Wednesday, 28 October 2015

Kiln Forming Myths 8

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”.