A query
about reducing a bubble appeared on the internet recently. The bubble was from between the shelf and the
single layer glass. It was a relatively
shallow dome that did not seem to have thinned the glass much.
There is
quite a bit of information on reducing the incidence of bubbles. Among them are
my blog posts on the subject.
My view is
that large thin bubbles cannot be repaired successfully. As the bubble forms and grows, it pushes a
proportion of glass to the side. This
thickens the glass at the edge of the bubble.
Bursting the bubble and filling it with something (e.g., a piece of
glass, or frit) leaves marks at the thickened edge of the bubble, so it remains
a mark in the finished piece.
Method 1
However,
glass with a low uprising between the shelf and the glass can be successfully
repaired, if the uprising is low and the glass has not thinned. In the case
mentioned, the risk in simply re-firing right side up is that the bubble will
increase in size. The weight of the glass may not be sufficient to pull it down
except at higher temperatures – which is where the risk of increasing the size
of the bubble occurs.
Instead, flip
the piece over. Allow the weight of the glass to flatten the uprising.
You can use a much lower temperature to flatten the glass by taking advantage
of the weight of the whole piece. This
lower temperature means that you will not mark the surface so much as at higher
temperatures. Don’t worry if the uprising is not central, you do not have to balance
the glass on the point of the bubble for this process to work.
Take the
piece to 620°C maximum for as long as it takes to flatten. The rate of
advance should be slow – not more than 100°C per hour. This steady, slow input of temperature will
allow the glass to relax at lower temperatures than rapid increases.
You should
use the smoothest separator surface that you can – Thinfire or Papyros, or a
smoothed kiln wash. This together with a
low temperature will give minimum markings.
You must
observe the process from about 560°C to be able to stop the slump when the
piece is flat and advance to the annealing segment of the firing.
This post gives a further alternative. Use two shelves to compress the uprising
flat. Although the post is talking about thinning a pot melt, the principle is the same. Place fibre paper around the edge equal to the thickness of the glass piece and place another prepared kiln shelf on top. You do not need to invert the surface of the piece to do this. It may be that you will need a fire polish to remove any marks on top.
A plea
Do not
drill holes. Especially not in the case of a shallow bubble. The glass has not significantly thinned and
so can be rescued. Drilling a hole will
only leave an unwanted mark.
No comments:
Post a Comment