Avoiding Large Bubbles

“I tried small projects and they turned out fine. I have a 12" square with an emblem in the centre and a border set in slightly from the sides. Most of the glass is only the one layer. Both firings produced huge bubbles in the areas where the glass was only one thickness.”

Scale does matter. What can be done at a small scale does not always directly transfer to a larger scale.

The first problem this project created was using only one layer as the base. Glass has a surface tension which means that it tries to become 6-7 mm thick. One layer is only half that. As it thickens at the edges, it traps the air under the other parts of the glass, and as the glass continues to soften the expanding air bubbles come up through the thin parts of the glass.

Using two layers of glass with the design on top will ease the problem.

The design is the second problem. The weight of the border makes it even more difficult for the air to get out from under the glass.

Although having two layers of glass will reduce the problem, think about ways to make the border incorporated with the second layer of glass, so the weight of the glass at the perimeter is not greater than the interior.

The third problem is that there is not a bubble squeeze in the schedule (elsewhere in the query). The soak of 10 minutes at 538C is not necessary. You do need a soak at a point between 650C and 677C - this is the bubble squeeze temperature range. It is also the slump temperature, so you can determine what the bubble squeeze should be for your glass by what the slump temperature is.

The bubble squeeze can be accomplished by a half hour soak at the slump temperature, or by a slow rise from 50C below the slump temperature – taking an hour or so, depending on the size of the piece.

A fourth problem is the separator is batt (kiln) wash, the edges of the glass conform to the batt wash, resisting the movement of air from under the glass.

You may need to change to fibre paper for single layer pieces, as that allows more air out. Thinfire may be enough, but you can put it over 0.5 mm fibre paper. For a smoother finish, sprinkle powdered batt wash over the fibre paper and smooth it with a plasterer's float or a piece of window glass if you don't want to use Thinfire.

Lastly, don't use the pre-programmed schedules in your kiln's controller.

Look at the glass manufacturer's website. Bullseye, Spectrum and Uroboros give basic firing schedules that work with minimal adjustment. I don't understand why kin manufacturers don't simply refer to the sites to give their customers good advice, instead of the pre-programmed stuff.