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Author: Subject: Crucibles and Metalcasting
Democritus of Abdera
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[*] posted on 13-8-2004 at 16:44


Quote:
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)




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[*] posted on 13-8-2004 at 17:01


No.

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.




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[*] posted on 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.
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[*] posted on 14-8-2004 at 02:12
Re: Crucibles


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.

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[*] posted on 14-8-2004 at 07:05


Quote:
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




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[*] posted on 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.
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[*] posted on 17-8-2004 at 15:37
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. :D

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!




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[*] posted on 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.
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[*] posted on 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...:)




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[*] posted on 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.




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[*] posted on 26-8-2004 at 14:18
Stainless


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...
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[*] posted on 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

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]




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[*] posted on 31-8-2004 at 09:28


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
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[*] posted on 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?




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[*] posted on 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?




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[*] posted on 3-9-2004 at 18:12


Bromic,

Wouldn't that material insulate the melt?

[Edited on 4-9-2004 by FrankRizzo]
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[*] posted on 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.



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[*] posted on 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. :(




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[*] posted on 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.
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smile.gif posted on 8-9-2004 at 01:32
Final bit


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
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[*] posted on 8-9-2004 at 09:35


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]




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[*] posted on 13-9-2004 at 13:14
More stuff


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)




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[*] posted on 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.
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[*] posted on 13-9-2004 at 14:17


Quote:
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 m.pt. 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.
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[*] posted on 13-9-2004 at 15:59
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]




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