Wednesday, 20 May 2015

Firing Quickly

Firing quickly is often our desire, in spite of the mantra of the experienced – slow and low. How to do this safely – without fractures or bubbles – is the requirement.

Firing quickly on smaller things (say, up to 100 mm) is not normally a problem.  Difficulties can arise due to the kind of layup, but usually the mass is not great enough to be thermally shocked, nor the size enough to trap air that would cause bubbles.

Firing quickly for larger pieces is where difficulties arise. These relate to the initial rate of advance to the softening point, the bubble squeeze, advance to top temperature, and annealing.

Advance to softening point
The first place this occurs is to the upper strain or softening point.  This is the range where the glass is solid and does not transmit heat well, leading to the risk of thermal shock.  You need to find a rate of advance that is a little slower than that which would cause the glass to break.  My guidance is to use no more than three times the annealing rate for the glass of that thickness to reach the softening point.  This temperature is approximately 40C above the annealing point. The glass is certainly plastic above that temperature, so the rate of advance can be faster.

Bubble squeeze
Strategies from the softening point to bubble squeeze vary.  You can go quickly, say 1.5 times the previous rate of advance, to the bubble squeeze heat soak of around 30 minutes.  The other is to go quickly to 50C below that temperature and advance at 50C per hour to the bubble squeeze heat soak.  This is often used on more complex and thicker lay ups. There are numerous variations upon these two strategies depending on the circumstances.

Top temperature
Ways to get to top temperature from the bubble squeeze vary, but as fast as possible risks bubbles due to excessive softening the surface, over firing due to the controller not shutting off quickly enough, and a lack of control of the surface texture.  Twice the initial rate of advance is quick enough, but still allowing the controller to shut off when the top temperature is being reached.

The s
oak at top temperature does not need to be more than 10 minutes.  If you can achieve the desired results in less that time, you should consider reducing your top temperature.

Annealing cool
A s
oak at the bottom end of the annealing range will reduce the anneal cooling time.  The lower temperature of the annealing range is about 40C below the annealing point, so to be safe the annealing soak can be set to be 30C below the annealing temperature.  This reduces the range of temperature over which the slow anneal cool takes place. 

The initial anneal cool should be to 55C below the soak, the second stage of the anneal cool to 110C below the soak can be at twice the initial anneal cool rate.  The rate of cool can be increased to 370C, where for pieces of 9mm or less, the kiln can be turned off.

However, you need to think carefully about firing quickly.  When realistically will you be able or actually need to take the piece out of the kiln?  If it is the next day or after work, then a slower firing reduces the risks of rapid firing and still enables you to take the piece out when needed.