The purpose of glass bending is to achieve a mark-free
curved surface. This is usually done with glass that has smooth
surfaces - normally clear but it can be applied to any smooth
coloured glass too.
There are at least two methods –
moulds and free bends.
Mould
Using a mould enables you to achieve
the shape you want with the least observation. However, you need to
be careful to use the lowest temperature to achieve the shape to
avoid marking the glass. You also need to measure the outer
circumference of the original shape, as you are bending, not
stretching the glass as in most other slumping operations. A mould
is most useful when the shape is not a radius curve. Metal can work
very well and because you are using low temperatures, a dusting of
alumina hydrate or talc will act as an effective separator. Of course
you can use ceramic or fibre as a mould too.
To be able to use the low temperatures required, you need to take advantage of the weight of the glass. This means the glass needs to fall into the mould, not drape over it. Relatively fast rates of advance can be used, as you are normally bending one layer of glass. However from the annealing point upwards the rate should be slowed to allow all the glass to heat throughout and enable the bend to occur at low temperatures. Observation will be required to determine when the bend fully conforms to the mould.
To be able to use the low temperatures required, you need to take advantage of the weight of the glass. This means the glass needs to fall into the mould, not drape over it. Relatively fast rates of advance can be used, as you are normally bending one layer of glass. However from the annealing point upwards the rate should be slowed to allow all the glass to heat throughout and enable the bend to occur at low temperatures. Observation will be required to determine when the bend fully conforms to the mould.
Free drop
A free drop is similar to an aperture
drop, but using a channel rather than a bounded opening. To do this
arrange a channel of the appropriate width plus 10mm to allow the
full curve to form at the edge. Kiln shelves that are cut into
strips, or lengths of fibre board - both supported on kiln furniture
- will provide a good channel. Apply kiln wash to horizontal pieces
forming the channel or cover in fibre paper. Place a witness at the
appropriate height to ensure you can see when the glass has reached
the depth/curve required. Initially, this seems to be extra work,
but the expense of making a metal mould far outweighs the time taken
to set up a free bend for simple radius curves.
Temperature
The temperatures required for glass
bending are 40C-80C above the annealing point of the glass. For
simple curves start with the 40C above annealing whether using a
mould or channel . If after two hours, nothing has been achieved,
advance 10C and soak for another two hours. Repeat as necessary.
Lots of observation is required. Remember that the wider the
aperture and the thicker the glass the less time and temperature is
required to achieve the result. After the first bend you will know
the combination of temperature and time required for the depth and
width of any other piece like this.
Stopping the bend
If you are using a mould, you simply
advance to next segment. With a free drop you can also simply
advance to next segment as the movement is so slow. But if it is a
deep bend - more than a simple radius curve - advance to next segment
and open kiln until annealing temperature is reached. Then close the
kiln and anneal.
Anneal for the thickness of the glass.
No special annealing is required.
For the free drop, when cold, cut off
the excess to the size required for the opening.
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