, and tomorrow when I test some other things, I want to fire them fully in my furnace.


Next, I took some of my 25 pounds of raku clay (even though I have more than I will ever need probably, I try not to waste any for some reason.;)) and formed a crucible out of it on an old potter's wheel.

Well, that's not quite true, first I spent an hour making very artsy blobs, and watching my friend make some of the same, and then someone told me the proper method, and after that I started having success, but the next try failed, and then my friend finally make a crucible (OK, I helped a little:))!

It is about 1 cm thick, 10 cm wide, and 8 cm tall, and is still drying. The bottom is thicker though, we will see how that works.


I also did a test on several coating for the propane tank crucible. The steel can was sanded clean, cleaned with isopropanol, then 5%bleach soln., and then 4 coatings were applied.

1, pure bentonite and some water, this hardened into a nice heat resistant coating, the problem was that the clay fell off of the crucible in one nice sheet as it dried. :(

2, 1200deg. F enamel, I know, the temp rating is not high enough, I just wanted to see what would happen. Guess what, it burnt and flaked off. :(

3, fireclay for fireplaces. This has yet to be heated, the coating adheres well but is soft.

4, Dap fireplace crack sealant, this stuff is rock hard, and sticks very very well, also yet to be heated

Part 2, maybe it should be chapter 2, Research.

1- Graphite crucibles.

There are 2 types of graphite crucibles. The first is the clay bonded graphite crucible ($10 to $40 for a #6 crucible, check ebay), and the second is the kind that Democritus describes, which costs 100 dollars, and is impossible to make on your own. I have not found any info on this kind, maybe it is just a block of pure graphite that is carved into the shape needed. Maybe graphite powder is pressed until it fuses into a graphite chunk?

Anyways, a clay bonded graphite crucible is, you guessed it, graphite bonded with clay. Now there are various ways to do this of course, one type of crucible is called cold isostatic pressed. Ooh, long name, must be hard to do! The basic idea is that a rubber mold is filled with clay and graphite, and immersed in water, which is then put under a whole lot of pressure. Ok, it is hard to do. :) Something that might be nearly as good would be to put the clay/graphite mixture in a mold and then ram/compact it, the benefit of isostatic pressing is that the graphite is compacted from all sides.

Someone with a big press (which I lack) and a furnace might try this. Hmm, who could that be?

I will try it in small amounts and perhaps different ratios, but I need a better source of graphite powder, the only stuff I can find is lock lubricant. :( It might work.

Also, I found the answer to my own question "why don't graphite crucibles burn in a furnace?" The answer is that they do! The solution is to use a reducing atmosphere of course. Democritus, I know that technically if there is enough charcoal the atmostphere will not be oxidizing, but a little oxygen always seems to get through to my precious crucibles. :(


Silicon carbide.

Silicon carbide crucibles are made in several ways, the silicon carbide can be premade, or it can be created in situ. in the crucible with the following reaction.

SiO2 + 3C -> SiC +2 CO

(I may be wrong but I believe CO is favored over CO2)

The temperatures needed are from 1600 to 2500 deg. C. It looks like I'm going to have to buy my silicon carbide. :( But yes, I'm going to still try to make my own anyways. :)

Anyways, now that we've got the SiC for the bulk of the crucible, we need something to bind it together with. Here are several options, none of which I made up.

1, mix the SiC with pitch and/or tar, and then heat. The pitch/tar will hopefully decompose into carbon, now the mix is a carbon bonded SiC crucible.

2, use starch, which also decomposes. In pat. # 5,538,675 and # 5,525,556, urea can also be used with the starch, when heated somehow this produces Si3N4, which binds the SiC together.

3, use SiO2, created in situ from the decomposition of ethyl silicate. The SiO2 melts/acts as a flux to fuse the SiC together. I don't see why very fine SiO2 powder wouldn't work though.

4, use clay. Enough said.

Stay tuned, I'll try some more of these out!

Jen - 18-8-2004 at 13:27

I've used graphite before - (for work - machined graphite, very expensive , but they could afford it, heh) - and it needed coating, mostly to prevent it reacting with the metal, seeing as we were running in a reducing atmosphere at the time.

Popular coatings were alumina, zirconia, yttria, and also BN. Particularly for nonferrous metals BN and zirconia are good, I think.

If all else fails, try using a slurry method - think lost-wax casting, only instead of letting the metal solidify and then busting the cast apart, try making a thicker shell and keeping it. They used to use clay instead of ceramic slurries for the lost-wax method as well.

Cyrus - 18-8-2004 at 19:36

Thanks, I should add that one site recommended a 75% alumina 25% kaolin wash to protect SiC shelves in kilns, should work for SiC crucibles, probably graphite too. I will try it. :) I need some SiC! And C in the form of graphite!

I am pretty sure that "ceramic" is about the same as clay usually, and I think that some lost wax castings are made from CaSO4 . 2H2O so they were reusable.

Pottery is interesting, I am going to make some things with the extra clay and try to do some glazes on them...:)

Cyrus - 25-8-2004 at 10:27

Hmm, this is turning into a monologue.

Anyways, dap stove and fireplace mortar has great potential as a covering for steel crucibles, that I have found out.

