Wednesday, 2 September 2020

Altering Annealing Temperatures

Sometimes  it is discovered that a kiln is firing hotter than other kilns, and you need to alter your process temperatures from the generally presented ones.  That your kiln is firing hotter than others is when you recognise the tack fusing profile of your tack fused piece is rounder than expected. 

Altering process temperature and soak times

There are two things you can do.

1)  Reduce the time at the temperature.  If the recommended schedule has the process work being done at 780°C for 15 minutes and the glass is too rounded or more like a contour fuse, you can reduce the soak time to 5 minutes, depending on how over-done the pieces are. 

2)  If the reduction in soak at process temperature does not work, then you can begin to reduce the process temperature.  Often only 5°C with a 10-minute soak is enough.  For some kilns it may be as much as 20°C again with a 10-minute soak.

Remember that the speed at which you advance to the process temperature will have an effect.  The slower you go the lower the temperature can be.  The faster you go, generally the higher the temperature needs to be.  There several factors combining to determine which is the right process temperature and soak.  Experimentation and record keeping are required to find just the right combination.

Annealing temperatures in a “hot” kiln

If your kiln fires hot, you do not need to alter the annealing soak temperature.  I have seen the recommendation that when you need to reduce the process temperature you also need to reduce the annealing temperature by the same amount.  This is not so for several reasons.

The first is that reducing the temperature of the annealing soak runs the risk of trying to anneal below the acceptable range.  These are a few paragraphs to explain.

Annealing occurs over a range.  The annealing point is the temperature at which annealing can most quickly occur.  But there is a range during which annealing can occur.  It is generally around 43°C either side of the annealing point.

If you follow the recommendations to anneal in the lower end of the annealing range, it is possible that you will start the annealing soak at too low a temperature by reducing the annealing soak temperature in line with the reduction of the top temperature.

The second is that the temperature measurement is of the air, not the glass.  On cooling, the glass is hotter than the air temperature in the kiln.  The recommendations for the annealing temperature take that into account.  So, reducing the temperature risks straying outside the annealing range.

Example of the annealing of a tack fused piece comparing temperatures of the air to the under tack stack and exposed base during the anneal soak and first cool

You should note that if you are using the Bullseye recommendations to do the anneal soak at 482°C, you already are in the lower end of the annealing range.  The average annealing point of Bullseye remains at 516°C. This new recommendation for the annealing soak is 34°C below the annealing point and any reduction of more than 9°C will put your anneal soak outside the annealing range, meaning that your anneal will be inadequate, no matter how long you soak there.

The third element relates to the annealing range.  The anneal soak can occur anywhere within that range. But the practical measure is to soak at, or below, the annealing point.  If your kiln fires hot, you do not need to alter the annealing soak temperature.  It will not matter if the glass is in fact hotter at the annealing soak than in some other kilns. 

It does not matter, because the soak at the annealing point, or lower in the range, is to equalise the temperature throughout the glass piece. The annealing point is not some magic number or temperature that sees to producing a sound piece of glass.  The soak at annealing point is to equalise the temperature to + or - 5°C within the glass.  This is referred to by the technically minded as Delta T = 5°C, or in symbols as Δ T = 5°C.  Bullseye has published a table that gives practical information on the length of soak required for this temperature equalisation for different thicknesses.

Once the temperature is equalised within these limits, you can begin the anneal cool.  This is an essential part of annealing and is designed to maintain the equality of temperature differentials during the cooling.  The rate of cooling is directly related to the length of the temperature equalisation soak required for the piece which in turn is related to the thickness of the piece.  This forms the fourth reason that starting the anneal soak slightly higher than recommendations, will not affect the annealing process adversely. The first slow cool is essential to achieving a sound piece as it maintains this small differential in temperature during the early part of the cooling into the brittle phase of the glass.

Annealing Temperatures in a Cool Kiln

Exactly the same reasoning process is applied to both hot and cool firing kilns.  You do not need to alter the anneal soak, even though it means you will start the temperature equalisation at a slightly lower temperature than the published schedules.  This is because you have to increase the top temperature to get the effect you want and so would also be annealing in a cooler kiln.  Since you are measuring the air temperature, the glass temperature will be above the air temperature and will still be in the safe annealing range.


The reasons annealing temperatures do not need to be altered if you kiln fires hot or cool are related to:
·        annealing range
·        air temperature measurements
·        rate of the anneal cool

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