Odd Schedules

Schedules appear on the internet which do not seem to have a logical sequence in the firing schedule.  Some have multiple soaks at intervals up to 540°C.  Others have kind of dance toward the top temperature – slow, quick, slow.  Some initially cool at a given rate and then slow to about half that initial rate.

Multiple soaks

These schedules have been referred to as catch-up schedules.  They tend to look something like this:

200°C to 150°C for 20 minutes

250°C to 300°C for 20 minutes

300°C to 590°C for 20 minutes

50°C   to 677°C for 30 minutes

330°C to 804°C for 10 minutes

AFAP   to 482°C for 60 minutes

60°C   to 370°C for  0 minutes

Off

The justification for the first two soaks is given as allowing the glass to catch up to the air temperature.  It would be much safer for the glass to have a moderate steady advance in temperature rather than risking the heat shocking of the glass.  You could achieve the same work in the same amount of time by altering the rate of advance to a single one of 198°C to 590°C.  This achieves the same temperature in the same amount of time, but has less risk of heat shock, as there is a steady input of heat.  

Secondly, if the glass can survive the initial rate of heat up without breaking, there is no need to soak at an arbitrary temperature.  The first relevant point where a change in temperature makes sense is above the softening point, which for most fusing glasses is about 540°C. The equivalent softening point for float glass is about 700°C

Slow, quick, slow

This kind of schedule alters rates up and down with little justification as far as I can see.  This is an example:

139°C  to 560°C  for 30 minutes

222°C  to 621°C  for 30 minutes

139°C  to 786°C  for 15 minutes

9999 to  515°C  for 120 minutes  

60°C   to 427°C  for 10 minutes

115°C  to 350°C  for 10  minutes

The question for me is why the slow down to top temperature. There is a lot of heat work being put into the glass, so that the higher top temperature may not be required.  The slower rate from 621°C does allow a form of a bubble squeeze to occur, but is not the traditional one.  A 139°C rate from 621°C to 677°C with a soak would be faster than usual, but may be acceptable.  I would prefer 50°C per hour with a 30-minute soak at the end.  Then advancing at 300°C per hour to top temperature.  The anneal soak and cool of this schedule are acceptable, even though different than I would do it.

Erratic Slumping Schedule

The fusing schedule above was followed by this slumping schedule:

83°C to 148°C  15 minutes

167°C to 590°C  10 minutes

83°C to 720°C  10 minutes

222°C to 410°C  120 minutes

83°C to 427°C  10 minutes

This schedule seems to have a catch-up phase in that it goes at half speed for the first 148°C and then doubles the speed to 590°C (a little above the brittle phase of the glass).  It then slows the rate and continues that to a very high slump temperature.  It is, of course, necessary to have a slower rate of advance in the slumping than the fusing, as the piece is now thicker. Slowing the rate of advance as much as in this should be able to achieve the slump at around 620°C (100°C) less than the target temperature used by the schedule. 

Once the top temperature soak is finished, a very slow cool to the annealing soak is used in this schedule.  This is not ideal as it invites devitrification to form.  The kiln and its contents should be allowed to cool as quickly as possible to the temperature equalisation soak at the annealing point.

The schedule then uses an annealing soak temperature 100°C below that used for the fusing. This does not make sense. The annealing soak should be at the same temperature for both firings.  The length of the soak is not in question, but the early turn off the kiln at 427°C is questionable. The anneal cool of the fused piece extended down to 350°C.  The anneal cool on slumping should be almost the same as the fuse.  Almost all anneal cools extend to 370°C at least.

Anneal Cools

Some anneal cools have erratic rather than progressive cooling.  In this example the early part of the schedule is eliminated:

……………..

AFAP to 482°C 120 minutes

110°C to 427°C 0 minutes

55°C to 370°C 0 minutes

200°C to 100°C 0 minutes

off

Here the schedule is faster in the most critical part of the anneal cool than in the next, cooler part.  This will not provide as good an anneal as if the first two segments after the equalisation soak were reversed.  Start slowly in the anneal cool and then you can speed up (approximately twice the previous segment rate) on each of the following segments.

Rationale

This critique of the schedules above is not to batter anyone.  It is to make clear that there needs to be a conscious rationale for each of the segments in relation to the others.  If you take a schedule from a source, it is a good idea to see if there is a reason for each segment and how it relates to the next. 

·        The scheduling must take account of the nature of the glass.  Glass is a poor conductor of heat and needs steady moderate input of heat.

·        Glass is brittle until approximately 55°C above the annealing temperature when you can accelerate the rate of advance.

·        Time is required to allow air out from between the layers of glass. This usually done in the range of 620°C to 675°C and is known as the bubble squeeze.

·        You need to go relatively quickly through the devitrification range of temperatures – approximately 700°C to 760°C - both up and down.

·        Glass needs a temperature equalisation soak at the annealing point (or nearby) related to its thickness.

·        The rate of cooling needs to be progressive.  The first 55°C below the annealing soak is the most important.

·        Cooling rates must be related to thickness.

·        The second cooling rate can be up to double the initial one.

·        The final cooling rate can be double the previous one.

·        The rate of firing will affect the required top temperature.