Questions about when it is
possible to open the kiln during the cool down to avoid thermal shock get the
answer, “it depends….”
These dependent variables include:
Temperature Differentials
Thermal shock is related to how
quickly a piece can cool without developing stress that cannot be contained
within the piece. So, when the
temperature differential is a few tens of degrees between room and kiln air
temperature it is less risky than when the difference is hundreds of degrees.
This means that there is a relation
between room temperature and when you can open the kiln safely. If the room is at sub-zero temperatures, you
will need to wait for a lower temperature in the kiln, so the temperature
differentials are no greater than when the room is warm. Remember the glass can be much hotter than
the air that the thermocouple measures.
Cooling rate of the kiln
The natural cooling rate of the kiln
(that is, in the unpowered state) will affect when you open. If your kiln cools very slowly from 150°C,
you may feel confident to open the kiln a little to speed the cooling from that
temperature. If you kiln cools quickly -
usually in smaller kilns - then you need to wait longer for a lower temperature
to be achieved.
Size of the piece
The size of the piece(s) relative to
the kiln size has a bearing on when it is safe to open the kiln to speed
cooling. The more space the piece takes
up in the kiln the cooler the temperature reading needs to be before you open
the kiln.
Placing
The placing of the glass has an affect
too. If the glass is at the front of a front
opening or top hat kiln, it will cool more quickly and unevenly than one at the
back. A large piece placed more to one edge than another will also require
lower temperatures before opening.
Thickness
The thickness of the glass also needs
consideration. The thicker the glass,
the hotter it will be in relation to the measured air temperature, and so the
longer it needs to be left to cool before opening.
Type of kiln
Your kiln may cool slowly or quickly,
but the style of the kiln is important too.
The kiln may be brick lined or fibre lined, or a combination. The greater the mass of the insulation, the
earlier you can open, as the dense brick will radiate heat back toward the
glass.
If you have a top hat kiln it is
probable that you can open earlier than if you have a top opening or front door
opening kiln, as they will dump hot air slower than top and front opening kilns.
The venting method
The way you open the kiln to increase
the cooling rate is important. If you
open vents, that provides a gentler flow of cooler air than opening the lid or
door. If you open lids or doors, you
need to wait for a lower temperature than for opening vents.
And I am sure there are other
considerations. But these are enough to
show that there is not a single answer.
The answer is in relation to the kiln and its contents.
Acceptable Cooling Rates
The speed of cooling that a glass can
sustain is indicated by charts giving the rate of cooling for the final rate of
decrease to room temperature. Faster
rates might be induced by turning the kiln off at 370°C and opening the door/lid
at some slightly lower temperature.
This means that you need to know how
fast a cooling rate is acceptable. The
bullseye research suggests that 300°C per hour for the final cooling is as fast as you would want to
cool a 12mm thick piece. This is in a
closed environment. Therefore, you will
want to be slower – at least half the speed for a partially opened kiln of say 5cm.
My predictions for acceptable cooling
rates are (with a room temperature of 20°C; a piece evenly thick and 30cm
square, but less than half the area of the kiln floor; and a top hat kiln):
6mm - 300°C per hour (although I never use more
than 200°C per hour)
12mm - 150°C per hour
19mm - 75°C per hour
25mm – 45°C per hour
Note: Tack fused items with these total heights need to have these rates
halved, or use the rate suitable for a piece twice the thickest part.
But!
You cannot open the kiln until the
natural cooling rate is at the predicted acceptable rate of cooling or less, to be safe.
The natural cooling rate at various
temperatures can be determined by observing temperature falls in relation to
time intervals between those observations.
You can make a chart to indicate the cooling rate at different
temperatures. The kiln will naturally cool more slowly at lower temperatures.
Schedule to room temperature
A protection against too rapid
cooling is programming to room temperature.
If your kiln is cooling less rapidly than you predict is acceptable, you
are using no electricity – OK, maybe a tiny fraction of a kilowatt to keep the
controller operating. But there is no worry of using excess electricity.
The point of programming to room
temperature is that if the air temperature in the kiln cools faster than
predicted, the controller will turn the kiln on. You will need to be
present for a while after venting the kiln to hear if it turns on and you can lower the lid to a point where the kiln
does not turn on, indicating the rate of cooling is less than put into the schedule.
An example:
Assume you predict that 150°C per
hour is the appropriate rate of cooling from 370°C. Also assume you open the kiln at
100°C and a minute or so later you hear the kiln start. Then you know that you have opened the kiln too
far causing a more rapid cooling than 150°C per hour and you need to close the
opening to less than the current state.
This probably will be a progressive thing. You will come back, say, half an hour later
and open a little more. Everything seems
fine, but 10 minutes later you hear the kiln switch on again. Oops! You opened too much – you need to close
the kiln a little. This may repeat several times.
The real answer to when you can open
your cooling kiln is dependent on many variables. You will have to decide on how critical these
are in relation to the piece(s) you have in the kiln. Once you have decided on the appropriate
rate, you should program that into your schedule for the final segment. This means when you partially or fully open
the kiln the controller will switch the kiln on when the cooling rate is faster
than you wanted.
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