Wednesday, 14 March 2018

Cooling the Kiln

“Why does my kiln take so long from boiling point to room temperature?”

The rate at which a kiln cools is dependent several factors:
  • ·        The mass of the kiln. Some kilns have dense insulation bricks.  These are very good at holding heat, and release it slowly.
  • ·        Its insulation characteristics. Other kilns have light weight bricks or fibre insulation. Both these materials have less mass and can release heat quickly at high temperatures, but much less slowly at lower temperatures.
  • ·        The environment. The temperature of the surroundings has a big effect at lower temperatures.  The amount of air movement around the kiln also influences the cooling rate at these lower temperatures.

The physics of heat transfer determine the cooling rate. if all other factors are the same, the rate of temperature fall is faster when there is a greater temperature differential.  And it is slower where the temperatures are closer together.  You can see this by comparing the rates of fall at 800⁰C and at 300⁰C.  It is much faster at the higher temperature and slower at 300⁰C.  You will also notice that the kiln cools more slowly at the lower regions when the outside temperature is high than when low.

Rather than waiting hours or days for the kiln to get 
to room temperature, there are some things you can 

·        Open any vents or peep holes your kiln has. Not only are peep holes good for observing the progress of the kiln work, they are important in cooling.  Their relatively small size insures that there is not such a great air exchange that could cause thermal shock.  The temperature at which you do this is relative to the thickness or variation in thickness of the pieces in the kiln, of course.

·        Open the kiln lid/door a little. As the temperature fall rate reduces, you can crack the kiln open a little.  Many times, you need to put a prop under the lid to keep it open only a little.  Again, this should only be done at a low enough temperature to avoid any thermal shock to the glass.

·        Create greater air movement around the kiln.  You can of course create greater air circulation around the kiln by opening doors and windows, or by a fan.  If you use a fan, it is best to avoid direct air current from the fan onto the kiln. This is because when the vents or lid are open, dust can be spread over the glass and throughout the studio.  If using a fan, it is best to have the kiln closed.  Some kilns have powered ventilation to speed cooling, but these are usually industrial.

How do I tell if I am cooling too fast?

The risk of opening your kiln after the end of the second part of the annealing cool (generally around 370⁰C) is thermal shock from the relatively cool air contacting the glass and cooling one part too much, causing a break or fracture.

You can select how fast a cool rate is safe for your piece and programme that into the controller down to room temperature.  Doing this does not use any more electricity than simply turning the kiln off.  The controller will only put more energy into the kiln if it is cooling more quickly than the rate you set. 

And this is the point of programming to room temperature.

When you vent your kiln, and have the controller set for a cooling rate, it will only add more heat if you have opened the kiln too much.  If you hear the controller switch on the elements, you know to reduce the size of the opening, because it is cooling faster than you set the rate to be.  This makes for a safe, but more rapid cooling than just letting the kiln cool with no ventilation.

"My controller shuts off when I open the kiln."

If your kiln does not allow any opening of the lid/door without the controller switching off, you need an alternative.  In this case, you will need to take note of the temperature drop over set periods to learn if the temperature is falling too fast or too slowly.  Usually 15-minute intervals are all that is required.  Record the temperature at the switch off and before venting the kiln. Vent the kiln. Fifteen minutes later record the temperature. Multiply the difference by four to get the hourly rate.  If that rate is above the one you intended, close the venting a little.  If it is less, open the venting a little more. Then record the temperature after another quarter of an hour. You continue to do this until you are satisfied you have settled on the rate of cooling you intended.

You must exercise patience with the cooling.  

The larger, thicker, more important the piece is, the more caution is required. 

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