Casting into Plaster/Silica Moia*
Introduction to Kiln Casting
The term “kiln casting” covers any number of methods of casting glass into plaster/silica molds as well as molds fabricated from other materials. It is a technique practiced by many glass artists today.
Before casting, one should dry the mold properly. The glass is then placed into the mold. Bring the mold and the glass up to temperature according to both the thickness of the mold and/or the thickness of the glass being cast. You do not want to raise the temperature so quickly that the mold will prematurely crack or the glass violently break and fly all over the annealer. (See Libensky annealing chart on pg. 53. This chart also has the drying time for molds.)
Once up to casting temperature, the mold is held at that specific temperature until the glass has flowed to all parts of your mold. The thicker the mold, the longer it will take. The temperatures needed to get the glass to flow properly is usually 1475° F - 1550° F (801°-843° C). I do not recommend going much past 1550° F (843° C) as the mold really begins to break down above that temperature.
Chunk casting is perhaps the simplest method of casting and is usually done with simple open faced molds or with the flower pot system. If you’re using the chunk method for the first time, you may think that you’ve calculated the quantity of glass wrong as there will appear to be a great deal more glass than will comfortably fill your mold. Fear not, since half of what you have piled up in your mold is air spaces between the pieces of glass. The first time I used this process I estimated correctly the amount of glass needed but second guessed myself when I saw
what looked like a huge amount of glass in the mold.1 I removed what I thought was an excess; after firings however, I realized my calculation was correct but my ] instinct was wrong. “Measure twice, cut once.” Seei photo on page 87.
Flower Pot Casting
If a closed mold or a container form is being cast,! other methods of filling the mold must be utilized. One j of the cleverest methods employed is the following! flower pot method. Expand the hole in pot to about ] an inch (2.54 cm). Suspend a terra-cotta flower pot | over the opening of your mold to a distance of about ] an inch or so. Use pieces of brick or pieces of kiln shelf to suspend the pot. Fill the flower pot with] casting glass. Use at least 10-15% more glass than you’ve calculated, for a good quantity of glass will adhere to the sides of the flower pot. When the mold reaches casting temperature, the glass will flow out of the flower pot hole and begin to fill the mold. If the flower pot does not hold enough glass to fill your mold, it will be necessary to add glass to the flower pot until the mold is completely filled. If the pieces you’re adding are large, you should pre-heat the glass in order to avoid shattering the additional cullet. It takes quite a bit longer than you think for the glass to flow from the flower pot. I’ve taken up to 4 hours for a simple small mold at 1500° F (815° C) using Bullseye casting glass. If your mold is constructed well, and if you’ve put chicken wire in the mold, it will easily take extended casting times. The book: Techniques of Kiln-formed Glass, by Keith Cummings, published by Penn/Black, is okay and worth th
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