For the last couple of years I have been making a lot of designs in glass clay. I have found it limiting in the fact that you cannot produce transparent work.
So last year I embarked into kiln casting glass . I made some wax models of small designs for pendants and made rubber moulds of these so I could produce many wax models for making moulds using the lost wax process.
My first attempts were not good, the surfaces were rough and parts did not cast at all. I tried many different firing programs, but nothing really worked. The heat seemed to reach into the top of the mould, but not into the more encased areas. Also the surface of the casting was never smooth.. I had to spend hours grinding and polishing the glass on the pieces that did come out whole. I noticed pictures of others work before cold-working was also rough.
I joined many forums, but the conclusion from these, was that small pieces take as long to cast as big pieces.
I started to think was it the plaster , after all it is an insulator and above 830C it starts to interact with the glass preventing it from flowing properly. It must also be heated slowly to prevent cracking.
Why is it plaster been used at all for glass casting, I began to ask myself?
When I was making peices in glass clay I found a material that did not seem to stick to glass. So I made some into a putty and pressed this around one of my wax models. I melted the wax out and was able to see it had picked up all the detail. A good sign, so I heated up the mould to burn out the binder.
After the firing it remained as strong as a plaster mould and I was able to drill small holes in it by hand with a small drill bit to allow air to escape.
I reheated the mould to melt the glass frit. I was able to blast this up as fast as I could without the mould been effected. I fired around 800 C for a long time to melt the glass. After firing I was able to break open the mould and the glass had not stuck to the mould . Great, so I did some more testes. These I fired higher to speed up the process. The higher I fired the more successful the casts , the more he moulds glowed, the more detail was in the cast and the stronger the mould became.
However the higher I fired the more of a residue was left on the surface of the mould. This took time to clean off as with the polishing the casts from the plaster moulds .
So although I ended up with a much higher success rate in terms of complete castings with more detail, there was still a lot of time being spent cold-working.
I started to search for a releasing agent used in the manufacture of glass. I found one that applied thickly prevented the glass from sticking to the moulds I was using. However it would come off and would go into the glass .
It did not prevent the glass from sticking unless it was applied very thickly . Applied thinly there was some sort of interaction between my mould putty and the releasing agent. It seems to fuse to the putty and the glass. There by making it difficult to remove.
So with two new problems , I firstly found a binder to prevent it coming off into the glass, which I am still tweaking. Then because the releasing agent is fairly pricy , I thought can I put some sort of thin layer between the releasing agent and my putty moulding material. This last process is not necessary if you are not worried about the cost. However if you are making a very large object then you might what to consider this.
I have gone back to the plaster for this , but just a very thin layer .
My first experiments have been very successful, please see pictures below of two similar sized objects of about 1.5 cm in size. The pictures are not very good, but you can see the rough surface of the item cast in plaster and the one cast by my new technique. Which by the way need no real firing schedule, just to go up slow to burn out the binder , and then to blast it up and down a few times before maintaining a constant temperature for a hour or two to allow bubbles to come to the surface and the top to even out.