Absolute Firing Temperatures - Kiln Forming Myths 9

There is a given temperature for each level of fusing – slump, tack, full, etc.

You will often see statements about the temperature for achieving a particular effect.  It is as if all glass under all circumstances does the same thing at a given temperature. These temperatures can only be understood in relation to several things.

• ·         Kiln characteristics
• ·         Speed of firing – i.e., heat work
• ·         Time at forming temperature

The relevant factors about the kiln are:

·         Insulation.  The two main types of insulation in kilns are fibre blanket and insulating brick.  Fibre blanket is often the main insulating element in kilns as it does not absorb a lot of heat. It of course loses heat more quickly than refractory brick.  Most often the floors of kilns are made of brick for rigidity and resistance to damage.  (They also can be replaced individually if one is damaged.)  Refractory brick comes in two densities.  The light weight one is not rated to such a high temperature and loses heat more quickly than the higher temperature rated dense brick.  Both lose heat much more slowly than fibre blanket.  This means the top temperature can be reached more quickly in a fibre insulted kiln than in brick insulated kilns. The brick insulated kilns radiate the heat back into the kiln upon cooling, making for long safe anneal cools without much effort in controlling the cooling rate. Thus the temperatures for an effect are different for kilns with bricks all around than with fibre blanket, and no comparison is easy between kilns with different insulations.

·         Size.  The size of the kiln has an effect on the temperature cited to achieve an effect.  A small kiln can heat up very rapidly, but the glass cannot heat evenly as quickly.  A large kiln takes more time to heat up, as there is more insulation absorbing the heat input.  So working temperatures for small and large kilns are different.  The size of the piece(s) of glass also have an effect.  Small pieces can be heated much more quickly than large or thick pieces, so the top temperature for an effect will be different for the two sizes.

·         Temperature variation across the kiln shelf affects the rate of firing possible and (as noted later) will affect the top temperature.  The more even the heat the faster it is possible to go and that affects the temperature chosen.

·         Element placement.  Some kilns have only side elements, some only top elements, and some have both.  All these variations affect the temperature required to obtain an effect.  In general, top fired kilns can be fired faster than side fired kilns.  Kilns with both, require an intermediate rate, unless the side and top elements can be fired independently.

Speed of firing, i.e., heat work

·         Heat work factors make the top temperature different in different circumstances.  This is mainly about the speed at which you fire the glass.  Generally, the slower you fire, the lower temperature you need.  Allowing the glass to absorb the heat gradually usually means that you can achieve a particular effect at a lower temperature.  A fast rise in temperature requires a higher temperature.

Soak times 

·         The amount of time you soak at the working temperature will also affect the temperature chosen.  A longer soak allows a lower temperature to be used (although that can get into the risk of devitrification from spending too long at the top temperature – it is a balancing act).  A higher temperature can be used to keep the soak time reduced. 

All these variables mean that without being given the kiln characteristics and a schedule, you cannot evaluate the temperatures and rates of firing that are given out by others.  You need to know how closely their kiln fits with your kiln in its characteristics as outlined above.  When asking for a temperature or a schedule, you should indicate what kind of kiln you are using.  You need to know in any schedule what the ramp speeds are and the soak times.  They can then, of course, form the basis for your experimentation.

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”.