It forms a solid hard tough grey coating on steel, the only problem is that as it is heated, the glue softens, and bubbles ALWAYS FORM and harden!!! The areas where the bubbles form are weak and easily crushed. Also, the crucible looks like it has a very bad case of acne or something. :) I even tried heating the crucible at a temp. increase of "1 deg. F per minute" starting at 170 deg. F and going to about 270 F. (actually I just upped the temp. 15 deg every 15 min.:))

I thought, after going to about 270 F, all the residual water should be boiled off, eh? Let's just heat this thing up to 400 F for 18 min until crust is lightly browned or desired appearance is obtained. A pizza was being cooked too.;)

The coating started bubbling as it reached 400 deg F, and I took it out as soon as I saw all of the bubbles forming. :(

I think the solution is to start heating at about 280, and THEN slowly heat to >400 deg. F.

I made some clay graphite things, 1 rod and 1 sphere, they will be fired soon. I just ordered some graphite powder, more to come here (of course).

Also I made 2 small clay pinch pot crucibles. Pinch pot means I just take a blob of clay and work it with my fingers until it looks crucibleish.


uber luminal - 26-8-2004 at 14:18

I bought some expensive graphite crucibles for copper and Al. They worked ok, but were difficult to get out of the furnace (10 lbs Al, noting to grasp onto... 1400 F.. ) anyway, I went to Walmart... and bought a cheap $2-3 Stainless steel bowl. Hammered a spout onto it, and welded on some notches to use as a handle.

This thin stainless steel bowl worked better than anything I tried before that (Iron pipe, graphite, etc).

It dawned on me that the cheap walmart bowl was just that, cheap. But If I could use a better grade Stainless steel... say 409 or something with more than 12% Cr. And just have it a thin wall, this would work awsome. Typicaly when you heat stainless the heat stays were you put it, but since it does stay put, it would heat your load more quickly? Assuming enough heat was evenly distributed on the bowl (have 2 burners). This means your crucible stays hot between castings, dosent melt, or form oxides, and fires up quickly... Granted this would only work for melting things with a MP below Iron...

Cyrus - 27-8-2004 at 09:24

Hmm, stainless might work, I'll have to try THAT too. :)

But you haven't seen my demonic furnace! It EATS METAL! Tin cans are toast in 1 run. I assume you are using a properly tuned propane furnace, while I am limited to charcoal for the time being.
(I might build a better furnace:P)

Yesterday, as I attempted to try making phosphorus, I also fired the clay pieces I described above. Wood tends to heat up quickly when its burned, so I had very little gradual heating control. :( In an attempt to slow and even out the heating, I put the ceramic stuff in between 2 tin cans wired together, which were then placed inside of half of a 1 lb propane can. By the end of the 2 hr run, the propane can was so weakened that I could grab the edge and bend it in with 2 fingers. The "tin" cans were mostly gone.
I think I heated the the stuff WAY too rapidly, and even though it was dry before firing, most of the pieces had sections pop off because of the steam produced during firing. The large crucible shattered, the small one is still usable, a little "cookie" of clay had the bottom fly off, but the interesting part is that


Even though one of the clay graphite pieces was the thickest of all, the only "damage" that happened was that the outer 0.2 mm of graphite got burnt off by the furnace, leaving clay on the outside.

This stuff is strong, and now that I have 4 lbs of fine graphite, more tests can be done.

I also heated up (to a dull red to orange heat) the alumina/sodium silicate "cookies" the 1 to 1 ratio piece turned into a yellow sticky goo, and then bubbled, leaving a very low density but weak foam, the 3 to 1 ratio piece stayed solid and hardened, same with the 5 to 1 piece. They were not nearly as hard as the clay pieces, being easily broken in one hand, next time I will dehydrate the sodium silicate and powder it before mixing for a more homogenous mixture, and they will be fired to a higher temperature.

Some small (1-2 cm wide) clay, clay graphite, and alumina pieces all withstood the thermal shock of being thrown into cold water while at a bright yellow heat! However, the alumina pieces began to lose some sodium silicate underwater- they felt slimy. Perhaps this is a good thing- fuse the alumina using sodium silicate as a flux, then remove the alumina.

You guessed it, more tests are coming.

Edit- uber luminal, about the graphite crucibles, do you know if they were clay-graphite or what the coating on the outside was? After a few runs, did the outside of the crucible turn lighter like the color of clay, or did it stay the same color throughout?

[Edited on 27-8-2004 by Cyrus]

ignaro - 31-8-2004 at 09:28

Hi all! I am a newcomer, not too good writing english.

I read sometime ago in a book by Ure. I have this note:
"Crucibles: ure, pag 554; clay-graphite; coke can also de used"

NB: "coke" has the meaning of carbon fuel, not other cokes. Well... I presume that such is the meaning, I read it a long time ago and not remember it

Unfortunately, bnf don't allows now book downloads, reading must be done in a page by page mode. :(
Fortunately, I downloaded the entire book. ;)
Unfortunately, I can't find that CD. :(

If I find the CD I could send, upload, or whatever such image pages, or the entire book

ignaro - 1-9-2004 at 07:14

The book is:
A dictionary of arts, manufactures, and mines; containing a clear exposition of their principles and practice. By Andrew Ure

It is in MOA, also:
p. 554
p. 555

And I found the CD! Does somebody wants the book?

BromicAcid - 2-9-2004 at 21:16

Look at the exceptional properties of this mix along with its preparation!

Couldn't you just form a crucible out of this?

FrankRizzo - 3-9-2004 at 18:12


Wouldn't that material insulate the melt?

