Heat Work is Cumulative

“…. the first fuse (contour) I brought it up to 1385°F and held for 5 minutes - it did not contour as much as I would like - do I re-fire at same temp and hold longer or go up in temp and hold same amount of time or something else?”

Observe

Of course, the smart answer is “Observe to get it right first time”.   Observation will enable you to determine when the piece is fully fired.  To observe you need only peek at 5-minute intervals to determine if the piece is as wanted. 

Know your Controller

In combination with this you will need to know your controller well enough to be able to advance to the next segment if the piece is done before the segment finishes; or how to stay on the same segment until it is finished and then advance to the next segment.

Of course, there are circumstance when you cannot or do not want to be present at the top temperature of the firing.  Then consider using the delay function to enable you to be present. This gives a countdown until the kiln starts.  The practice is fully described in this blog entry.

Time or Temperature

If you are experiencing an under-fired piece and want to re-fire it to get a better finish, the usual question is whether to fire for longer or at a higher temperature.

The response is – “Neither”.

Re-fire to the same temperature and time as before, unless you are looking for a radically different appearance.  Heat work is cumulative.  You have put heat into the glass to get the (under fired) result.  By firing it again, the heat will begin to work on the glass as it rises in temperature.  The piece, in this instance, is already a slight contour.  The additional heat of this second firing will begin to work just where the first firing did, and will additionally change the existing surface just as the first firing did.  The degree of contour achieved by the first firing will be added to equally in the second firing.  It is of course, a good idea to peek in near the top temperature to be sure you are getting what you want. More information on heat work is available here with its links. 

Rate of Advance

It is important to remember that on the second firing the glass is thicker, and you need to schedule a slower rate of advance until you get past the strain point – about 540°C for fusing glasses, higher for float and bottle glasses.

Future firings

At the finish of the second firing you will have soaked at the top temperature for twice the scheduled time.  You can use this extra time for the next similar firing, or increase the temperature slightly and keep the original firing’s length of soak. 

As pointed out earlier, observation for new layups, sizes, thicknesses, etc., is important to getting the effect you want the first time.