Wednesday, 30 August 2017

Firing Schedules for Wissmach 96

Petra Kaiser is reporting that there are people finding cracks in white W96, which she cannot be replicate.  However, they are using strange firing schedules.

The most popular one appears as follows, in Celsius, with my comments.

166°C per hour to 232°C and hold 20
166°C is relatively slow. It is a rate I would use for a fused 6mm piece.  An unfired two-layer piece I would fire at 200°C to the bubble squeeze.  There is no effect in soaking for 20 minutes at this temperature.  If there is a worry (often expressed) that there will be thermal shock unless you let the glass catch up, slow the rate of advance to 134°C.  This is of course excessively slow for a two-layer piece. 

If, however, you are tack fusing onto two un-fused layers, then 166°C may be appropriate, as you are shading parts of the base from the heat of the kiln. But the soak is not necessary.  It does not do anything useful.

166°C per hour to 538°C and hold 20
As the rate for this segment is the same as for the first, I repeat the soak is not necessary.  If the glass survived the first 200°C at this rate, it will survive the next 300°C too. 

This rate for two layer pieces could be increased to 200°C without damage.

The 20-minute soak at this temperature again does nothing useful.  If the glass survived to this point, you can continue the temperature rise to the bubble squeeze at the same rate as in this segment.

278°C per hour to 621°C and hold 30
Although this rate is not excessive, there is no real reason to speed the temperature rise.  If you use 200°C from the outset to the bottom of the bubble squeeze, no time will be lost in getting to the bottom of the bubble squeeze.

However, this schedule leaves out the important second part of the bubble squeeze.  This is a slow rise to about 50°C above the start of the bubble squeeze process. 

Insert an advance of 50°C per hour to 670°C with a 30-minute soak

278°C per hour to 788°C and hold 15
788°C is a temperature given in the Wissmach tutorial on firing schedules.  However, Petra Kaiser has found that 771°C with a 10-minute soak is sufficient for a full fuse (or 765°C with a 12-minute soak).

The speed at which you reach the top temperature affects what you need to use as the top temperature.  This rate of less than 300°C will not require more than 771 as a top temperature. However a faster rate will require a higher temperature, and with it potential bubble problems, over firing, needling, and inconsistent results.

afap to 527°C and hold 120
This seems to come from the old Spectrum 96 schedules where a temperature equalisation soak was established above the annealing point.  Even if it were necessary, two hours is excessive.

The temperature equalisation of the glass should occur at the annealing point. Therefore, this segment is unnecessary.  And should be replaced by an AFAP to 510°C

55°C per hour to 510°C and hold 120
If the previous segment is eliminated, the rate in this one should be AFAP to 510°C with a soak of 30 minutes for a full flat fuse of 6mm.  There is no need for a longer temperature equalisation soak, as this is enough time for all the glass to be within 5°C of each part.

If you were tack fusing, a soak of an hour would be sufficient for a single layer of tack on a 6mm base.

28°C per hour to 399°C and hold 1
This rate is appropriate for a piece of 19mm.  A 6mm piece could use a rate of 80°C per hour.  A tack fused piece as described above could have an annealing cool of 60°C per hour.

Depending on the natural cooling rate of your kiln, it is possible to turn the kiln off at this point.  If you kiln cools off faster than the cooling rates given above, then you do need to programme a second stage cool.
55°C per hour to 93°C and hold 1
This is excessively slow for a 6mm thick full fused piece – a possible rate would be 200°C per hour.

The one-minute holds in these two down rates are only required where your kiln controller will not accept “0” as the number.  If the controller will accept 0, then use that, as 1 minute will not do much of anything, except confuse.

Writing and evaluating  schedules

When you are writing or looking at others’ schedules, review what is happening to the glass at various temperatures.  This excellent guide tells you what is happening to fusing glass at various temperature ranges.  Float glass has some different characteristics.

Combine that knowledge with what you are trying to achieve in the firing.