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.
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 5ºC 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, just 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 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.