[Edited on 4-9-2004 by FrankRizzo]

BromicAcid - 3-9-2004 at 18:59

It probably does possess insulating properties, however if the crucible thus formed were totally enclosed in the furnace heat would raise the temperature of the inside of the crucible sufficiently, considering it can go to 1800 C a flame could be run straight into the side.

Cyrus - 7-9-2004 at 12:50

Ok, I'll have to try that too!!! :cool:

If I am understanding the site correctly, I just have to mix about 70 % Al2O3 with CaO and H3PO4 so that when heated, Ca3(PO4)2
is formed as a binder for the alumina.

Sounds nice.

I found a site where the exact compostion of commercial clay-graphite crucibles was described, as in 0.5 % Fe2O3, 27 % SiO2, etc... but now I can't find it. :(

Twospoons - 7-9-2004 at 14:18

Bloody brilliant find Bromic! I just examined the patent, and it looks as though all thats needed is some phosphoric acid, some superphosphate, and maybe a bit of Portland cement.

Adding a bit of grog (fired, crushed clay) for body and I think we might have a winner.

Final bit

IvX - 8-9-2004 at 01:32

Yesterday I molt and made a small 'object' of Al from pieces of a can in a small clay dish(kinda like a soup bowl).

Just had the pieces in there and put the whole thing in some charcoal and lit(in a hole though).See it acutally works :p

Cyrus - 8-9-2004 at 09:35

I think the recommended body was alumina, grog will probably work too.


This might be the best for diy crucibles, but I still think a SiC C bonded crucible would be the "holy grail crucible."

[Edited on 8-9-2004 by Cyrus]

More stuff

Cyrus - 13-9-2004 at 13:14

I will get some dental cement soon, which should be calcium phosphate, and I think the advantage of BromicAcid's find's method mostly applied to the first firing.

Also, I tried another alumina mix, 10 g Al2O3, 1.5 g sodium metasilicate, 0.75 g bentonite, enough water to make it workable, I think I added too much.
This is a good one too, nice and tough when fired, because this time the sodium metasilicate was dried in an oven and then powdered, the mix was more uniform, and the final product better. I added the bentonite for workability during the wet phase, it did help a little, because w/o the bentonite the mix tends to run and become unworkable, kind of like starch and water, but not THAT bad. The stuff is sticky, much too sticky to form like normal clay, and not very strong until dry, also it shrinks some while drying, maybe I should add some raku clay.

One thing I noticed while heating this w/ a flame torch was that the very edges would glow bright WHITE, but when cooling down would not go white -> yellow-> orange -> red -> dark, like I might have suspected, they went from white to dark, just like that. Wierd.

I am also working on a light insulating firebrick formula, test batch # 1 is drying, composed of 2 tablespoons of Al2O3 (14.5 g), 2 tbs "wet" raku clay (41.5 g)
1.5 tbs rough to fine grog from smashed prefired clay (15 g) and 3 tbs sawdust (6 g)

Twospoons - 13-9-2004 at 13:55

Use 'ball clay' instead of bentonite, if you want to increase the plasticity of your mix. This will be available as a dry powder at your ceramic supplier.

JohnWW - 13-9-2004 at 14:17

Originally posted by uber luminal I went to Walmart... and bought a cheap $2-3 Stainless steel bowl. Hammered a spout onto it, and welded on some notches to use as a handle. This thin stainless steel bowl worked better than anything I tried before that (Iron pipe, graphite, etc).

It dawned on me that the cheap walmart bowl was just that, cheap. But If I could use a better grade Stainless steel... say 409 or something with more than 12% Cr. And just have it a thin wall, this would work awsome. Typicaly when you heat stainless the heat stays were you put it, but since it does stay put, it would heat your load more quickly? Assuming enough heat was evenly distributed on the bowl (have 2 burners). This means your crucible stays hot between castings, dosent melt, or form oxides, and fires up quickly... this would only work for melting things with a MP below Iron...

Good grades of stainless steel may be resistant to chemical corrosion, especially where the Cr and Ni contents are both high. HOWEVER, the problem is that almost every sort of steel alloy in common use has a significantly lower melting point than pure Fe, 1,535ºC. Almost the only exceptions are some high Fe-Mo and Fe-W alloys for highly specialized purposes (e.g. mechanical parts, and for exceptional corrosion resistance combined with hardness); and cast iron with the highest possible carbon content i.e. pure cementite, Fe3C (6.68% C by weight, which would be unusual - normally not more than 3.4%), which has a of 1,837ºC. The m.pts. of mild steels are around 1,350ºC. Those of stainless steels are between 1,370ºC and 1,530ºC, with the high-Ni ones having the lower m.pts. Hastelloys, Inconel, and Monel alloys, which are mostly Ni, Co, and Cr, some having Mn, Mo, Cu, Si, with only between 2% and 30% Fe, for exceptional corrosion resistance in process plants, have even lower m.pts., between about 1,230ºC and 1,430ºC, also being lowered by Ni.

As for "the heat in stainless staying where you want it", this would be the case if it had an exceptionally low thermal conductivity but high specific heat for a metal. This is true to some extent, having on average a 10% higher specific heat than mild steels, and only about 1/3 the thermal conductivity of mild steels.

Reference: Perrys Chemical Engineers Handbook, chapters 3 and 23.

John W.

