“Double the annealing soak time for each firing” and “Slow the rate of advance each time you fire” are common responses as a diagnosis when a piece breaks in the slumping process. It may come from the fact that once fired, It is now a single piece that needs a slower rate of advance on the second firing. I’m not sure where the idea of doubling the annealing process originates.
You need to think about why you would slow the rate of advance and double the anneal for each subsequent firing of the piece. This is an investigation of the proposals.
Thickness determines ramp rates and annealing
Annealing soak lengths and cooling rates are related to
thickness and complexity. If no
additions or complications are added between the previous and the current
firing, there is no reason to extend the soak or decrease the rate of cooling.
You of course, need to consider what lay-up and process you
are using in the additional firing. Have
you added any complexity to the piece in the previous or the current firing? If so, you do need to consider how those
changes will affect the firing requirements.
Fire polishing
The question to be asked is, “if the piece was properly annealed in the first firing and shows no significant stress, why do I need to change the firing?”
The answer is, “you only need to slow the heat up because it is a single piece now.” You do need to know that the existing stress is minimal, of course. A note on stress testing is here. If there is little or no stress from the previous firing, the annealing and cooling can be the same as the previous firing. Nothing has changed. You are only softening the surface to a shine. The anneal was adequate on the first firing, and it will be on the second.
If you are firing a pot or screen melt, you have added a complexity into the firing. This is because of the high temperatures used in the first firing. It means you may wish to be more cautious about a re-firing to eliminate bubbles, or for a fire polish for the surface.
Frit layers
If you are adding confetti or thin layers of frit or powder you have not significantly changed the piece. You can re-fire the piece as though you are fire polishing any other piece of the same dimensions.
Additional layers
If you are adding more full layers in subsequent firings, you need to reduce the rate of advance to top temperature. You also need to extend the soak and reduce the cooling rate according to the new thickness of the piece. This is because the piece is thicker, so the rate of advance needs to be slower, the time required to adequately anneal is longer, and the cooling rate needs to be slower. All of these changes in scheduling are to accommodate the additional thickness.
Tack fusing additional pieces
If you are tack fusing pieces to the top of an already fired piece, you need to go slower than you would by just adding a full layer. Tack fusing pieces to an existing piece adds a significant complication to the firing. Tack fusing requires a firing for thickness between 1.5 and 2.5 times the actual total height of the piece. The complexity added is the shading of the base glass from the heat radiating from the elements.
For example, if your piece from the melt is 9mm/0.375", it would have been annealed with a 90 minutes soak. The first cool would be at 69C/127F per hour, and the second at 125C/225F per hour with the cool to room temperature at 415C/750F. If it shows no significant stress, you can fire polish and anneal in the same way as your initial firing.
But
If you tack fuse pieces on top, then you need to treat the piece as though it were between 15mm/0.625" (a little over 1.5 the thickness) and 25mm/1.0" (a little over 2.5 times) thick. This would require a soak of 3 or 4 hours. A cooling rate of between 40C/72F and 15C/27F per hour for the first cooling stage is needed. The second stage between will need a rate between 72C/130F and 27C/49F per hour. The final cooling to room temperature will be between 90C/162F and 240C/432F to room temperature.
Conclusion
If you have made no significant changes in thickness or complexity, the second firing can be the same annealing as the first firing. If you have altered the thickness or complexity of the piece, the second firing will need to be slower.
Further information is available in the ebook Low Temperature Kiln Forming.
