Scheduling for Size or for Thickness

When scheduling a firing, which is most important, size or thickness?

As usual in kilnforming - it depends.

Generally, the thickness is the most important consideration.  The concept is that the heat needs to be put into and released from the glass through its thickness as it is larger than it is thick.  This means the shortest distance for the heat to travel is through the thickness.

But, if the pieces are small, the heat can be released from the sides too. In this case, size is important.  Small pieces, say under 100mm, can be fired quickly.

If the piece is very large relative to your kiln, you need to slow the heating and cooling as the hotter and cooler areas of your kiln will be brought into consideration. Large pieces are those that occupy almost the whole of your kiln. This is especially important in side fired kilns but has application in top fired kilns.  The heat is uneven in all kilns to some extent.  To overcome the limitations caused by this, you need to slow the rates.

The general rule

But, in general, you fire for the thickness of the piece (as determined l factors such as uneven thicknesses, tack fusing, stress points, etc.) because that is the important variable for absorbing and releasing heat.

Always consider whether things needing different firing conditions should be fired together or in separate firings.  This applies to slumping as well as the fusing of pieces.

Further information is available in the ebook Low Temperature Kiln Forming.