homemade crucibles

[Matt22191]

HAVEHEATWILLCAST posted this article over in the foundry engineering forum: http://www.copperstarways.com/Downlo...gCrucibles.pdf

I'm intrigued, but the idea of finding hard firebricks and crushing them into such fine grit doesn't appeal to me much. What about just getting some 80 grit silicon carbide or aluminum oxide sandblasting media and substituting that for the crushed fire brick?

[Tim]

What about just getting some 80 grit silicon carbide or aluminum oxide sandblasting media and substituting that for the crushed fire brick?

And bonding them with what? Clay?

Industry uses graphite bonding with SiC, by decomposing pitch and phenolic (among other types of) resins to carbon in a reducing atmosphere and firing up to 2000C or so, where the carbon turns to graphite and sticks the stuff together.

Alumina can be bonded by clay and directly substitutes sand, grog, etc. in a recipie. I recommend looking up your clay's composition and calculating how much of each will make mullite.

I've heard of SiC bonded with clay. It could work. As I recall, clay-graphite crucibles have a natural carbide content, which would be due to reaction of graphite with the silica in the clay.

In any case, crucibles have to be fired near their melting point for maximum strength and density. This is the main problem with anything like graphite or SiC crucibles -- better to leave those to industry.

Tim

[Matt22191]

Yes, clay, just as suggested in the article. Just thinking of ways to avoid having to pulverize firebricks. Since firebricks are mostly alumina and silica, from what I understand, I figured either would work. But who am I kidding? Alumina's cheaper, so that'd be my choice!

[Anon]

Normally, silica sand would be cheaper. If you have a cheap source for alumina, though, the alumina should be somewhat stronger and more resistant to thermal shock. (Depending on what else is in the mix, it may also be more heat-resistant.)

[Matt22191]

I was comparing sandblasting media, based on McMaster-Carr prices. Might not reflect the typical case.

[acoop499]

Isn't Alumina just aluminum oxide?

[Anon]

It is indeed.

[welder19]

Can't you get some from the dross skimed from al melts?
I read where (combined with some other ingrediants) it can be used as a crucible coating.
I'll try and find where I read it at.

welder19

[welder19]

Here it is, I found it, it was Frank at theworkshop.ca

Again the Electric Wheel chair mixer is put to use... The dark powdery substance is the powderized dross from aluminum melts.

The dross is actually Aluminum oxide or alumina... Alumina is a rather robust chemical in that it's closer to a ceramic than a metal and can withstand a considerable amount of heat.

Here is a link to the page so you can see the rest of the ingrediants and how he made it.
http://www.theworkshop.ca/casting/Fo...15/Fndry15.htm
I just wonder how much of what is skimed off is actually al oxide?
Is it everything minus the actual al, or is there other junk that you would need to get out to make it usable?

welder19

[HAVEHEATWILLCAST1]

I just wonder how much of what is skimed off is actually al oxide?
Is it everything minus the actual al, or is there other junk that you would need to get out to make it usable?

Tim put up a reply when I ask about refining Al dross (you can search) and it went something like: Melt all the dross with a lot of salt to get the Al out. Then wash the dross/oxide from that a few times to get the salt out. Then dry, use and enjoy. About the same as Frank said but in Tim's great full learned style. ( He throwed in some "if you want'a and betters".)

I'm saveing my dross(what little I have) even though I now have some used Aluminum oxide blasting sand that does have junk in it.
That's why you can use wood ash too, its already been burnt. Wood ash does have some acid in it (fluxs I think).

Aluminum oxide really "holds" heat but in doing so will not let go of it and reflects it back. Heat will not go to a hot place. The walls of the furnace get hot and reflects heat on the melting pot. And if you paint the pot? It trys to reflect the heat too but the furnace walls are bigger so the pot gives in and takes the heat.

Let me link that I found last night. I was going to put it in a post of Anon's anyway.
"According to the Second Law of Thermodynamics, heat cannot flow from a colder body to a hotter body, so the additional heat absorbed by the coated crucible must be conducted through the coating and the crucible to the colder load inside the crucible."
http://www.hwr.com/products/Product_...-rev_03-07.pdf