Here is some further detail about the problem ...
- I used Spruce Pine batch with various oxide colorants to try my hand at my own colors
- I mixed small batches in crucibles for melting into glass in a high temperature kiln at 2150F.
- after the glass was formed, I moved the crucibles to another kiln, positioned to flow the glass into molds.
- after the glass was shaped, I compared results, and adjusted recipes to tweak the colors.
- after arriving at a couple pleasing colors, a larger batch was prepared, then added into an empty furnace, with a large crucible reservoir of about 120 lb capacity.
- once melted, this glass was ladled into a steel frame on a marver table, then transferred to an annealer.
- the two colors made, a yellow and a blue, both turned out beautifully at this stage.
These billets were then used to cast several shapes in large plaster/silica molds. Biggest, 10 " dia solid cast sphere, smallest 3 " cube. They were all fired together, and in order for the big spheres to be properly annealed, the whole cycle was necessarily slow, and there was a prolonged time about 1250F.. where devit can occur.
Some of the small cubes were not this glass, but were glass made deliberately for casting.. One was a Bullseye blue, which was one of the clearest of the resultant cast shapes.
The big spheres were badly devitrified... the outer surfaces of all the starting billet pieces were visible and clearly covered with devit that looked like thick flannel. In addition, in areas away from the edges and faces of the starting pieces, there were spherical burst-like growths of crystals scattered around. A mess, as I said.
In addition, there were an unusual number of large bubbles trapped against the surface of the mold wall, particularly at a level that seemed to be when the internally packed glass inside the sphere leveled off, and the reservoir glass flowed in on top of it..
Can anyone tell me:
-what is the fundamental difference between this glass and the glass made specifically for casting?
-I had hoped to make a whole array of my own colors, and then select certain ones to make for casting. Is this really not possible based on glass batch formulated for blowing? I am disappointed that if I make my own color this way there is nothing I can make with it by casting.
- is there any patch I can do to keep this process going? I am conducting tests to see if there is a heat work level that the glass can withstand (perhaps up to 1 " thickness, with fastest forming and annealing possible). Are there any additional additives I can blend in back at the batch stage (I mean here add to the already formulated Spruce Pine batch, since I am not making my own) that would have a chance to produce a workable casting glass?
Any comments or questions would be welcome, and might save me a ton of wasted effort in useless work. Thanks!