Taken from "THE SPIRIT OF CLAY" by Robert Piepenburg

Cyrus - 13-9-2004 at 15:59

~start quote



This secondary clay is extremely plastic. In fact, it is so "sticky" that it is often referred to as a plasticizer and need only be added to clay bodies in amounts of 2% or less to significantly improve their working qualities. The results are immediate and there is no need to wait for the beneficial effects of bacterial growth achieved through aging to keep the clay from being short and unworkable. Bentonite, like ball clay, also adds to the dry strength of clay in the greenware state. Both of these fine-grained clays contain some iron impurities but bentonite is far more plastic than ball clay.

~end quote

Btw, I have given up on iron for crucibles (except for small amounts of Al) , it contaminates the melt, melts too easily, etc, etc, etc.


The formula for firebricks I was using has barely any shrinkage while drying. It still fits the form well, as far as I can see. :)

Remember the black "rock hard" dap fireplace cement I coated my furnace with? It all turned yellow and is flaking off in large soft flakes. Also, the furnace lid has cracked into 4 symmetrical pieces, the gaps from the cracks are about 0.3 cm in some places. :( Lousy concrete refractory. Axehandle, is your furnace still holding up?

How about ramming the refractory in, immediately taking out the forms, then taking sheet metal and "slicing" the monolithic refractory into a bunch of "bricks"? Of course, they would be much better than normal bricks becaue they would fit the furnace shape perfectly? This ought to stop lots of cracking.

[Edited on 14-9-2004 by Cyrus]

Twospoons - 13-9-2004 at 21:59

You're right, bugger it! Should have checked my references first. Did find one addtional note about bentonite though - too much can cause problems with excessive shrinkage. No idea how much is too much - minimum effective suggested is 5%, so the 7% you used should be about right.

Cyrus - 14-9-2004 at 19:23

I fired the test formula for refractory insulating firebricks today, using a wood fire in my backyard ( dug a pit and made a nice little fire.) I was too lazy to drag out the furnace. :)

The brick turned out well, the only noticable problem was that I could scratch the stuff with my fingernail.:(

Maybe I have diamond fingernails or something. ;)

I coudn't break the very small, about 1 cm thick brick with one hand though, so it's not too weak, it has a nice ring when dropped (lightly :)). No cracks though.
I fired a similar sized piece of normal red brick, which cracked in half, but kept its hardness. Also, I made another brick from 10 parts "fireclay" 4 parts sand, and 1 part bentonite, this was a lousy mix, weak, hard to work, low temp. capability, it wasn't quite dry when fired, it also cracked halfway through, later I broke it in half, but it was too hard to scratch w/ my fingernail.

I then tested the bricks in an extremely scientific way for thermal conductivity/insulation. Take torch, put on med. setting, point at brick, and count how many seconds it takes to glow bright yellow. My good formula took 20 seconds, the bad one took 43 seconds, and the red brick never got above dull red/orange, even after 65 seconds. I would add more sawdust or something like perlite, the brick is plenty crush resistant, it just isn't as scratch resistant as I would like. I then took the bricks to test them for thermal shock. (my standard test, heat an edge as hot as I can and throw in bucket of cold water) They all passed. Nothing has ever failed that test, I need a better one.

However, after the lousy formula brick was heated up to yellow heat, it turned soft and crumbly. The "fireclay" I got for free is worthless. Oh well. Dental cement coming soon.

Another thing, supposedly kyanite or mullite, which is calcined kyanite, hold the clay body together well and prevent cracking from thermal shock. I'll test that too, both for crucibles and for insulating refractory.

Metalcasting molds

uber luminal - 14-9-2004 at 22:09

Fair enough.

I didnt post this here becuase molds is something totaly different than crucibles... as you can cast glass or polymers... but anyway,

I wondered... what do you use to cast into?
The typical answer I read about is, to use a fireclay(and sand) which I assume is a Mg based clay. well turns out, no one in my area sells this magical Fireclay. I was told by distributors that it no longer had a safety or use rating, and thus no one used it anymore. all that was sold was fire cement, or premade bricks. One vendor told me they could ship in 2 bags of it... for $10 a bag... and $108 shipping.

I have used sand/clay dug up out of river banks, and this seems to work ok, except when i cast with this, the castings form cracks and look like crap.

I am looking for something that can replace the "green sand" that is typically used, such that I can mold something, cast it, and tear down the mold and reuse the same mold material (it doesnt perminatly harden)

Cyrus - 15-9-2004 at 08:40

Originally posted by uber luminal
I am looking for something that can replace the "green sand" that is typically used, such that I can mold something, cast it, and tear down the mold and reuse the same mold material (it doesnt
perminatly harden)

Greensand is the way to go.
It's good.
That's why everyone uses it.
You don't WANT to use fireclay, it permanently hardens at high temps.
Bentonite is much better. Any pottery supplier near you should have some.
If not, you only need like 5-10 lbs per 100 lbs of sand, so shipping woudn't be that bad. Use as little water as you can, and yet have it hold together well, and as fine of SILICA sand as you can, ok 400 mesh is too small, 100 mesh should be fine.

If for some reason this doesn't work, make investment mold stuff from plaster of paris, sand, maybe perlite, etc. This will harden, but if you crush it up, and heat it in an oven, it can be regenerated.

Anyone have any other suggestions?

