A frequently asked
question is how to get a smooth shiny surface to the outside of slumped bowls.
There are two certain ways – have the shape blown, or do a free drop.
Avoid Moulds
A free drop is the
process where the glass blank is placed over an opening which allows the glass
to fall without touching any mould. You
need to observe periodically during the firing to arrest the drop when it is at
the stage and shape you want. You then
need to remove and polish the rim that rested on the elevated ring that
supported the glass during the drop.
Failing
these techniques, you need to use a mould
The surface of the
glass that is in contact with the mould will take up the texture of the mould surface. When
the glass is hot enough to take up the shape of the mould, it will be soft
enough to take up some texture from the mould. The hotter you fire, the more
texture will be imparted to the glass.
You can
minimise the texture of a mould
Prepare the mould
with the smoothest surface you can. If the shape is simple enough, you
can use very fine sandpaper - 6000 grit is useful. This will give the smoothest possible mould
surface.
Use the finest kiln
wash you can find to coat the mould. The finer the powder is ground, the
less texture is present. You can also
smooth the kiln washed surface with a balled-up piece of soft cloth or
tights. Do this very lightly, so that
you do not rub off the kiln wash. Remove the excess powder before firing.
Minimise the temperature
A major way to
reduce the texture is to fire at a slow rate to the lowest temperature you can, using a 30 to
90 minute soak. This will give you less texture than a fast rate to a higher temperature with
a shorter soak. To determine how long is required at a low temperature , peek periodically to see if the slump
is finished.
The principle is to fire as slowly and to as low a temperature as is practical. This will reduce the chances of marking as long as the glass does not slip down steep sides.
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