Friday, 5 October 2012

Glass bending


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.

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.