Skuttle Indoor Air Quality Products Klin Critical Temperature Ranges, Firing Processes, G L A S S

24 GLASS 101

G L A S S 1 0 1

Critical Temperature Ranges

A “Critical Temperature Range” is any temperature or temperature range in the firing cycle that has a high level of potential for limiting the success of the project. Limited success can be expressed as overfired, underfired, breakage, devitrification or bubbles, just to name a few. It can be argued that there are numerous critical temperature ranges. To keep things simple we are going to discuss the primary four: Heating Range , Process Range, Pre- Annealing Cooling Range, and Annealing Range.

HEATING RANGE

The Heating Range goes from room temperature to the first set of data in the Process Range. The only concern during this range is heating the pieces too fast. Small pieces can normally be heated faster than larger pieces.

PROCESS RANGE

The Process Range is the temperature range where the material begins to visibly change. It is this stage that deter- mines the final shape of the piece. During the Process Range temperatures and hold times are key. If you are unsure of the desired peak temperature you may want to start on the low end of the range with a longer hold time. This will help insure that thicker pieces receive the proper heatwork throughout the entire piece.

PRE-ANNEALING COOLING RANGE

After the process range is through, it is desirable to cool the piece quickly for several reasons. The first reason is to stop the heatwork. This is especially important on a project such as a less then 100% fuse or a drop mold.

The second reason is that an undesirable reaction known as devitrification can occur during this cooling period if the kiln is cooled to slow. Devitrification is a scummy white crystallization on the glass surface that is difficult if not impossible to remove. Be sure to slow down the cooling before you enter the Annealing Range.

Opening the kiln lid to increase the rate of cooling, while practiced, is not always recommended. On certain models the thermocouple is in the rear of the kiln and the temperature from front to back can vary causing one part of the piece to enter the annealing phase before another.

ANNEALING RANGE

The final critical range is the Annealing Range. This is the temperature range where it is important to cool the glass slow so it can even-out in temperature. This process prevents stress from building in the glass which could cause the piece to break.

Our pre-programmed firing schedules anneal from 960 °F (516 °C) to 800 °F (427 °C), which should be adequate for most tested compatible glass. By incorporating such a broad range, the risk of breakage is limited. Be sure to keep the lid or door of the kiln closed until the kiln reaches room temperature. Opening the lid too soon can cause pieces to break.

Firing Processes

There are many different processes or techniques used for manipulating glass with heat inside a kiln. In this manual we will focus on two, Fusing and Slumping. Other techniques include but are not limited to Drop Molds, Pate de Verre, Casting, Painting, and Combing. For more information on using your kiln with these techniques please consult your glass supplier.