Tacho - 16-9-2004 at 03:41

Originally posted by Cyrus
If I am understanding the site correctly, I just have to mix about 70 % Al2O3 with CaO and H3PO4 so that when heated, Ca3(PO4)2
is formed as a binder for the alumina.

I did a test with hardware store lime (hopefully CaOH). I figured since common phosphoric acid has 20% water anyway, it didn't matter if it was CaO or its hydratated form.

The reaction is one of the most exotermic I have seen without fire. It melted the plastic pot immediately.

Gas evolved, I don't know why. Maybe my lime was mixed with carbonate, maybe it was the water boiling away under the heat.

I had to add some water to continue the reaction. There was NO imediate hardening. I'll check tonight.

Twospoons - 16-9-2004 at 14:16

Its possible it was all carbonate. Did it say 'lime' (the carbonate) or 'slaked lime' (the hydroxide) ?

Tacho - 16-9-2004 at 16:42

Originally posted by Twospoons
Its possible it was all carbonate. Did it say 'lime' (the carbonate) or 'slaked lime' (the hydroxide) ?

Oh, there is no such ambiguity in portugese. It was the hydroxide, but not reagent or technical grade, just construction stuff.

ignaro - 17-9-2004 at 07:26


Oh, there is no such ambiguity in portugese. It was the hydroxide, but not reagent or technical grade, just construction stuff.

There is another ambiguity (a similar one exists in spanish):

There are three limes, I don't know the english names:

-Cal Gorda: made form nearly pure calcium carbonate
-Cal Hidráulica: made from calcium carbonate + 10% to 20% clay
-Cal Magra: made from calcium carbonate + silicium oxide + iron oxides + clay

Tacho - 17-9-2004 at 09:07

Originally posted by ignaro

There is another ambiguity (a similar one exists in spanish):

There are three limes, I don't know the english names:

-Cal Gorda: made form nearly pure calcium carbonate
-Cal Hidráulica: made from calcium carbonate + 10% to 20% clay
-Cal Magra: made from calcium carbonate + silicium oxide + iron oxides + clay

Well, here it goes like this:

"Cal virgem": Calcium oxide - or xxxx-lime
"Cal hidratada" : Calcium hidroxide- or yyyy-lime

I believe they are made FROM calcium carbonate by calcination ( CALcination... humm... I see historical roots here...), and they become hard by reacting with CO2 in air to become carbonates again.

Carbonate rocks are "calcários" - or zzzz-lime.

ignaro - 17-9-2004 at 09:42

I'am sorry, I don't stated it: Gorda, hidráulica and magra are portuguese words.
(They are also spanish words, with similar meaning, but not strictly related to masonry)

Gorda is used for walls, because it uses CO2 from air.
Hidráulica is used for walls and understructures, because it don't need CO2; it is similar to a portland cement.

Cyrus - 17-9-2004 at 12:07

Hmm, nice little language discussion.

Anyways, I've been in my "lab" doing more tests, see backyardmetalcasting, but I have to say that the phosphate bonded dental investment stuff is GREAT!
An hour or two after mixing it, I was hitting some with a hammer to get it from its mold, and it wouldn't budge, I had to drill it out! Today I'll mix up some clay/graphite/kyanite/grog batches, more details to come. (Of course)

sarcosuchus - 21-9-2004 at 18:16

some were on a cd ive got is the step by step process for making rocket nozzels useing the al2o3-sodium silicate mix.thought about makeing my own grinding wheels but it should make real good cruibles for melting all kinds of stuff.l will track it down and upload to ftp as soon as possible

JohnWW - 21-9-2004 at 18:47

Actually, Tacho, Portland cement, the principal ingredient of which is burnt lime, or quicklime, CaO, forms concrete with an aggregate of crushed rock and sand, or bonds with sand alone to form mortar, on the addition of water (which hydrates it to slaked lime), does so by reaction with the silica in the sand and crushed rock. It forms calcium silicates, e.g. Ca2SiO4 and CaSiO3 (wollastonite) along with mixed silicates, which crystallize when the stuff sets. Burnt lime or quicklime is "cal viva" in Spanish and Portuguese.

Other ingredients in Portland cement include gypsum, which is sparingly soluble CaSO4.2H20 which dissolves and then resets; and clinker (slag left over from steelmaking). Some types also contain powdered clays.

John W.

ignaro - 22-9-2004 at 09:12

And because of such calcium silicates "cal hidráulica" is similar to portland cement.
I find the english name for it: "hidraulic lime". A little obvious, but the earlier term used by Smeaton was "water lime".

General Casting and Crucible info

Gyro - 15-10-2004 at 08:24

I'm enjoying this thread -- the range of inquiry is fascinating, and revealing some useful chemical and formulation info.

I'd like to add, from a practical standpoint, a few comments for anyone wishing to cast right now, using simple equipment.

I've built a Gingery lathe from scratch, a milling attachment, and a Tesla disk turbine -- all from castings in a charcoal furnace. Some of the info is available on my website .

1.) I've cast probably a hundred aluminum parts using a cast iron crucible -- an ordinary plumber's lead pot. It shows little deterioration.

2.) For crucibles, I've also used at times, food cans, stainless steel canisters (ala Wal-Mart), and a 5" long section of 4" diameter iron pipe, with a steel bottom welded in.

3.) I have used both fireclay and bentonite for greensand formulation. Both have produced excellent results.

There are a few things to note which I believe have affected the varied results reported here.

