Frequently Asked Questions
What is an example firing schedule that will help me to fuse glass successfully?
There are several variables to consider--i.e. size of the piece, thickness and layers of glass, colors being used, etc.--that can change the answer to this often asked question. With that in mind, here is an example schedule for small, jewelry sized pieces (1"-1.5" and smaller), 2-3 layers thick, with which we have found great success.
The following values are typically what you will see on the majority of the digital controllers currently on the market. There are a couple of things with which you should be familiar. For example, in the following schedule, there is a repeating theme of 'ra, °F, and hld' followed by a number. These three components--ra, °F, and hld--make up what is called a segment. The number after those letters--ra1, °F1, hld1--represent that particular segment. Also, 'ra' is short for 'rate per hour'; '°F' is short for 'degrees Fahrenheit'; and 'hld' is short for 'hold time'. So, with all of that in mind, let's take a look at our schedule.
ra1 300 Rate per hour in the first segment is 300° per hour.
°F1 1000 Our first stopping point is 1000°.
hld1 0.0 This schedule does not hold for any time at 1000°.
ra2 FULL Rate per hour of the second segment is full power.
°F2 1350 The schedule then climbs from 1000° to 1350° as fast as possible.
hld2 0.0 We will not hold at 1350° for any length of time.
ra3 500 Rate per hour of the third segment is 500° per hour.
°F3 1425 Our target full fuse temperature for this segment is 1425°.
hld3 0.10 We will hold at 1425° for 10 minutes, although we may not need that full amount.
ra4 FULL Rate per hour for the fourth segment is full, which essentially turns the kiln off so that it cools as fast as it can.
°F4 950 We now cool to our annealing temperature of 950°.
hld4 1.0 We will maintain the annealing temperature for one hour.
ra5 0000 This rate tells the digital controller that we are finished with the schedule. At this point, allow the kiln to cool naturally, as the bricks that comprise your kiln retain heat very well.
So, some of you may be asking--'Why do you have so many different rates and holds--why not just go to fuse temperature?' Well, from room temperature to approximately 1000° is the temperature range where your glass is most likely to thermal shock if heated or cooled too rapidly. This can be a very dynamic occurrence, so heat slowly through this range. Once you get past that temperature range, you can move a lot faster. This is precisely what you want to do through the temperature range of approximately 1200° to 1300°, which is the range where your glass will be most likely to succumb to the phenomenon known as devitrification, or crystal growth. To avoid this, be sure to clean your glass thoroughly before firing, and spend as little time as you can in the devitrification range. Once beyond that range, our schedule slows back down so that we now have more control over the rate per hour. This will avoid the potential for an overshoot of the target temperature. When the kiln reaches the fuse temperature, you should be there to keep an eye on what is going on inside the kiln. If you like what you see, and you are only at 1420°, or have reached 1425° and have held there for only one of the ten minutes programmed as the hold time, you do not have to let the kiln continue heating. Skip step to the next segment and begin the annealing segment. Glass, unlike ceramics, is very user interactive. It is tailored to the individual artist's tastes. This is why you will find a thousand different firing schedules out there. Our best advice is to run some test firings with smaller, less expensive glass samples, and document your results so that you develop some successful schedules for whichever application you prefer.