# Looking to build a alumina crucible.



## OMG (Nov 14, 2008)

How hard would it to make an alumina crucible?
I was thinking of taking a bar of aluminum, using electrolysis with water and a bit of hcl to dissolve the aluminum anode as Al(OH)3, then dehydrating it a bit to make more of a thick goop, then pouring the goup into a clay mold and cooking it until all the Al(OH)3 converts to Al2O3.
Would this work? Would the crucible end up being too porous to hold molten metal without leaching it into the crucible? I read that ethanol can be used to take the water away from Al(OH)3 goup and make powdered Al(OH)3. Should I be trying to do this instead?
Are there better ways of making a good alumina crucible from aluminum? I would be making quite a few different sizes and I'd rather learn to make them myself than just go buy some.
I also need a rather large/long one of specific dimensions with little tolerance to fit properly into my homemade reverb furnace.


----------



## Lou (Nov 14, 2008)

Huge waste of time!! I can just send/sell you one. They're dirt cheap in bulk, I've got hundreds of them. I think mine are mullite, but they work for platinum.

Lou


----------



## OMG (Nov 14, 2008)

PM sent Lou.
So would it actually work to dry out aluminum hydroxide or would the crucible end up being too porous? 
Anyone know how they are actually made?


----------



## Lou (Nov 14, 2008)

Aluminum hydroxide is pressed under pressure and heat and perhaps a bit of binding agent (kaolin/mullilte perhaps), then its sintered at high temperature.

Not an undertaking for anyone here, myself included (I even have a furnace that could push that temp, but it's not worth the trouble).


Mullite is much more within reach, and it'll hold up to platinum for a melt or two.

Alumina will hold up to rhodium but it vitrifies and you have to boil the button in lye to remove the alumina veneer.


Lou


----------



## Joe (Nov 15, 2008)

OMG, I think you have a great question. Allumina crucibles I've seen are over $100 for larger sizes. It would be nice to be able to take an abundent material and make custom sized refactory castings. 

Sorry, I can't be of techinical help. However, this site sells castable allumina and other materials. In the links it shows a casted funace tray. So, what you are onto might be possible. 

http://www.cotronics.com/vo/cotr/cm_castable.htm


----------



## Lou (Nov 15, 2008)

Very impressive Joe. 
In their case, they're using some sort of chemical bond to do it. My guess is that it's based on a phosphate binder (much like platinum investment, for any of you jewelers out there).


I came at this thinking of him wanting pure alumina, which must be made by pressing and sintering. It's just a very difficult process. Truth be told, you treat a good alumina crucible well, and you will melt platinum with it for a long time. I recommend alumina over fused silica (quartz) for platinum for the following reasons:

Alumina has superior temperature resistance (less vitrification at temp.)
Alumina has superior mechanical strength (less phase modification)
Alumina's vapor pressure is less than silica at platinum's temperature.
This is important because very hot silica will fume silicon dioxide, which will freeze in air to very fine particles, which you will breathe in, and these might cause problems, i.e. silicosis. I was told this by glassblowers that deal in quartz apparatus. They take precautions, and so should any of us that melt our platinum in quartz!!




Lou


----------