First, "fireclay" is a nonspecific term. I recently bought a bag of so-called fireclay from a masonry supplier, which was completely useless. Yet I earlier bought a bag of AP Green Co. fireclay from a pottery supplier which enabled me to build both my foundry, produce a good quality greensand and cast my lathe.

The "bad" fireclay wasn't even all clay -- it contained what looked like grog, fines, and other diluents. When I called the manufacturer (Koppel) the person I spoke to said that it was intended only for plugging old wells, and did contain broken chimney tiles from their casting operation. Obviously, it's a stretch to call something "clay" that is already vitrified! The greensand I mixed with it had no bond strength to speak of. A waste of time and money.

If you want to be sure of obtaining a real fireclay, potters use specific named clays which are equivalent. One of them is called Hawthorn 30 or 35. This is available at ceramic suppliers.

If you want to use a steel or stainless steel crucible to melt aluminum (which I do) and do not mind the somewhat lower purity and quality of the castings (which I do not), then there are two factors to pay attention to.

The first is that pressure and volume of blast will determine the relative lifespan of any crucible. The second is that crucible wall thickness will do the same.

With a high pressure blast you can easily exceed 1500 C locally, and in fact burn your crucible right up. Steel burns. Stainless steel burns. You can put a hole in a crucible of thin stock quite easily, with a high directed blast. If the high localized pressure is coupled high blast volume, then you will oxidize not only your crucible wall but also your melt.

Blast pressure should be modulated to produce a somewhat reducing atmosphere to protect both the crucible and the melt. Throttle down to get good mileage! I do this by making the blast pipe discontinuous, and simply moving my air source pipe away from the tuyere source pipe. I adjust continually to keep a moderate heat going -- especially for aluminum. Heck, it melts at a low enough temp. You don't need steel melting temps. In fact you can even melt it without a blast in a simple wood fire pit outdoors.

The second factor -- crucible wall thickness also adds to the lifespan. A thicker crucible will spread the heat better than a thin one. Generally there's a hotspot where fuel and blast come together, and most simple charcoal furnaces will have a crucible sitting on top of fuel, and only one tuyere blast source. This creates a hot spot that, in a thin can type crucible, will melt through in short order. Again reducing the blast will help some, and baffling or orienting the blast tangentially will also improve crucible longevity.

I like bentonite a little better than fireclay for greensand, because it has higher bond strength, and requires less water, however, perfectly good aluminum castings can be made from fireclay greensand as well. Just be sure to get good fireclay. I have never had the post-melt "fire hardening" problems some others have with fireclay with aluminum melts. I can crumble the greensand fairly easily and break out the casting without problems -- but perhaps I don't ram as hard as others -- or my fireclay is different than theirs. In fact, if I return the lumps to the greensand bin, overnight the humidity softens the lumps to sand consistency again. Yes, I return all sand. Eventually the bond strength reduces, and I rejuvenate with a little more pure clay. The day will come when this won't work, but a LOT of castings will have come off the line before I reach that point, in my experience.

I would strongly recommend than anyone considering casting read Dave Gingery's book on building a charcoal foundry. It's inexpensive, and contains most of what you need to know.

Steve Redmond

Theoretic - 15-10-2004 at 13:52

Just a suggestion: to get a very hot flame, cheat and use H2 + air/H2 + O2. :)

Cyrus - 20-10-2004 at 11:22

That's not very practical for most of us, right now I'm working on an oil burner, it'll probably be forced air powered, but a natural aspirated oil burner is my goal.

I'm still testing ceramic crucible mixes.
BTW clay graphite has better thermal shock resistance than SiC, so that's what I'm working on mostly.

The cera-fina works ok, but is not quite as strong as clay when fired. The best results so far are with about 30% graphite.

And here's a page that is just beautiful!


One thing I'd like to clarify about the alumina crucible method proposed by BromicAcid (sodium silicate bonded)- Alumina isn't fluxed by sodium silicate.
That means the flux is just melting and holding the alumina particles in a glass.

I'm also experimenting with different insulation methods, since perlite melts at too low of temperatures, I'm testing using little styrofoam spheres to provide porosity.

More to come of course...

[Edited on 20-10-2004 by Cyrus]

Oxydro - 20-10-2004 at 13:00

(a bit off topic)

Oil burner you say Cyrus... I actually managed to make one of those this past summer. It worked, to a limited extent -- there were problems of fine tuning that I never got around to fixing.

I already had a charcoal fired furnace (really crappy one, made it out of what I had lying around which meant just "heatsink" concrete), and to convert it to oil I fed a spiral of copper pipe down the inside, hammered to a small nozzle at the bottom. It was fed from a pop-bottle funnel with a valve connecting the pipe to the cap of the bottle.

Initially, it worked ok, but it was limited, because after a while, the copper would heat up so much it would start boiling the oil inside it, and the pressure kept the oil out of the pipe. I really needed a pressurized system or a pump.

Then I screwed up the nozzle design trying to tune it, and I haven't been back to it since... work got in the way and now I've moved away from my parent's house where it lives. I'm looking to aquire a burner from an oil-fired water-heater and build a better system around that.

[Edited on 20-10-2004 by Oxydro]

Cyrus - 20-10-2004 at 18:15

Yes, I thought of doing something like that, but the copper pipe would be in a burner tube, not the furnace. The problem I discovered is that oil decomposes around its b.p. or lower to form a black shiny (carbon containing of course) lacquer that sticks to copper very well. :(

In other words, the pipe will clog over time.

