This is a technique that will obtain a random, organic feel to glass that would otherwise be scrap (cullet) – remembering that you have to use compatible glass throughout. The principle is to take the temperature up high enough for the glass to begin to flow easily and bubbles to blow through and burst.
The results can be used as they come out, or they can be cut to provide points of interest in other work, or the glass pieces can be damed before firing to obtain thick pieces which can be cut into slices for other work. And I am sure, there are numerous other ways to use the resulting glass too.
The effects are rather like colourful molten rock with gases bubbling through. These bubbles mix the glass colours. So you need to be sure you do not use a wide variety of colours, or your result will be similar to the molten rock - muddy. Use a few contrasting colours, and ensure you include a significant proportion of white to maintain bright colours. Also remember that the hot colours – reds, yellows, oranges – opalise at high temperatures, so the transparents can be used as opals.
You can use whole sheets of glass or scraps. In either case, it is useful to start with a clear base to help avoid picking up kiln wash when the glass is moving about. The glass must be clean to reduce the incidence of devitrification. Stack you glass on top of the base glass in what ever order you like. Contrasting colours alternated give a strong result.
You can put shelf paper of 0.5 mm or thicker on the shelf or simply kiln wash the shelf with several layers of wash until the shelf surface is no longer visible through the wash. Use of thinfire is not recommended as the powder can be pulled into the glass.
If you do not dam the area to contain the glass calculate how far the glass will expand on the shelf, so that you do not put down too much glass and have it spill over the edge of the shelf.
You can use bubble powder onto the base layer to promote the bubbling during the firing. However, if you are using cullet, you can just take the temperature up rapidly without a bubble squeeze, which will give you plenty of air pockets to burst through the layers of glass.
You can take the temperature up at about 300ºC per hour to 925ºC with no bubble squeeze and soak for 10 – 15 minutes. Then allow the kiln to drop the temperature as fast as possible to about 815 and soak there for around 30 minutes to allow the little bubbles to rise to the surface an burst too. Then reduce to the annealing temperature and soak for the thickness you calculated in preparation for the firing.
You need to be careful in firing and annealing pieces using this glass. Any glass that has been fired to a high temperature tends to begin changing compatibility. So you need to be careful on your rates of advance, and on the annealing and cooling portions of the firing when using the glass in other projects. You may want to consider using a schedule for twice the thickness of the piece on subsequent firings.
There may be devitrification on the surface. You should sandblast or abrade away this devitrification in some way to be able to get a shining surface when you fire polish.
There may also be a number of pin hole sized bubbles at or just below the surface. These will close with a fire polish also.