As you move up from smaller pieces to pieces that occupy most of the shelf, you sometimes begin to get large rounded bubbles at tack fuse and burst ones at full fuse.
|Image from B Stiverson|
You have to go back to basics to discover the cause.
It is not likely to be the schedule. It has worked for smaller items. But it is important to review the schedule. Is it like others you have seen? Is it similar to what the glass manufacturer recommends? Both these will reassure you that the schedule is OK, if not perfect, or to revise it.
Going back to the basics relates to the cleanliness of your kiln, among other things. Even a small speck of material under the glass can result in a bubble. Although the grit lifts the glass off the shelf only a fraction, as it heats up the glass slumps around that and creates an air pocket. That grows as the glass heats up and creates a large diameter bubble. If there is no grit in evidence, you need to check another element of your kilnforming practice.
The large bubble might often occur in the same relative place in the kiln, although different places on the glass pieces, depending where they are placed. This is an indication that you may have a hollow in the shelf. It may not have been obvious with smaller pieces. You need to check the shelf with a straight edge. If any light is seen between shelf and edge, you have a depression in the shelf. It may only be a sliver of light, but that indicates a depression which is enough to create a large bubble. That must be fixed.
There are temporary and permanent fixes for avoiding bubbles due to depressions in the shelf.
The temporary fix is to use 1mm fibre paper on the shelf, to allow air out from under the glass. This can be topped with Thinfire or Papyros. Alternatively, a thin layer of powdered kiln wash can be smoothed over the fibre paper to give the smoothest back possible in the circumstances. You can use a plasterer’s float, or simply a piece of float glass.
The permanent fix is to sand the shelf smooth and level. A method for doing this is here.
Single Layer Bases
If you are firing with single layer bases, there may be nothing wrong with the shelf. It is typical in tack fusing to use single layers with glass placed decoratively around the surface of the base. This leaves gaps where the base glass is exposed. Even though the whole piece may survive the differential heat up of the exposed base glass and the covered parts, there is the possibility of creating an air pocket under the exposed base. This comes from the weight of the stacked glass pressing any air out to the side. If the design is unable to provide a route out for the air, the possibility of creating an air bubble increases.
It is possible to create conditions to reduce the possibility of these large bubbles developing.
One solution is to use a layer of fibre paper as for a shelf with slight depressions. This allows air out from under the glass, even with a single layer layup.
The other solution is to change the rate and temperature of the firing. By using the low and slow principle, you can reduce the risk of bubbles. Use a much slower rate of advance to a lower temperature with a longer soak you can achieve the look you want without bubbles. This utilises the concept of heat work. It does require observation to determine when the effect you desire is achieved and then advance to the next segment.