Refiring and Annealing

A question about re-fusing was posted:

 I have just taken a large [rounded tack] piece, with … A small piece … flipped and showing the white side…. If I cover this with a thin layer of coloured powder frit, does the piece need the long anneal process when I fire it again... I will be taking it up to the lowest tack fuse temperature possible, so the rest doesn’t change too much.

When considering the re-firing of a fused piece, even with minimal changes, the schedule needs re-evaluation of both ramp rates and annealing. 

Ramp Rates

Previously the piece was in several layers. The piece is now a thicker single piece and needs more careful ramp rates. You cannot fire as quickly from cold as the original unfired piece. Previously, the sheets could be heated as though separate. They were not hot enough to stick together until beyond the strain point. They could withstand the differential expansion that rapid heating causes. 

The thicker, previously rounded tack piece will need a slower initial ramp rate. Looking at Stone* and the Bullseye chart for Annealing Thick Slabs indicates the rate should be halved for each doubling of calculated thickness. A rounded tack firing of two layers should be fired as though twice its actual thickness. This means using a schedule for 12mm/.05” thick rather than 6mm/0.25”. This would be at a rate of 330°C/595°F. 

The first firing was of two layers of 3mm/0.125”. Now you are firing a tack fused piece of 6mm/0.25”. It requires a rate of 165°C/297°F as the first ramp rate. If you started with a rounded tack of two base layers and one tack layer, you may have been using a first ramp rate of 150°C/270°F (for 18mm/.075”). Now you will need to be thinking of 75°C/135°F as your first ramp rate. 

Annealing

The annealing time and cool rate will not be affected in the same way. In the first firing you are already annealing for the two layers forming a single piece of 6mm/0.25”. As there is no change in the profile or thickness of the piece, it can be annealed as previously. The cooling rates are the same as for the first firing. 

Credit: Bullseye Glass Company

Refiring with Additions

Ramp rate

If there are additions to the thickness, a slower ramp rate will be required. For example, if an additional 3mm layer is placed on top of a 6mm/0.25” base for a full fuse the ramp rate will need to be reduced to that for 9mm/0.375”, i.e., 415˚C/747˚F according to various charts. However, I never fire faster than 330˚C/595˚F.  There is too much risk in breaking the glass through differential expansion with fast rates.

 

In this case the firing is for a rounded tack. You will need to schedule as for 18mm/0.75”. The rationale for this doubling of the thickness is in my ebook Low Temperature Kilnforming. This initial rate for 18mm/0.75” will be 150°C/270°F. 

Annealing

This time the annealing will need to be longer than the first firing. The thickness has changed with the additions of pieces for a rounded tack firing. Instead of annealing for 6mm/0.25” you will be annealing as for 18mm/0.75”. This requires a hold of three hours at the annealing point and cooling over three stages. The first two of these stages are 55°C/100°F each. The first cool rate is 25°C/45°F per hour and the second is 45°C/81°F per hour. The last is at 90C°C/162°F per hour to room temperature. 

These examples show how dramatically later additions in thickness can add to the length of the firing to get a well-annealed piece without breaking it on the heat-up. 

 

*Graham Stone. Firing Schedules for Glass; the Kiln Companion. 2000, Melbourne. ISBN 0-646-397733-8

As a side note, Stone’s book has become a collectable.