Alumina slip casting - burner head and crucible

[thijsvh]

Hello all,

First post here to show some results of experimenting with the slip casting of aluminiumoxide (alumina). https://en.wikipedia.org/wiki/Slipcasting

Got interested to make shapes with refractory oxides by the posts of MonkeyForge and Mellin although they were using zirconia and veegum/bentone and were forming the mass by hand. Zirconia is a little too expensive to mess around with so i went with alumina. Using a combination of digitalfire.com, wikipedia, google patents, google scholar I found a lot of info which I all read. But a lot of it was above what was achievable at home. And a lot of it is about achieving the "perfect" results. I just want a useable result...

So... what did I do. I already had some alumina-bentone clay (97%/3%) because at first my idea was to make shapes by hand but I found that really difficult. So I made a batch of slip from this clay by adding lots of water and mixing it with a blender. Aiming for a consistency of heavy cream as was mentioned on DigitalFire. This I poured into a simple plaster of paris mold made out of a plastic drink cup. It worked but was incredibly slow in forming the walls. The plaster was sucking in the slip very fast at first but after a millimeter or so it slowed almost to a stop. Dried my plaster mold in the oven and tried again, same result. Not great, not terrible. Remembering what I read somewhere that bentonite has a "problem" with releasing water. The bentone in my slip was creating a layer and allowing almost zero water through. Created a new batch with 99% alumina and water. Poured into my plaster mold and behold incredible fast wall forming. Now almost too fast to keep the mold full. It dries really fast in the mold and releases itself from the mold walls within a couple hours. Then let it air dry for a couple days.

With this succesfull "recipe" I made a larger batch of alumina slip, specific gravity was around 2.40 and the bucket was really heavy. But the slip was still really fluid. It will settle if you leave it sitting for a while so you have to mix it good before use. Also made a mold for a crucible and a burner head. These can be seen in the pictures I added. The crucible was some random cup I found somewhere and the burner head was 3D printed. Also 3D printed a hole pattern as a guide to drill out the holes in the burner head when it was dry enough to handle. 

The crucible I have not fired yet. The two burner heads are already fired and 1 of them sintered well because it has a nice ring to it when you hit it with a metal item. The other one is strong but has a somewhat empty sound when you hit it. I didn't really had a procedure to fire them so maybe that's the problem. I fired slowly for 3 hours to about 1000 degrees celsius and then went full power for about 1,5 hours, no idea what temperature, then slowly back to 800 degrees in about 3 hours, then i turned off the burner and closed up with ceremic blanket to cool down during the night. Measured temperatures were at the exhaust of my furnace (my thermocouple wire will melt inside). I hope the inside reached at least 1200 degrees celsius. (Using natural gas and a forced air burner)

The same method can be used to cast items with zirconia, magnesia, etc. But you will need a higher temperature to sinter it well. Hope you guys can use some of the info and use it to create more cool stuff.

[MonkeyForge]

Hoi Thijs,

This looks very promising, slip casting was on my list but never got around to trying it. I added the molochite clay to my mixture to counteract the shrinkage and water repelling properties associated with bentone somewhat. This worked for a clay body but I don't know if it would for casting slip. I'd have to try. Also this means less % zirconium silicate. It would be interesting to see your plaster molds in detail.

All in all a nice contribution to the homebrew ceramics part of this forum. Keep us posted. 

[thijsvh]

Will make some pictures of my two plaster molds for you tomorrow. They are pretty easy to make, if you watch some Youtube videos about the subject it easy to understand the basics.

There is a lot of information about refractories, ceramics, IR reflective coatings, etc to be found inside old patents. But it is a bit difficult to find what you are looking for because there are so many. In the patent below they are using zirconium silicate and tiny amount of sodium silicate and water. No veegum, bentone or molochite. Have seen others were they did use some additives but from what I remember those were to keep the zirconium silicate slip in suspension longer.

ZIRCON REFRACTORY COMPOSTION AND METHOD OF MAKING IT

https://patentimages.storage.googleapis.com/cd/17/73/f2d904b5c6f474/US2880097.pdf

 

If you want to read more.... In this one they talk more about the forming of crucibles with various refractories and about how it's different with clay slip casting.

PREPARATION OF CRUCIBLES FROM SPECIAL REFRACTORIES BY SLIP-CASTING

https://nvlpubs.nist.gov/nistpubs/jres/23/jresv23n2p319_A1b.pdf

[thijsvh]

The mold for the burner is a two part mold and for the crucible a simple one part mold. The casting time for the burner was around 10 to 20 seconds after which the wall thickness was between 2 - 3 millimeters. The crucible took about a minute to reach 1 centimeter thickness. If you wait too long before dumping out the slip and the part walls are too thick you can recycle it by blending it with some water and mixing it back in with the slip.