Crucibles and Metalcasting

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!

Don't tell me use iron (fails) or just fireclay (too weak) or porcelain (too easy) or a teacup (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... not likely though

[Edited on 8-8-2004 by Cyrus]

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

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 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

Cm'on Cyrus!

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 ]

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

THANK YOU BROMIC ACID!!!

BEAUTIFUL! EXACTLY WHAT I WAS LOOKING FOR! IT WILL BE TRIED!

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

The Nickel Carbonyl idea just scares me...

-start rant-

Democritus, did you write or market that booklet? 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, 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-

IT IS A JOKE!

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.

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

Actually, the idea was mostly Theoretic's.

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!

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

Next I am going to do some tests!

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...

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.

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!

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

Here's a pic. to illustrate.

well...uh...actually...

Quote:

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.... http://www.miniworlddolls.com/evenheat/ConeInfo.htm

Quote:

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! 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

Thanks, Democritus. Interesting...

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

Hmm.

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.

anyway;

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 )

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.

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.

Aw, those stupid laws.

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?

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)

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)

Re: Crucibles

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!

Results of research and testing

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

1-

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.

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


2-

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.

3-

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.

2-

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!

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...

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.

Stainless

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)

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

NOTHING HAPPENED TO THE CLAY GRAPHITE ONES!

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]

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

I read sometime ago in gallica.bnf.fr 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

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

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.

Final bit

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

NO, I AM NOT GOING TO TRY THAT.


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

Taken from "THE SPIRIT OF CLAY" by Robert Piepenburg

~start quote

Bentonite

---------------------------

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.

Edit--

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]

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

General Casting and Crucible info

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

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!

Enjoy!

http://budgetcastingsupply.com/images/Crucible_Data_Bartley-...

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]

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 backyardmetalcasting.com (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.

I STILL think sugar will work for a phenolic resin! 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! The only other oil available was waste car oil, which isn't nearly as energetic.

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.

Thanks,
Paul

"Corelle ware" crucibles?

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

Good luck with our casting project!

[Edited on 22-3-2008 by microcosmicus]