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.
Precautions
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.
