I hear the comment "you can't over
anneal" all the time. Is it true?
My response to this may be controversial, and I do expect
there will be some dispute with aspects of what follows.
My view of the statement “you can’t over anneal” is that it
results from a lazy approach to thinking about the process.
The short answer is,
in my view “yes, you can over anneal”.
- · Lengthy anneal soaks can induce stress in certain circumstances. More later.
- · Excessive annealing soaks waste time and money.
- · Annealing is more than the soak. It is a combination of equalisation of the heat within the glass (not just temperature) and the gradual cooling of the glass to below the lower strain point to ensure the glass does not incorporate differences of temperature of plus or minus 5°C.
There is both tradition and research to assist in
determining the length of the anneal soak.
The tradition seems to embrace 30 minutes anneal soak for each layer of
glass. The research has been done by Bullseye and they have developed a table to assist in accurately determining annealing soaks for thick glass.
Although this is for thicker pieces, it will inform users of
the relationship between thickness and annealing soaks. The table starts at 12mm thick, but you can
extrapolate that a 6mm flat piece cooling from both sides will need a one-hour
soak, an initial cooling rate of 110°C, a secondary rate of 200°C. It is safe to turn the kiln off at 370°C, as
the kiln is unlikely to be able to cool faster than the 330°C suggested
(although I programme to room temperature). The temperatures used need to be
altered for glass other than Bullseye, but the rates remain valid. My advice is
to use the research, rather than tradition.
Other considerations include the nature of the kiln. If your kiln has significant temperature
differentials across the shelf, long annealing soaks will incorporate those
differences during the annealing cool and result in a stressed piece. You do
know the temperature distribution within your kiln, don’t you? This Tech Note #1 from Bullseye will give you
the information to test for the temperature distribution. Using this information will enable you to avoid the cool
spots when placing your pieces and utilise the areas where the heat is even.
Economy is another reason that it is possible to over anneal. Soaking at the annealing temperature uses a
significant proportion of the electricity consumed in a firing. This means an overly long temperature
equalisation soak will use more electricity than necessary. It also uses more kiln time than necessary,
by delaying the anneal cooling and the following natural cooling rate of the
kiln.
It is possible to
under anneal
You need to learn about the effects of your project on
annealing requirements, because it is possible to under anneal. The research on annealing is based glass of
uniform thickness. The most popular style of kilnforming appears to be tack
fusing of one degree or another. This is
unfortunate for the novice, as it is the most difficult of styles to anneal
adequately. There are a lot of factors to consider when setting the annealing schedule.
I feel this is the origin of “can’t over anneal” thinking. Instead of thinking about the specific
annealing difficulties, many seem to just add more time in a generally random
manner. The post on tack fusing
considerations (the link above) is designed to help in thinking about the
requirements of the lay-up of your piece. The cumulation of factors can easily
treble the annealing soak and slow the rates by three times. In some extreme
cases, the annealing time can be extended by as much as five times.
What is the anneal?
Another problem is that most often annealing is thought of
as merely a soak at the annealing point of the glass. It is much more than that. The annealing point is usually the
temperature at which the heat within the glass is equalised in preparation for
the anneal cool. This is because the
annealing temperature is that at which the glass will most quickly anneal. Since the anneal is temperature sensitive,
the equalisation of the temperatures within the glass will be most successful
at getting a good anneal throughout the cool.
For two-layer flat fused items, the annealing point can be
used as the heat equalisation temperature.
The soak is to get the glass to be + or - 5°C throughout the piece.
Sometimes, especially with thicker or more difficult pieces,
the annealing is done closer to the lower strain point. The reason for this is
to save time in the annealing cool. If
you look at the Bullseye annealing chart, you will see how slowly thick pieces need to be
cooled, so starting 35°C below the annealing point can save many hours of cooling.
Once the glass has equalised in temperature, the object is
to cool the glass at a rate that ensures the internal temperatures do not vary
more than plus or minus 5°C across and through the piece. The rate can increase by approximately twice after
the lower strain point has been reached (approximately 55C below annealing
soak). This second stage rate should
take the glass to around 370C, where the rate can again be doubled to room
temperature.
Difficult pieces
Tack fused and other pieces with uneven thicknesses require
more care in the annealing to ensure even cooling of the whole without a
greater variation in temperature than +/- 5°C.
As said above, tack fusing is one of the most difficult of styles to
anneal adequately. The blog entry for
tack fusing considerations indicates some factors that increase the
requirements for more careful annealing.
As an example, I cite a piece 6mm thick, with two layers of
rectangular and pointed pieces that are just barely rounded. This adds five factors of complications for
the fusing - two levels of tack fusing, rectangular pieces, pointed pieces,
laminated tack fusing. This number of
complications increases the practical thickness to 21mm – 6mm of flat base, 3mm
each layer of tack (6mm), 3mm for rectangles, 3mm for pointed pieces, 3mm for
laminated fuse. Because this is tack
fused, the next practical step up in the table needs to be used. That is the
one for 25mm, which requires a four-hour temperature equalisation soak, and
15°C per hour initial anneal cool rate.
Glass other than
Bullseye
I have so far talked about Bullseye. It is possible to apply these times and rates
to any glass of which you know the annealing point. The annealing soak can be set above the lower
strain point, which is approximately 55°C below the annealing point. To be safe, a point 35°C below the annealing
point is used.
E.g., if you are annealing a 12mm slab of float glass, the
annealing point of which (in the UK) is 540°C, you chose a temperature of 505°C
to do your two-hour soak, followed by a cool rate of 55°C for the first 55°C
and then 99°C for the second stage cool to 370°C. The final cool of 330°C per hour to room
temperature remains the same too. So,
you can see the rates and soak times remain the same regardless of the glass
type. It is only the temperatures that
change.
A summary of this can be seen here.
Bullseye chart for Annealing Thick Slabs
Unfamiliar terms can be searched for on the blog: www.glasstips.blogspot.co.uk
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