The only solution is to either inject the oil at a low temperature using an atomizing nozzle or injecting it into a chamber at very high temperatures (the burner) - high enough that the carbon formed will burn off. For this method you will also need an oxidizing atmosphere ;) and lots of turbulence IMO.

I'm making a "test bed" burner to test both designs. For an example of the latter design, see (the ursutz burner)
Also, see the forum on the same site for more of my and others ideas for oil burners.
You know, I ought to start an oil burner thread.

Cyrus - 15-1-2005 at 22:55

I STILL think sugar will work for a phenolic resin! :P I did some tests today, and if there were only a way too keep it from bubbling so much, it would be very nice- maybe using a lot of aggregate and little sugar will help.

I've been slip casting a few crucibles out of clay, and even coil-building them! (artistic, I know;)) They are about 1.5 mm thick after 10 minutes, I think the clay is too fine, I'll have to try some EPK. I haven't tried slip casting clay-graphite yet.

The oil burner got fired today, it's sooo cold outside, I think that may have been part of the problem- there was "burning" going on, but the oil wouldn't vaporize fast enough to create a good flame. I wasn't using my McDonald's oil because it's a solid at STP! :(:(:mad: The only other oil available was waste car oil, which isn't nearly as energetic.

rift valley - 23-3-2005 at 15:29

Today I was in a rush to get some metal melted so I could cast a sculpture for a school project. With no suitable crucible in sight I noticed my nickel crucible. Nickel melts at +1400 degrees so it should be fine to melt some Al right? I only paused to wonder if the liquid Al might form an amalgam with the nickel, but it wasn't long before I continued on anyway. Once my furnace got cooking I left to heat up for about a half an hour. When I got back the furnace was at an extremely bright orange heat, almost yellow. This is what I found.

What was once this

was now this

With this in my furnace (cleaning this is going to be a B****, any ideas? I will probably heat the metal up just below its MP so I can easily break it out)

Any ideas on why this happened did the aluminum form an amalgam with the nickel (I thought nickel was very corrosion resistent, or does that not matter if we're talking about amalgams?) The hottest the furnace could have got up to is around the 1100C range and it was probably well short of that.


Polverone - 23-3-2005 at 19:11

Ellern's discussion on p. 279 of "Military and Civilian

"Mellor cites the phenomenon of a temperature rise of 30 g of molten gold at
1155 deg. C. after addition of only 0.3 g of aluminum. The latter, being cold,
caused an initial drop to 1045 deg., whereupon the tepmerature rose to 1380
deg. C. A perusal of Circular 500, as well as of the bnooks by Kubachewski and
Evans and Kubachewski and Caterall, shows numerous strongly exothermic reactions
between metals as expressed by the heat of formation of arsenides, antimonides,
bismuthides, aluminides, and others."

Palladium and aluminum alloy exothermically, the basis of the article called
Pyrofuze, described ibid., p. 207. Nickel and aluminum alloy exothermically, the
basis of the Pyronol torch.
S. H. Fischer and M. C. Grubelich have listed
extensive data on exothermic intermetallic reactions in their article
"Theoretical Energy Release of Thermites, Intermetallics, and Combustible
Metals," pp. 231-286, Proc. Twenty-Fourth International Pyrotechnics Seminar

Note that Ellern's book is available on the FTP site, though I don't recall if it contains any additional information.

12AX7 - 23-3-2005 at 20:40

Referring to:
You can clearly see that aluminum and nickel have pretty good solubility. Contrast to something like iron, commonly used for crucibles:

"Bright orange almost yellow" I would call 1800-1900°F = 1000°C. Iron is barely 20% soluble, while nickel is up at 40%. Especially with such a thin crucible, it simply passed right on through!

I've had tin cans look the same way, just a cylinder left above the liquid level. The trick is to oxidize it before use so you get a layering of Fe:FeO:Al2O3:Al, rather than Fe immediately touching the Al melt, allowing it to dissolve. Now, I don't know how much nickel will oxidize, but if it doesn't, I can promise trouble.

So um, yeah... not much else to say besides condolences to your crucible.

BTW, nickel (or more accurately, the Al3Ni particles) will probably strengthen the metal, if you intend to keep it. I'm not sure how much, more than 1% might embrittle it.


chemoleo - 24-3-2005 at 12:40

That's very interesting, to see this happening at a large scale like this. There's been a thread on intermetallic reactions, have a look at this (for lateron, thread ID 2202). It also lists the energies generated between the different metals when fusing.

[Edited on 24-3-2005 by chemoleo]

"Corelle ware" crucibles?

fizzy - 24-3-2005 at 17:51

I seem to remember that Corning Corelle ware is basically fused silica that has been allowed to do some crystallization.
Is that correct?
Could corelle coffee cups be used as small crucibles?
Up to what temperature?

After all these are only a dollar apiece at the local Salvation Army or Goodwill store.

12AX7 - 24-3-2005 at 19:13

Ah, partially crystalline glasses are fun bits of stuff.. didja know that Macor machinable ceramic is basically glass with mica plates grown in it?

