A question was asked of me about schedules for tack fusing multiple pieces – three layers thick in places – as a single unit, then placing on a 6mm fused base and tack fusing. Special interest was in how the different thicknesses and the tack fusing would affect the scheduling of the annealing.
My response – edited – was as follows.
This is going to be a long reply. I have written a general guide to tack fusing that will be useful, but this response will try to be more specific to your project.
First, tack fusing of pointed things is more sensitive to annealing than rounded things. For up to 3 layers of triangles, I would be thinking of annealing for at least 12mm (four layers). This means a 2hour soak at 482⁰C, followed by a cooling rate of 55⁰C for the first 55⁰C degrees and then 99⁰C for the next 55⁰C. After this 110⁰C degrees of cooling the rate can be as fast as 330⁰C/hr. This will apply whether Bullseye or System 96 is involved.
Second, from the description I take it that a 6mm clear under a 3mm layer of two colours side by side is being fused as a base. [This was confirmed], so you could fire at 200⁰C/hr to a bubble squeeze of 30mins and then 300⁰C/hr to top temperature. Anneal at 482⁰C for 60-90mins and cool for first 55⁰C at 65⁰C/hr and the next 55⁰C at 150⁰C/hr, followed by 300⁰C/hr to room temperature.
The third stage is to combine them. Think about how thick this is physically – ca.18mm. Then think about the differences in thickness – 9mm. My rule of thumb is to add the difference between thicknesses to the thickest part – in this case to 18 plus 9 equals 27mm. This is the “scheduling thickness” for this variation with rounded elements. As your piece has lots of triangles, you need more care. It is an additional level of difficulty. So I add another 3mm to my “scheduling thickness” to accommodate the angular aspect of the piece, making a total of 30mm for putting the two fused pieces together.
This thickness leads me to propose a relatively complicated schedule. I suggest 70⁰C/hr to 250⁰C, 100⁰C/hr to 540, 120⁰C/hr to 620 and then 150⁰C/hr to top temperature. The top temperature will be lower than your normal tack fuse temperature because this is a much slower rate of advance than normal. This in turn, means that you will want to be checking at intervals on the tack fuse progress from at least 720⁰C.
The annealing will be long and slow. About 5 hours at 482⁰C, 11⁰C/hr to 427⁰C, 20⁰C /hr to 370⁰C and 65⁰C /hr to 30⁰C. This will be a schedule of about 35+ hours.
The two sources mentioned earlier give the rationale for this kind of schedule. Think about the considerations I have listed, and then decide whether I am being too cautious or not. The principle remains - as you increase the risk factors, you
· slow down rates of advance and cooling rates, and
· extend soak times.
You should note that I have used Graham Stone’s Firing Schedules for Glass, the Kiln Companion and the Bullseye chart for Annealing Thick Slabs in preparing the proposed schedule, although you will not find this exact schedule in either of them.