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I've purchased methyl cellulose powder from art supply stores- mix to whatever consistency you like- it's used to thicken the water for marblizing paper... also used for stage blood since it coagulates so nicely. I've not tried the powdered form for glass, but the Klyr-fire works best for me when it's in "partial coagulation"- not too slippery and dries much faster...

Ralph wrote:

I wonder if it could be used in sand casting as a binder. You would have to dry the mix completely before firing and not get the glass hot enough to blow bubbles as there might be some gasses kicking around from the burnout.

You're right on the money with this idea, Bert and I think it would work better than you might expect.

I've done quite a bit of work with shallow-relief casting and have now abondoned traditional plaster/silica in favor of pure calcined alumina bound with CMC.

Be very surprised if you have any burnout problems. I've worked with CMC and glass for 2 years (and in ceramics for 20 before that) and never had a burnout problem.

Another advantage in my work is the reduction in volume of mold material used. I use (or even make) alumina molds right on mullite shelves. The mullite provides support and the mold can be very thin, often less than 1/8".

By all means try CMC with sand. If you want to avoid the silica thing you can get cheap alumina in various meshes. Anywhere from #60 to #100 should provide a similar texture to fine sand. The best thing about alumina is its incredible refractoriness - so reluctant to stick to glass.

A reminder - CMC has little strength unless it is completely dry. It's weak when there is any moisture present.

R

Ralph

This sounds like a very worthy research project!

For a long time, I worked with alumina mixed with diatomeacous earth and some plaster. I had to mix the D-earth in to make the alumina stand up and hold some dimples. I have never seen larger particles of alumina here in the US. I guess I'll have to look in to that.

I did try aluminum oxide sand blasting grit and it sticks big time. I am now using olivine sand mixed with plaster. It doesn't stick on the bottom, but if it gets on the top it seems sink in and stick.

Thanks again!

I did try aluminum oxide sand blasting grit and it sticks big time

That's a surprise. I haven't used blasting grit for molds but always assumed it would work. Aluminum oxide blasting grit should be Al2O3, as is calcined alumina; alumina hydrate should be Al2O3 with some water of crystallization. At firing temperature they should all be similarly refractory.

I use two grades of calcined alumina (#100 and #300) from the pottery supply and they have the same firing behaviors. Now I'll take care if trying aluminum oxide abrader.

BTW CMC/alumina mixes won't replace large dimensional plaster investments - simply not strong enough. It's fine for flat, open face molds with shelf support.

R

Ralph wrote:

I did try aluminum oxide sand blasting grit and it sticks big time

That's a surprise. I haven't used blasting grit for molds but always assumed it would work. Aluminum oxide blasting grit should be Al2O3, as is calcined alumina; alumina hydrate should be Al2O3 with some water of crystallization. At firing temperature they should all be similarly refractory.

I use two grades of calcined alumina (#100 and #300) from the pottery supply and they have the same firing behaviors. Now I'll take care if trying aluminum oxide abrader.

BTW CMC/alumina mixes won't replace large dimensional plaster investments - simply not strong enough. It's fine for flat, open face molds with shelf support.

R

Ralph

I learned a lot today about alumina. I did a search and found a company that manufactures both alumina hydrate and calcined alumina. The calcined alumina is more pure and it has no attached water. On the other hand once alumina hydrate has been fired it no longer has the water attached. It is less expensive.

I also learned that Australia has the best bauxite deposits in the world which explains why you have more alternatives and calcined alumina is not too expensive.

He is sending me samples of 3 grades of alumina. One is multi sized with all but 10 - 15% going through a 100 mesh screen. This is the coarsest he supplies. He is also sending a superfine powder. I think this is the find for making top notch kiln wash. I'm not sure about the 3rd grade.

I am really interested in discovering how a kiln wash, made of alumina hydrate mixed with CMC in the water, will work. If my picture of it is correct, the CMC will burn off each time and the powder can be wiped off and saved for reuse. This would be a great system if it works.

I understand what you mean about it not being strong, but it is likely stronger than no binder at all. I have been having a problem with sand getting on the top surface of my glass. I'm not exactly sure when or how it gets there, but it leaves bumpys that I don't want.

I can see mixing up some olivine sand, some alumina of varying particle sized, perhaps some bentonite, and some CMC in water and wetting the mix, sifting it out, making the mold, drying it out, placing the glass and firing.

I have not messed around with the bentonite yet. I am hoping that it doesn't leave scum on glass like kaolin does (after the second firing).

Bert, the alumina files are growing. Good thing because it's a useful material.

I am really interested in discovering how a kiln wash, made of alumina hydrate mixed with CMC in the water, will work. If my picture of it is correct, the CMC will burn off each time and the powder can be wiped off and saved for reuse.

Wish it was this simple. I've found that 300# alumina/CMC kilnwash degrades. Yes, you can recover it for re-use but the performance reduces. You may remember my postings about flux leaching. Some of the flux-leach theory came from observing the behavior of thin alumina shelf washes. (Incidently I think flux leaching would be better known as "flux-fume contamination").

If alumina is expensive in the U.S., re-use by all means, but mix in some new stuff with the old to work against the flux being picked up in the alumina. I've used and re-used 100# alumina completely dry as a shelf wash and it works OK -- fine-sieve it on. However the alumina particles are very mobile and you have to take great care not to get them on the wrong side of your glass - I fire multi-level shelves so eventually canned this approach.

but it is likely stronger than no binder at all

For sure.

You may get some pleasant surprises playing around with bentonite and CMC. Wyoming bentonite, probably the stickiest of them all, synergises with CMC. It's likely that you'll get a whole new level of binding strength in combination. I haven't done anything in this area yet, but I remember the effect of them together in pottery glazes. Good bentonites are just as hard to mix with water as CMC. Try a kitchen blender or some other hard mechanical agitation.

Your concern about scum from bentonite could be well-founded. If you get scum from kaolin I wouldn't be surprised to see scum from bentonite. Depends... Kaolin is a "clean" large-particle primary clay. Bentonite is a "dirty" highly weathered clay that may even contain organic residues.

Did you notice Ellen Abbott's comment in another post that she doesn't use CMC in her beautiful PDV because she gets a burnout residue. Isn't it amazing how we all find our own ways through the limitations and pitfalls of our individual materials and processes?

Ralph