Anyways, if that's what it is, it'll basically act like glass, and probably melt at usual temperatures, be sticky and not very shock resistant (although that depends).


tumadre - 18-10-2006 at 20:55

I have found that my local natural clay does hold up to iron at a temperature upwards of 1700C but the strength is to be laughed at.
although it will absorb 20% its weight in water, melting steel in it via an arc under sand was a quick and dirty success.

and on the production of graphite:

"two carbon electrodes in a horisontal furnace and a mix of carbon and carborundum, heated through the application of 9000 amperes at 80 volts alternating current....for at least 30 hours"

"Electrical engineer's Pocket-Book" seventh edition copyright 1913

StevenRS - 22-3-2008 at 14:39

I have read over the thread, and decided to resurrect it. I have been working on a very small crucible for melting precious metals, and need ideas. I have made a Titanium oxide and sodium silicate crucible, and it is rock hard, but I still want to be sure it is good before melting any of my expensive stuff. Any thoughts?

Also, it might make a decent normal crucible if sized up.

microcosmicus - 22-3-2008 at 14:46

Try it out with cheap scrap metal before melting the expensive stuff.

StevenRS - 22-3-2008 at 15:03

I just did, it worked fine. Zero weight change.

microcosmicus - 22-3-2008 at 15:37

Zero cracks too, hopefully --- I doubt you want the
expensive stuff spilling all over the place should
the crucible break :o

Good luck with our casting project!

[Edited on 22-3-2008 by microcosmicus]

StevenRS - 20-9-2008 at 15:34

I have recently been using my microwave as a heat source to melt metals, up to brass. I am using a kaowool insulated brick of silicon carbide bonded with sodium silicate, and a ceramic crucible sits on top of it.

While this works, the problem is that the crucible is slow to heat as it only sits on the SiC. 30 sec in the microwave, the SiC is orange hot, but the crucible is still cold. What I need is a small SiC crucible, so the container for the metal is directly heated by the microwaves.

I have tried to make a SiC crucible bound with waterglass, but this does not work, it is to soft at high temp and bubbles form in it; I also have tried decomposition of Pitch in the SiC to bind it with carbon, and the result was a soft powdery clay. No good.

Does anyone have any idea how to make a SiC crucible without pressure? Heating is no problem, I have achieved temperatures hot enough to melt the kaowool into a puddle.

12AX7 - 20-9-2008 at 17:32

Have you tried sintering with clay? Certainly more refractory than Na2SiO4.


StevenRS - 20-9-2008 at 17:38

I have not, as I assumed most sintering processes used high pressure, but on research, I realize this is not so. What type of would be best? Regular pottery clay? I have some of that so i will try it.

12AX7 - 20-9-2008 at 17:50

Ball clay would be reasonable... high fire and sticky. Not as sticky as bentonite, but doesn't melt as easily; stickier than kaolin. Random pottery clay may be a mixture, check to be sure.


not_important - 20-9-2008 at 18:28

Alternatively consider making a sandwich from two sheets of rolled clay with a filling of SiC with a couple percent of clay as the filling. Cut out a circle for the crucible bottom, trim the remaining part and roll it into the crucible wall, attach the bottom, then seal exposed SiC with more clay. Obviously pick a high fire clay. End result is a clay crucible that heats well in a microwave.

If you want stickyness and are going to try a fully SiC crucible, use a plastic resin that goes to char without much bubbling, or a thick tar softened with a drying oil, as the binder; fire the stuff as hot as you can so as to get some bonding of the Sic to itself. The SiC should be at least 1/3 very fine grained material.

StevenRS - 20-9-2008 at 18:39

I tried charring various organics, and none of them were very effective. The resulting structure (or lack of) heated very well though, red heat in under 30 seconds. Tar worked for a while, I might try that again.

Also, I ball mill my SiC into dust, like 350 mesh. Is this to fine, or should there be coarser material mixed in? I could get it even finer if that is needed.

12AX7 - 20-9-2008 at 19:07

That's probably fine, it should make a lovely consistency with clay.

I'd beware of differential expansion building a layered stucture like that. Better to have it evenly distributed I think.

The problem with pitch is, it needs to be fired ca. 2000C to sinter and recrystallize the graphite.


[Edited on 9-20-2008 by 12AX7]

StevenRS - 20-9-2008 at 19:25

Yea, I guess I did not get it hot enough. (the tar attempt). Will definitely try again. How much tar or clay should be used? I was thinking about 12% by weight tar, but I have no idea how much clay to use.

EDIT: I tried with tar, and had poor results. The resulting amphorous carbon burned off before it sintered. I thought I had kept O2 away, but I guess not.

[Edited on 20-9-2008 by StevenRS]

OMG - 17-10-2008 at 19:10

I had an idea for making aluminum oxide by heating the crap out of aluminum phosphate. It was also a possible way to make phosphoric oxide too.
Could this be doable? just make a paste of AlPO4 and form it into a shape, then cook it?

kclo4 - 17-10-2008 at 19:51

Hmm, and what temperature does AlPO4 decompose at? I bet its really high.

12AX7 - 18-10-2008 at 09:35

Phosphate refractories are quite resilient and used for specialty purposes (tundish etc.). The active phase usually consists of calcium, magnesium or aluminum phosphate, forming a matrix around dead burnt magnesia, silica, alumina, mullite or whatever. Phosphates withstand considerable temperatures, ultimately forming glassy melts as silicate does, but with somewhat different properties (IIRC, the two are immiscible, so a dispersion of calcium phosphate in a glassy silica matrix creates a translucent material, known as bone china.)