Sciencemadness Discussion Board

Magnesium Oxide Crucibles

runninfarmer - 9-11-2012 at 17:34

I've seen you can buy magnesium oxide crucibles on this website:

I'm thinking I could make one of these but was wondering if anyone has hand any experience with it? Do you just form Mg(OH)2 in a mold, dry it, and then fire it? Or do you need a binder and what binder do you use?

I've looked through the threads on here, particulary this one:

But I can't find the answers I'm looking for. I'm trying to make a MgO crucible/brick for an arc furnace project. I've tried some of the arc furnace threads to no avail.

I appreciate any help and sorry if I did miss this in a previous thread.

[Edited on 10-11-2012 by runninfarmer]

[Edited on 10-11-2012 by runninfarmer]

plante1999 - 9-11-2012 at 17:48

Well whats the temperature involved in your project?
Will you use inert atmosphere?
I have tried fused silica and carbon refractory but never MgO one, it could be interesting to try.

runninfarmer - 9-11-2012 at 18:02

I would like to be in 1500-2200C range in air. Mainly to melt iron. I would like to try to make silicon from SiO2 which will require an inert atmosphere in the future. I figured a MgO crucible/brick was my best bet.

m1tanker78 - 9-11-2012 at 19:06

I believe MgO crucibles are sintered for long periods (without binder). I don't know the process but recall reading about it in the literature. Sintering implies maintaining the temp just below the melting point which is rather high ~ 2800C. It should be able to handle molten iron provided it maintains good mechanical strength.

How big does the crucible need to be??


runninfarmer - 9-11-2012 at 20:18

It appears you're right and you don't use a binder. I found this article from Los Alamos that describes making MgO crucibles for plutonium production. They used an induction coil to fire the crucibles at 1750C:

The issue will prolly have to do with the firing step, since their induction furnace involves a coil around a graphite cylinder. Any ideas?

12AX7 - 10-11-2012 at 18:41

Sintering takes place very slowly at roughly half the melting point. Where exactly depends on the material, and particularly on impurities. If a lower ultimate melting point is tolerable, liquid phase sintering can be done at a much lower temperature. Typical application might be MgO bonded with 5% bentonite (a sticky, rather impure clay, primarily aluminosilicate) and fired at as high a cone as you can get a kiln to go.

Something expensive and brittle like MgO isn't needed for melting iron, a simple commercial clay-graphite crucible can be bought cheaply.

If you just want a brick to putz around in, why not locate a hunk of limestone, dolomite or magnesite rock, chuck it in a kiln at a much lower temperature (I believe around 1100C is typical for calcining, obviously more won't hurt), and work with that? Calcined material is easy to carve, yet not so soft that it just falls apart. CaO-MgO isn't ideal, but the eutectic still melts about 2100C.


platedish29 - 4-12-2012 at 16:35

I always end up breaking my crucibles apart. Last time some gold was lost among a piece of the crucible I could never find again.

Fleaker - 4-12-2012 at 17:37

MABOR brand cupels are MgO. We use them to melt small quantities of Ir, Pt, and Rh in.

hyfalcon - 4-12-2012 at 18:02

If you are going to be firing iron then you better learn to use green sand molds. Try searching "iron melting cupola furnace" on google and then follow where that leads.

ElectroWin - 9-8-2013 at 17:37

US bureau of standards circa 1917 had specifications for fire clays suitable for crucibles for melting iron; i'm recalling this from memory, but i believe they used kaolinite clay, (above 95% aluminosilicate) with a certain percentage hard-burned grog, to add strength during firing. they specify that the grog should pass a 32-mesh sieve, if i recall correctly.

as temperatures increase above 1600 C, crucible makers suggest moving to 99% alumina.

Fleaker - 22-8-2013 at 07:05

I'd still say buy some cupels. They're cheap.

Check out Legend Mining Supply in Reno, NV.