Help! Looks like
my shallow bowl wanted a boob. [16” diameter pot melt slumped into
shallow 20” mould, 4” above floor, fired at 175°F to 1100°F for 20
minutes, and 75°F to 1250°F for 15
minutes]
What happened?
Sometimes a slump results in an uprising at the bottom of
the slump. In this case, and many others, we know the vent at the bottom of the
mould was open and the piece was supported above the shelf. This indicates that everything should be set
up for a good result. Still, this
uprising occurred. It is not a bubble,
as the glass is apparently evenly thick throughout the “boob”.
The usual, and mostly unexpected, cause is too long or too
hot a slump. The firing, if allowed to
continue, would result in a larger uprising and eventually a thickening of the
piece at the bottom of the mould associated with a related reduction in the
dimensions of the final piece.
What has happened is that the glass has become soft enough
for it to slip down the sides of the mould. But it has not been hot enough for
long enough to allow the glass to thicken.
The glass at the bottom is pushed up to compensate for the slightly
hotter glass on the sides of the mould sliding down on the glass at the bottom.
In this case there is a moderate (97°C) rise from a soak of
20 minutes at 593°C to 676°C, soaking for 15 minutes. This is a lot of heat work at a relatively
high temperature for a shallow mould.
What to do in the future?
Rate of Advance
Consider what you are trying to achieve at each stage of the
scheduling of the process. In this
slumping there really is no need to soak at 593°C. It is a nowhere
temperature. The glass is no longer
brittle. It is at the lower end of the
temperature range where the glass is softening anyway. A simple, steady rise in temperature, as at
the beginning, of at or below 100°C will be sufficient to bring the whole
substance of the glass to the slumping temperature. If the piece is really
thick, consider an even slower but steady rate of advance without any soaks.
Soak Temperature
Also, if the slump can be achieved in 15 minutes, it is too
hot. If the slump is complete in such a
short time, it will be marked much more than needed by contact with the
mould.
The cooler the glass at the conclusion of the slump, the less marking there will be. Yes, the soak time needed to complete the
slump will be longer, but the bottom of the glass will be cooler than a hot
fast slump. You should always be trying
to achieve the effect you need at the lowest practical temperature. The slow rate of advance will assist in
completing the slump at a lower temperature, as the amount of heat work put
into the piece will be greater.
Observe the progress of the firing
Observation is necessary when doing something different. Some argue that it is necessary in every
slump. I admit that I do not always
observe every slump, but this case again illustrates the need to observe each
slump.
By observing at 10- or 15-minute intervals, you will see
when the slump is complete. You may feel
you do not have the time to wait for an hour or so for the slump to be
complete, or that it does not fit with your activities. The answer is to arrange the kiln’s schedule so
that when the critical part of the process is reached it will fit with a space
in your other activities. https://glasstips.blogspot.com/2016/12/diurnal-firing-practices.html
Can this piece be fixed?
You could put the piece on a shelf and take it to a high
temperature slump with a significant soak to flatten it. You will need to observe when the uprising is
flat again, and then proceed to anneal.
However, the pattern placed in or on the glass will be distorted to some
extent. The uprising will flatten with a
thicker rim around the base of the rise in the glass. This may be visible.
My view of these things is to learn as much as you can about
causes and prevention and move on. You
advance your practice more quickly by understanding what went wrong and why
than by trying (unsatisfactorily) to rectify a failed piece. Often you can cut the glass up and use the
pieces in other projects.
Uprisings at the bottom of slumps are often the result of
too much heat work (rate, temperature and time). Slow rates if advance to low temperatures
with long soaks backed by observation prevent the occurrence of these
bubble-like uprisings.
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