Is it Possible?
It
is possible to slump unknown glass. This glass might be art glass, window
glass, bottles, or any other glass whose characteristics are unknown by
you. There are some suggestions about
the characteristics of some glasses in this post that can be used as a starting point.
Preparation of the Glass
Prepare
the edges to their final finish before slumping. This because the slumping temperature will not
be enough to alter the finish of the edge significantly. This preparation can be done with diamond
hand pads, or wet and dry sandpapers.
Start with a relatively coarse grit. You may wish to do the initial
shaping on your grinder. This will be between 80 and 100 grit. Continuing with a 200 grit and working your
way through 400 and then 600 grit will give you an edge that will become shiny
during the slumping.
Cleaning
Clean
thoroughly. This is especially important
when using glass that is not formulated for fusing. Devitrification is more likely on these
glasses. Water with a drop of
dishwashing liquid can be enough unless your water has high mineral
content. Then distilled water or a purpose
made glass cleaner such as Bohle or Spartan should be substituted. Finish with a polish to dry with clean paper
towels. More here.
Firing the Slump
Fire up slowly. You should advance at about 100°C to 150°C
per hour. Set your top temperature around
630°C for a simple slump, for soda lime stained glass. For bottle or window glass you will need a
temperature closer to 720°C although the also are soda lime glasses.
It is best to start
with simple curves, as there are fewer difficulties in determining what the
glass is doing. It will help you to
learn the characteristics of the glass before you tackle the difficult stuff,
such as compound curves or texture moulds.
Observation
It is
necessary to observe the progress of the slump as you do not yet know the
slumping temperature. You want to know
when the glass begins to deform so that you do not over fire. Start watching the glass at about 10 minute
intervals from about 580°C for stained glass and
680°C for window and bottle glass. There
is not much light in the kiln at these temperatures, so an external light is
useful. You can also observe the
reflections of the elements on the glass.
When the image of the elements begins to curve, you know the glass is
beginning to bend.
Altering the
Schedule
Soak for
at least 30 mins at the temperature when the glass begins to visibly drop. This
may or may not be long enough. Continue
checking at 5-10 minute intervals to know when the slump is complete. If the glass is completely slumped before the
soak time is finished, advance to the next segment. If not fully slumped, you need to extend the
soak time. This means that you need to know how to alter your schedule in your
controller while firing. Consult your
controller manual to learn how to do these things.
Stop the
soak when complete and advance to the anneal. Continue the slumping soak if not
complete after the 30 mins. In some cases,
you may need to also increase the temperature by 5-10°C.
Annealing
The
annealing point will be about 40°C below
the point that the glass visibly starts the slump. If you want a more accurate
determination of the annealing point, this post gives information on how to
conduct a test to give you both the slump temperature and the annealing point. It also
helps to determine the lower part of the tack fusing range (the lamination
state), since it is not far above the slumping point that you will observe.
The
annealing soak for a single layer, 3mm glass need not be long – 15 to 30 minutes. The annealing cool can be as fast as 120°C down to 370°C.
For thicker glass and slumped bottle glass you will need a longer soak –
30 to 60 minutes – and a slower cool.
The annealing cool in this case could be about 60°C/hour to 370°C. You can turn the kiln off at 370°C, if you
wish, or keep the temperature controlled to about 50°C. The rate for the final cooling can be
approximately double the first cooling rate. For
a single layer of stained glass this could be 240°C, and for thicker glass
about 120°C
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