Low and Slow Approach to Kilnforming
We are often impatient in firing our pieces and fire much more quickly than we need. After all, our computerised controllers will look after the firing overnight. So there is no need to hurry more than that.
The concept of heat work is
essential to understanding why the slow and low method of firing works. Glass
is a poor conductor of heat which leads to many of our problems with quick
firings. The main one is stressing the glass so much by the temperature
differential between the top and the bottom that the glass breaks. We need to
get heat into the whole mass of the glass as evenly and with as smooth a
temperature gradient as possible. If we can do that, the kiln forming processes
work much better. If you add the heat to the glass quickly, you need to go to a
higher temperature to achieve the desired result than if you add the heat more
slowly to allow the heat to permeate the whole thickness of the piece.
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Graphs of the
difference (blue line) between upper and lower surfaces of glass of different
thicknesses against cooling time
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However, this slower heating means that the
glass at the bottom has absorbed the required heat at a lower temperature than
in a fast heat. This in turn means that you do not need to go to such a high
heat. This has a significant advantage in forming the glass, as the lower
temperature required to achieve the shape means that the bottom of the glass is
less marked. The glass will have less chance of stress at the annealing stage
of the kiln forming process as it will be of a more equal temperature even before
the temperature equalisation process begins at the annealing soak temperature.
Applying the principles of low and slow means:
- heat
is added evenly to the whole thickness of the piece
- there
is a reduction in risk of thermal shock
- the
glass will achieve the desired effect at a reduced temperature
The alternative - quick ramps with soaks –
leads to a range of difficulties:
- The
introduction of heat differentials within the glass. Bullseye research
shows that on cooling, a heat difference of greater than 5ÂșC between the
internal and external parts of glass lead to stresses that cannot be
resolved without re-heating to above the annealing point with a
significant soak to once again equalise the heat throughout the piece.
- It
does not save much if any time, As the glass reacts better to a steady
introduction of heat. Merely slowing the rate to occupy the same amount of
time as the ramp and soak together occupy, will lead to fewer problems.
- It
can soften some parts more quickly than others, e.g., edges soften and
stick trapping air.
- Quick
heating, with “catch up” soaks, of a piece with different types and
colours of glass is more likely to cause problems of shock, bubbles, and
uneven forming.
- Pieces
with uneven thicknesses, such as those intended for tack fusing, will have
significant differences in temperature at the bottom.
- Rapid
heating with soaks during slumping and draping processes can cause uneven
slumps through colour or thickness differences, or even a tear in the
bottom because the top is so much more plastic than the bottom.
However there are occasions where soaks during
the initial advance in heat are useful:
- for
really thick glass,
- For multiple - 3 or more - layers
of glass,
- for
glass on difficult moulds,
- for
glass supported at a single internal point with other glass free from
contact with mould as on many drapes.
Of course, if you are
doing small or jewellery scale work, then you can ignore these principles as
the heat is gained relatively easily. It is only when you increase the scale
that these principles will have an obvious effect.
Slow,
gradual input of heat to glass leads to the ability to fire at lower temperatures
to achieve the desired results, with less marking and less risk of breaking.
Further information is available in the ebook Low Temperature Kiln Forming.
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