These general guidelines explain how to fuse glass in a digital kiln. You will likely need to alter the firing schedule for your kiln and brand of glass.
The basic firing schedule:
Segment 1: rate 300°F to 750°F target temperature
Segment 2: rate 750°F to 1500°F target temperature
Segment 3: rate 9999°F to 900°F target temperature
Segment 4: rate 100°F to 700°F target temperature
1. Vent the kiln by propping the lid 1/2" and leaving the peephole plugs out. Press Start. Segment 1 will begin at a slow rate.
2. At 500°F, the alarm will sound as a reminder to close the lid or door from the venting position. Turn off the alarm (on most controllers, by pressing Enter). Program another alarm for 1300.
3. As the temperature rises, look at the glass through the peephole. (Always wear firing safety glasses.) When the glass has fused to your satisfaction, write down the temperature displayed on the controller. Press Skip Segment. That will take you from segment 2, which was programmed for 1500°, to segment 3.
(Note: if the glass needs more heat when segment 2 ends at 1500°, press Stop. Reprogram segment 2 to a higher temperature. Turn the kiln back on. At this point, most controllers will skip segment 1 and go right to segment 2 again. When the glass fuses, press Skip Segment.)
4. After you press Skip Segment, raise the lid or open the door 2” to flash cool the kiln. This will take a moment. Some artists prop the lid with a post during flash cooling. When the temperature display reads 900°F, close the lid/door. Insert the peephole plug and leave the lid closed until the glass cools to room temperature.
Digital Programming Explained
Most digital controllers have similar features, no matter which brand of kiln you buy. They divide the firing into sections, or segments. Each segment consists of three pieces of information: a temperature, a heating rate, and a hold time. This is entered into the controller through number keys on a keypad, or by pressing arrow keys. The heating rate is figured in degrees of temperature rise per hour. Not every segment needs a hold time (maintaining the temperature for a specified period). In that case, the hold time is not entered.
Most controllers can fire in multiple segments. Every time you need to change the heating rate, you will need an additional segment. The heating rates can be up or down to control cooling as well as heating.
In the program used for the above firing, segment 1 takes the kiln to 750°F in 2 ½ hours. Rate is figured in degrees per hour. I divided 750 by 2.5 to come up with a rate of 300°. (If you needed to be precise about firing rate, you would subtract room temperature from 750.)
The alarm set for 500° is to alert the artist to close the lid from the venting position. The alarm on most controllers is faint. To hear it you will have to be close to the kiln. Some controllers have an external outlet that you can plug a loud electric bell into. When the alarm sounds, turn it off and program it again for 1300°, which is to alert you to go back to the kiln to watch the glass through the peephole. (1300° is only an example. Use a temperature best suited for the brand of glass you fire.)
Segment 2 speeds up the firing. When the glass fuses to your satisfaction, write down the temperature. Next time you fire that glass, program segment 2 with the shutoff temperature from this first firing.
Segment 3 is a flash cooling segment. Notice that I used a high rate. (On some controllers, 9999 automatically becomes full speed.) If I had used a slow rate, the controller would have raised the temperature again from 900°. The fast rate will prevent that.
Segment 4 keeps the glass warm through the annealing range. One of the advantages of a digital controller is that you can fuse very thick glass without spending extra time monitoring the kiln.
Whether you fuse glass as the ancients did, with a fire in the desert sand, or you use a digital controller, the kiln is only a tool. No matter what type of control system you use, the results, ultimately, depend solely upon your own creative judgment.