Trouble kiln casting with billets made from Spruce
Hi everyone... I haven't posted in years, but I am in need of some wisdom, and decided to see what you thought.
I recently had the opportunity to make some of my own colors using SprucePine batch and various metal oxides in a high temperature kiln. At first I only made small test samples just to tweak the color. I made two colors that I really liked, a golden yellow and a blue, and invested in enough batch and chemicals to make perhaps 400 lbs of the two..
I had some time with a furnace, melted the batch, and then formed billets by ladling the glass into a frame on a marver table, and then putting them into an annealer. The billets were beeeuuuutiful.
Then I tried to kiln cast with this glass, using a flower pot as a reservoir, and flowing the glass into a plaster/silica mold. and it was a disaster... extreme veiling and devitrification, with a lot of bubble formation at the mold wall. The shapes were 10" diameter spheres, solid castings, so the top temp hold time and annealing time were necessarily quite long. Smaller castings with other glass in the same kiln run were clear. (glass specifically made for casting.) ((all molds were plaster/silica of similar mix))
It seems that I overlooked one TINY detail about this glass. I have now been informed that batch for blowing is formulated with a goal to make glass of even viscosity over a wide range of temperatures. Casting glass batch is formulated (I am told) to resist de-vit and flow at a lower temperature. And I clearly did not use it as intended....
So my questions are these...
What is a typical formula for a blowing glass batch?
And a casting glass batch?
Are the fundamental ratios of glass former, flux and stabilizer different in the two?
Or is one a kind of subset of the other, but with additional additives to provide the altered characteristics needed for casting?
Is there any feasible additive (or set of additives) that can be added to batch made for blowing (if possible, specifically Spruce Pine, since that is what I now have on hand) such that it will produce glass that is robust enough for casting?
The glass may not be a total loss, if I can find a threshold of heat work that it can tolerate that can still permit thin castings. So far all heat work tests, even as short as possible, show de-vit..
And one final request for input... Is the following analysis correct? Please correct/clarify it for me if you can.
blowing glass stays for days in a furnace, and resists cords and property changes over time, and yet does not perform well at the much lower temperature when it comes to components within the glass beginning to crystallize and cause devitrification. In hindsight I guess this is because at high temperature the heat is sufficient to keep everything molten and crystallization is not possible.. degradation happens over time by a slow loss of flux from the melt. When this same material is much lower in temperature it is in a temperature region where it's component elements can begin to crystalize out of the melt. Casting glass has added components that reduce the mobility of the materials that tend to crystallize so that the crystals do not form as readily and increases the complexity of the melt in a way that interferes with and slows the crystallization characteristic of de-vit.
Any thoughts would be helpful