Wednesday, 9 December 2015

Kiln Forming Myths 14

You don’t need to anneal as much on initial firings as on the last firing


This seems to be based on the theory that only the last firing matters to the soundness of the piece.  However, re-firing a poorly annealed piece may be at too great a risk from this kind of ill thought-out practice.

Even for properly annealed pieces, there is risk at each re-firing of a piece from thermal shock.  The rate of advance on the second firing of a fused piece needs to be reduced to accommodate the now thicker piece than the two or more original pieces.  Re-firing of tack fused pieces needs even slower rates of advance, because of the uneven thicknesses.   

The risk on each heat up to poorly annealed objects is even greater than indicated above for well annealed pieces.  Poorly annealed pieces can’t absorb temperature changes as easily, so are at greater risk of heat shock during temperature increases.  

When considering whether to reduce the annealing at the early stages of your project, you need to consider at least: 

  • time saved versus risk, 
  • the size of the piece,
  • The degree of fusing.

The time saved will depend on how you fire.  If you fire overnight, there usually is no saving in time, so why increase the risk of breakages?  Time savings normally relate to how many firings you can get out of your kiln in a day.  So the number you need is an element in thinking about the risk too.

If firing during the day when there might be time savings, consider the size.  Smaller sizes usually can survive being under-annealed more easily than larger ones.

The degree of fusing is an important element in the risk.  The closer you are to a lamination fusing, the greater the risk of under-annealing causing a break on the next firing.  A full fused piece is more likely to survive an inadequate annealing on the subsequent firing.  These factors indicate that it is less advisable to under-anneal tack fused pieces that are going to have several firings.

You also need to be careful about applying the practice of blown glass workers to flat glass.  Blown glass, because of its curved shapes can withstand more stress than flat glass.  Although blown glass pieces can be under annealed before going through another process of heating, it is more difficult to do this with flat glass.

If you decide to reduce the annealing process at the early stages of a project that must be fired many times you need to be careful to avoid breaks. To avoid thermal shock of inadequately annealed pieces, the rate of advance must be reduced.  This reduction should be at 75% or less of the normal rate of advance for a piece of the size and nature of your project.

To have the best chance of survival throughout the kiln forming process, each piece needs to be properly annealed every time the temperature rises to or beyond the annealing temperature.