GLASS FIRING TIPS
Glass COE refers to the Coefficient of Expansion. It refers to the rate at which glass expands/contracts when heating/cooling. You CAN NOT mix glass with different COE into the same project. Because the glass pieces contact at different rates when heating/cooling, the piece will crack apart.
Use colored dots on your glass pieces to keep track of whether they are COE90 or COE96. Or only buy one type of glass.
The heat-up portion of the glass firing goes quickly, usually less than 1 hour. It is best to attend the kiln and watch your pieces as they approach maximum temperature. Then you can leave it for the slow cool / annealing process which takes longer.
When firing glass, keep in mind that there may be a significant difference between the temperature of the thermocouple/kiln vs. the glass itself. This temperature gap can easily be 50 degrees or more. Glass is not a particularly good conductor of heat, so it will lag behind the temperature of the kiln when heating up. If you use a small kiln that heats up quickly, this effect will be exaggerated and the temperature difference will be greater. If you use a larger kiln, which tends to heat slower, the temperature gap is likely to be less. Remember that ultimately it's the temperature of the glass, not the kiln, that will dictate when the glass will tack and flow.
When you stack several layers of glass, they will seek their own level, which is about 1/4"-5/16” thick. (Unrestricted, glass just naturally wants to be about that thick. It's a characteristic of it's molecular properties.) To make thicker pieces, you need to contain the glass inside a mold. For example, if making glass tiles, use one of our glass tile molds.
A thin piece of glass will shrink more during firing than a thicker piece. (Again, this is the glass wanting to become approximately 1/4" thick.) Also, glass pieces tend to start "rounding" when fired (corners become blunt, etc). This is due to the natural properties of glass, as well as a fluids natural desire to flow into a less stressful, more efficient/natural shape.
If your glass piece cracks during firing in the kiln, check the edges. If round, it broke during temperature ramp up. If sharp, it broke during cool down.
When using molds, it is best to use ceramic (clay) molds for slumping into. For draping over, it is often better to use stainless steel molds. This is because of these materials relative thermal expansion to that of glass, and their willingness to release the glass, instead of breaking it, as it cools
You can use the different available thicknesses of shelf paper to cut out designs for relief patterns in your glass. Just place the glass on the cut-out paper shape and there will be a relief pattern after the glass has fired. With the thicker shelf paper (1/8” and 1/4”), you can also fill a cut out design with glass frit to make a thin piece of glass in the same shape of your design. This can then be fused to a larger piece of glass; ie similar to sprigging with clay. (Save the part you cut out, as that can be used to make a relief pattern as explained earlier.)
You can use fiber shelf paper to make a through-hole in a glass project, for example, should you want to thread a chain through a glass pendant. Just roll up some paper into a tube, and place it between two sheets of glass. There will be a gap where the shelf paper tube was. (Make sure you use Fiber Paper for this and not Thin Fire Shelf Paper.)
Did you like these tips? Want to try some glass projects? If so, we carry a great book called Introduction to Glass Fusing you might consider getting. It has great info and tips, firing schedules (ramp, hold, and annealing times), how-to's and projects that take you step-by-step through glass fusing. Highly recommended.