The
principle of slow and low always applies.
Although the
principle of attempting to get the effect you want at the lowest possible
temperature with the slowest practical rate of advance should always be considered, there are times when it is not wholly
applicable.
Among these
are when working with small scale pieces, such as jewellery, and in general
pieces below 100mm that are at least 50mm from the side of the kiln. In these cases you can fire much faster, as
the heat has less distance to travel through the glass to maintain an even
heat. You still should be using two
stages – the first and slower to rise to the strain point and the second much
faster one to reach the top temperature.
In these cases the target may have to be a little higher than in a
larger, slower firing.
Another case
is in fire polishing. Fire polishing can
often have a fast segment to avoid distorting the piece. In this case you fire appropriately slowly
for the thickness of the piece until you are past the upper strain point. This can usually be taken as 540°C. (For float and bottle glass the temperature
is around 690°C). As you have passed the
brittle phase of glass by this time, you can advance the temperature
quickly. The objective is to achieve
enough heat to change the surface, but avoid heating the interior to the
softening point. You may want to observe
the finish of the surface, so that you can switch to the cool down phase of the
firing as soon as the polish is achieved.
All myths have an element of truth in them otherwise they would not persist.
They also persist because people listen to the “rules” rather than thinking about the principles and applying them. It is when you understand the principles that you can successfully break the “rules”.
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