Fay & Ralph Loewenthal on sat 14 jul 01
Dear Clayarters who are interested in glass, a lot of the
knowledge you need will be gained from experience, either
your own or somebody else's. I raided the city library and
read and photocopied whatever I could on glass. I have=20
about six lever arch files full of photocopies and they still
do not give the full picture.
First off keep in mind that you are working with glass or in
our terminology glaze. Every different type of glass has
different rates of expansion and contraction. If you mix two
incompatible glasses, they do not just craze, they crack into
many pieces. Always do tests first on small samples to=20
make sure of compatibility and the temperatures.=20
There are two options with slumping, you can slump over
stainless steel or into clay. Glass contracts more than clay,
and less than stainless steel. Whatever mould you are using=20
you need to coat it with kiln wash. I use a kiln wash that I
got from Don Jung, also a Clayarter. I just dip my clay=20
moulds into the kiln wash. The stainless steel moulds I heat
up in the kiln or oven to about 200 C, then dip it in a=20
diluted kiln wash. I picked up a lot of cheap stainless steel
moulds in second hand stores. I slap roll my grogged clay=20
and then cut out doughnut like rings of different diameters.
I just bisque fire them a lttle higher than the normal bisque
temperatures, so that I have reasonable strong mould. I=20
slump through these rings to make some rather attractive
vases.=20
Always make sure your edges are clean and smooth before
you do any kiln work. They will look worse after firing.
I fire according to the size of the pieces. The larger the
piece the slower the firing up. If you have a kiln that is not=20
electric or has elements in the lid or roof, then you can fire=20
faster. I fire any piece over 35 centimetres, 14 inches, in=20
diameter, at 50 C for half an hour and up it by 50 C every
half an hour till I reach the required slumping temperature.
I normally use window pane glass, slumping starts at about
750 C and it is usually finished at about 850 C. I flash the
kiln once I feel the piece is ready. I leave the door open till
the the tempeature had dropped to 500 C. I soak for two
hours at 500 C. I leave the kiln closed till the temperature
has dropped to just above room temperature. This is to=20
anneal the glass. If you do not anneal properly the glass
will eventually crack and break. This can happen even=20
months after the firing, so be warned. My temperatures are
all for our local window pane glass and in centigrade. You
have to check with your local glass manufacturer what=20
temperatures to use. Bottle glass uses higher temperatures,
and art glass usually uses lower temperatures.
Here is the kiln wash recipe:
Combo Kiln Wash:
Alumina hydrate 40
EPK 30
Kyanite (35 mesh) 8
Silica 22
Hope this helps if you need any more info just email me,
and I will help if I can.
Kind regards from Ralph Loewenthal in Cape Town
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