FILLING THE MOLD WITH FRIT OR CHUNKS
The easiest way to kiln cast is to simply fill the mold with frit. Unfortunately, this is not as straightforward as it sounds. That’s because frit compacts as it melts and settles down into the mold. How much it compacts depends on the size of the frit (or chunks) used. The smaller the individual pieces of glass, the less it will compact. A good general guideline is that it will compact to about two thirds of the original size.
There are several ways to deal with the settling of the glass. The most straightforward is simply to top off the mold with fresh frit as needed. This method is commonly used in connection with a reservoir at the top of the mold. This approach requires that you fire the glass, soak a while to allow for settling, then open the kiln to add more glass to the top of the mold. Obviously this has the potential to be an unsafe operation, so care must be taken to prevent burning or electrocuting yourself. Always turn the kiln off before reaching inside and always protect yourself with safety equipment and common sense.
If necessary, topping off a mold can be done several times. Frit is small enough that thermal shock will not be a problem. Moreover, at casting temperatures (about 1500 to 1550 F) frit will melt quickly, so just let it soak for 15 to 30 minutes, then check to see if more is required. You can keep adding fresh frit until the mold is full.
Another way of dealing with the settling of the glass is to take advantage of a reservoir to hold extra glass at the top of the mold. If your casting allows it, you can use large chunks of glass that sit on top of the mold and drip into the mold as the temperature inside the kiln increases. This is easier (and can be done with frit of any size) if you build a reservoir into your mold to hold glass until it melts and falls down into the main part of the mold.
Finally, if you plan to cast by filling the mold with frit or chunks of the same color (or mixed indiscriminately) you can estimate how much glass will be required by using this simple technique.
First, get a jar and fill it with water. On the outside of the jar, mark the top level of the the water inside the jar.
Second, pour water from the jar into the mold until the mold fills. The mold may not be watertight, so you’ll need to pour quickly before the water wicks away into the mold.
Third, fill the jar (which should still have some water remaining) with the glass frit you want to use until the water level in the jar reaches the original mark you made in step one. The amount of frit required is the amount needed to fill the mold cavity.
Finally, allow the mold and glass to dry thoroughly before firing.
(This method can also be used to estimate the amount of glass needed for crucible casting, but you will need to add some additional glass to account for the glass that will stick to the crucible.)
Regardless of the method you use to fill the mold with frit, you'll follow the same steps once the mold is finally full. Soak a few moments to allow the casting to fully form, then begin the annealing and cooling process.