Bubbles
are often blown through frit castings and other thin pieces. This
most results from insufficient volume of glass in the mould or on the
shelf. Also the design can induce bubbles where there are thinner
parts surrounded by thicker parts. As the glass softens, the
surface tension of glass - from around 730 - causes it to pull up to
equalise at about 6-7mm thick. This causes thinning in certain areas
to allow thickening in other areas. This then leads to the risk of
blowing bubbles through the glass where the glass has become thinner.
If
thinner work is required, you can fire an over-sized piece to about
750C for a short time and then cut it back to the final size. If you
want a flat thin sheet, you can also place the glass between two kiln
shelves. You need to separate the shelves with a 3mm spacer to keep
the upper shelf from coming completely down on the shelf, giving an
extremely thin fragile piece of glass.
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