WarmGlass.com

Hello,

I have been fusing 7.5" shallow bowls. My firing schedule has been that, once the kiln reaches fusing temperature, it shuts down and cools by itself.

Have gotten a larger kiln for fusing 16" shallow bowls, and have been reading some of these posts for advice on how to begin experiments to develop a new firing schedule. I'll be adding a bubble soak and annealing phase. However I notice that, after the annealing phase, going to 750 degrees at the rate of 100 dph is recommended, before finally allowing the kiln to cool by itself.

What is the reason for controlling the rate of cooling until the kiln reaches 750 degrees?

To avoid thermal shock.

Glass expands as it's heated, contracts as it cools, sometimes quite a bit (depends on the COE, or coefficient of expansion, that everyone talks about). If the glass is heated (or cooled) unevenly, then the hotter areas will expand more than the cooler areas. If the difference in temperature gets to be too great, the areas will no longer "fit" together (literally). They can cause the glass to crack, break or explode, depending on how much glass you have and how big the temperature difference.

So the reason you go slowly after the anneal soak is to make sure that everything cools evenly, that the edges/top surface of the glass doesn't cool faster than other parts. If the temperature in one area exceeds another by 5C or more (about 10F), you potentially have enough stress to crack the glass. The thicker the glass, the more likely the center of the piece will retain heat...so the slower you go.

If you've ever had a chance to work with a glassblower, you'll get some graphic illustrations of glass and heat: The glass visibly expands and shrinks with heat as you work and reheat it. And if you accidentally leave a piece out of the heat too long, the force of the resulting thermal shock can explode the glass into sharp chunks of very hot shrapnel. (ask me how I know this)

Thanks. I expect to do a number of tests to find what works for this new kiln and larger work. Sure appreciate your help in determining a starting point.

Any piece over 12" i add that extra step to 750.

The reason for 750 F (425C) is that annealing takes place in two stages.
The first is annealing soak, it is only for cooling so that there remains no more than 5 C difference in temperature, that is 9 F, within the glass mass . The annealing proper takes place during F is small enough not to introduce stress.

The length of time depends of the square of thickness, or more precisely of the distance x time heat must travel from the core to the surface.

OK, Cynthia.
How do you know this ?
Jim


[quote="If you've ever had a chance to work with a glassblower, you'll get some graphic illustrations of glass and heat: The glass visibly expands and shrinks with heat as you work and reheat it. And if you accidentally leave a piece out of the heat too long, the force of the resulting thermal shock can explode the glass into sharp chunks of very hot shrapnel. (ask me how I know this)[/quote]

Morganica wrote:Glass expands as it's heated, contracts as it cools, sometimes quite a bit (depends on the COE, or coefficient of expansion, that everyone talks about). If the glass is heated (or cooled) unevenly, then the hotter areas will expand more than the cooler areas. If the difference in temperature gets to be too great, the areas will no longer "fit" together (literally). They can cause the glass to crack, break or explode, depending on how much glass you have and how big the temperature difference.

So the reason you go slowly after the anneal soak is to make sure that everything cools evenly, that the edges/top surface of the glass doesn't cool faster than other parts. If the temperature in one area exceeds another by 5C or more (about 10F), you potentially have enough stress to crack the glass. The thicker the glass, the more likely the center of the piece will retain heat...so the slower you go.

If you've ever had a chance to work with a glassblower, you'll get some graphic illustrations of glass and heat: The glass visibly expands and shrinks with heat as you work and reheat it. And if you accidentally leave a piece out of the heat too long, the force of the resulting thermal shock can explode the glass into sharp chunks of very hot shrapnel. (ask me how I know this)

As a former borosilicate lampworker for years, I can attest to this. Boro chunks are sharp and hot, and never fun. It doesnt explode most of the time just cracks, but you can have some fun ones from time to time.