Wednesday, 18 May 2016

Re-Firing Schedules

Pieces need to be fired after their initial firing for various reasons – additions, corrections, fire polishing, etc.  You need to think about how this next firing differs from the previous one when thinking about the schedule to use.

The most common need for re-firing is after the full fuse or tack fuse to do the slumping.  On the first firing you had two independent pieces, so they could be fired faster than the fused piece.  It is now at least six millimetres thick – at least in parts. As glass is a poor conductor of heat, it needs a slower initial rate of advance than the assembly of thinner pieces did.

If you have fused a blank and now want to add tack fused elements to it, you need to consider how the pieces on the top will shade the heat from the glass below.  Unless the upper pieces almost completely cover the base, you will need to go much slower than the two-layer piece.  The blank is not only thicker, it also is shaded from the heat by the upper pieces.  If they are of both dark and light tones over the same base, the differential shading will be even greater, requiring slower rates of advance.

If you are adding layers of powder, you are not adding much to the thickness or unevenness of the glass.  So no additional reduction, other than that used for previous powder layers, in firing rates is required.

You need to think about the changes you have made to an already fired piece.  If you have made significant changes in thickness or are going to a tack fuse, you need to slow the rates of advance.  Some advice is given on rates of advance for tack fused items here.  If you have added only a layer of powder or thin coating of frit evenly spread, you will not need reductions in rates of advance.

Of course, the annealing soak will need to be longer for thicker or more complicated pieces, and the annealing cool will need to be slower. This blog post gives information on annealing considerations.