Occasionally
a slumped piece will develop faint lines beginning about half to
two-thirds of the way from the centre and radiating toward the edge.
My
experience leads me to think that these marks come from the glass
moving too quickly at too hot a temperature. The glass softens as it
reaches its slump point. If the temperature is taken above that, the
glass conforms to the mould and then begins to slide downwards. The
mould is by its nature not perfectly smooth and so the high points
make marks on the glass as it moves.
This
is re-enforced by the fact that the glass at the centre of these
slumps does not have those marks. It deforms less than the edges of
the piece and so (at whatever temperature) does not get so marked as
the sides and edges.
To
avoid these stretch marks you need to slump at the lowest possible
temperature and ensure the glass is the same temperature throughout
by the time it gets to its slumping point.
Temperature
Finding
the lowest temperature for the slumps in a particular mould requires
experimentation and observation. A simple curve – circular, oval
or rectangular – requires less heat than one with a flat bottom and
much less than one with angles. For a simple curve you can set your
slumping temperature at say 620ºC with up to an hour soak. The
important element to remember is that each shape and curve of mould
will require different schedules. To determine this you need to make
observations.
From
about 600ºC you need to make periodic observations of the progress
of the slump. Note the temperature at which the glass begins to move
– the reflections in the glass will begin to be curved. This is
the minimum temperature you can use for this span and thickness of
glass on this mould. The length of time required to get a complete
slump may be so long as to make using this temperature impractical.
Slump not quite complete |
Now
observations need to become more frequent – possibly every 10
minutes or less. When you reach a temperature where the glass is visibly
distorting, it is time to cease the temperature advance and begin the
soak. Record this temperature and continue to observe, recording the
time it takes at this temperature to fully slump. Continue to the
anneal.
Inspect
the piece when cool. If you have the result you want, you have the
temperature and soak time needed for this thicknesses and size of
glass on this mould. Record this information. If it is not fully
slumped you can try either extending the time (if that is practical,
it is the best option) or increasing the temperature on another
piece. This increase should be by no more than 10ºC, so that you do
not over fire the piece.
Glass conforms to the bottom of the mould |
Of
course, it is possible that the piece was slumped at too high a
temperature as evidenced by stretch marks, mould marks, uprisings in
the centre, distortions on the edges. Then you need to reduce the
temperature on the next slumping of a piece of the same dimensions.
Start with 10ºC less than your first piece, and programme the same
amount of time. Observe, record and inspect as on the previous one.
This
process shows why it is important to have a kiln with observation
ports to be able to follow the progress of your work. In some ways,
it is more important to have observation ports than whether the kiln
is front or top loading, coffin or clam shell opening. But that is
by the way.
Heat
The
second important element in avoiding stretch marks is to enable the
glass to be at the same temperature throughout its thickness. This
involves the concept of heat work. In
general terms it means you can achieve the same result by putting the
heat in fast and at a high temperature or slowly and at a low
temperature. The “slow and low” approach
allows more control and allows the glass to be the same temperature
on top as on the bottom.
It is
important to heat the glass slowly and steadily all the way up to the
slumping temperature. The temptation to increase the temperature
rapidly after the strain point needs to be resisted. Getting the top
too hot can at the worst, cause a split
on the bottom of the glass as the tension from slumping glass on the
top splits the stiff glass at the bottom.
This
means there is no need for a soak at the strain point, nor a speed up
in the rate of advance up to the slumping temperature. Exactly the
opposite is indicated. Choose a rate of advance for the glass
according to its thickness – at 6mm a rate of 150ºC will be
adequate. Maintain that rate of advance all the way up to the slump
temperature. This also is required when you are making observations
to determine what the slump temperature should be. The moderate rate
of advance all the way to slumping temperature ensures the whole
thickness of the glass is at the same temperature.
Heating
the glass slowly to enable all of it to be at the same temperature,
allows the glass to change shape at the lowest possible temperature
and avoid picking up so much of the mould texture. The glass at the
edge and upper sides is in contact with mould longer than central
parts as it changes shape and slides along the surface of the mould
at elevated temperatures. The lower the temperature used with a long
soak, means that the glass is less likely to slide along the mould
and so adds to the avoidance of stretch marks.
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