Sometimes in fusing, the base layer can exhibit a crack or break without the upper layers being affected. This kind of break almost always occurs on the heat up. In a tack fuse, the top layers may still be horizontal and unaffected by the break beneath them. If a full fuse, the upper layers will slump into the gap, or apparently seal a crack that is apparent on either side.
|An example of tack fused elements on top of a previously fused base|
This is more likely to be seen where there is a large difference between thicknesses. If the base is a single or double layer and there are several layers of glass – especially opalescent – on top, there is a greater chance for this kind of break to occur.
The reason for this kind of break is that the upper layers insulate the part of the lower layers they are resting upon. Glass is an insulator, and so a poor conductor of heat. This means that the glass under the stack is cooler than the part(s) not covered. A break occurs when the stress of this temperature differential is too great to be contained.
|An example of stacked glass in a tack fusing|
This kind of break can also occur when there are strongly contrasting colours or glasses that absorb the heat of fusing at different rates. One case would be where the dark lower layer(s) were insulated by a stack of white or pale opalescent glass. The opalescent glass will absorb the heat more slowly than the dark base. This increases the risk of too great a temperature differential in the base.
Reducing the risk of these breaks.
One way to reduce the risk of base layer breaks is to keep the glass nearly the same thickness over the whole of the piece. Sometimes this will not give you the effect you wish to obtain.
Slow the firing rate
Another way is to slow down the temperature rise. Some would add in soaks at intervals to allow the glass under the stack to catch up in temperature. As we know from annealing, glass performs best when the temperature changes are gradual and steady. Rapid or even moderate rates of advance with soaks, do not provide the steady input of heat.
This prompts the question of how fast the rate of advance should be, and to what temperature.
Rate of advance
The rate of advance needs to take account of the thickness differential and the total thickness together. A safe, but conservative, approach is to add the difference in thickness between the thinner and the thickest parts of the piece to the thickest. Then program a rate of advance to accommodate that thickness. E.g., a 6mm base with a 9mm stack has a total height of 15mm. The difference is 9mm which added to 15mm means you want a rate of advance that will accommodate a 24mm piece.
The rate of advance can be estimated from the final annealing cool rate required for that thickness. In the example above, the rate would be about 100°C per hour. The more layers there are, the more you need to slow the heat up of the glass. The Bullseye table for Annealing Thick Slabs is the most useful guide here.
Firing already fused elements
If you were adding an already full fused piece of 9mm thick to a 6mm base, you could have a slightly more rapid heat up, bu not by a lot. This is because the heat will be transmitted more quickly through a single solid piece to the base glass. It is safer to maintain the initial calculation. If your stack is tack fused, this will not apply, as the heat will move more slowly through the layers of the tack fusing much the same way as independent layers on the initial firing.
The general point is that you need to dramatically slow the speed of firing when you have stacked elements on a relatively thin base. Even a two layer base can exhibit this kind of break when there is a lot of glass stacked on it.