One
of the uses of cullet (small pieces of glass) is in casting.
However, simply placing the glass into a mould and firing, leaves
many bubbles and often shows the edges of the original pieces of
glass. Billets (ingots of glass) are more useful because they have
fewer of the small bubbles and fewer edges than cullet.
It
is possible to make your own billets. This can be done in a fashion
similar to pot melts, although the temperature does not have to be so
high. And the results are easy to store, if the dimensions are kept
regular.
You
need to have a mould for the melting glass to be contained within.
These moulds can be made from plaster. A simple way is to use old
margarine tubs placed upside down and fastened to the base within a
dammed area. Pour the plaster of paris over the tubs to make the
moulds. An alternative is to use strips of refractory material (fibre
board or cut up kiln shelves) surrounded by heavy bricks to stop any
movement due to the weight of the glass.
The
glass to be formed is put into ceramic flower pots and can be
directly onto the plaster of paris or dammed areas. You should put
at least one piece of glass to cover the hole at the bottom of the
pot. All this glass must be clean. Calculate the amount of glass
required by determining the volume of the containment area (in cubic
centimetres) and multiply by the specific gravity to give the number
of grams required.
Don't
get too ambitious about size, as these billets need to be fitted into
the mould reservoir for filling the mould. A small margarine tub is
approximately 12 cm wide, 7 cm deep and 7 cm high. This is as large
as required, and smaller may be better. If you are making your own
from dams, something like 4 cm by 8cm by 2cm may be better. This
size is convenient for filling a reservoir, and has the advantage of
being able to compare the intensity of colour the different
thicknesses will give to the casting.
Remember
that the thicker you make the billets, the longer you have to anneal.
So the annealing time of the billet may be the factor that determines
time. A 2 cm billet will take at least 9 hours of annealing time;
one of 4 cm will take 28 hours of annealing.
When
setting up the kiln for making the billets, remember that in general
the higher the reservoir above the billet mould, the fewer bubbles
you will get in the billet, although you are confined by the height
of the kiln. Although there still will be some bubbles, these will
further reduce by the second flow of the glass during the casting
process.
To
fire the set up, you can advance the temperature rapidly to 650/670ºC
with a long soak there (possibly 3 hours). The final temperature can
be below pot melt temperatures, so a casting temperature of 830ºC
with a long soak (possibly 6 hours) will be sufficient. Take note of
your final thickness – including any containment material – to
determine the annealing soak and schedule.
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