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blackbeen

Questions regarding bisque

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Hello,

I've spent all of my recent free time perusing every page.  I have absorbed an incredible amount of information that directly addresses things I've been stumped by and also answering questions that I didn't even realize I had.

 

For now, 2 questions regarding bisque firing:

 

I am using a small electric test kiln and doing various types of small items (pendants, ceramic light switches, garden art designs, etc.)

 

I'm confused whether I need to prop the lid open a bit during an initial hour or two of firing for vapor release?

 

What difference does it make whether I do a "fast" or "slow" firing?

 

Thank you!

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Lid open or shut?  If you ask 12 potters, you will get 13 different answers.  Neither of the kilns I use will allow me to start firing without the lid being shut and a safety device activated.  So I have to fire with the lid/door shut.

Fast or slow?  I use slow if ware is not ABSOLUTELY bone dry.  I use fast when the ware has been sitting around in the summer for a while, and I know it is really dry.  Remember that ware does not blow up because it has air bubbles, it blows up because the water heats and expands.

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If your kiln has a pyrometer and a controller which it sounds like your kiln does (based on the fast/slow commentary) then you should fire with the lid closed, always.  It is common practice on a manually controlled kiln, with a kiln sitter/switches, to leave the lid open during the initial part of the firing. This is done because with switches you get a constant amount of heat from your elements, and even with only one switch on, it is possible that it could get over the boiling point of water (212*F) potentially damaging your wares inside. By cracking the lid by 1-3" this would allow the kiln to dry out the remaining moisture in your pots, but also not get too hot by allowing heat to escape. When I was taught in school to fire we would leave the lids cracked by a couple of inches and only the bottom switch on LOW overnight, and in the morning the lid was closed and then the firing would continue.

Kilns with controller sense the temperature in the kiln via a pyrometer which is basically a thermometer that is plugged into your kiln's computer.  Every time the kiln closes the relays (the "click" of the kiln going off/on) it is sending current through your elements which generates heat. The computer senses how much heat is in the kiln and adjusts the amount of on/off cycles to keep the temperature within the selected firing program's range. Essentially, it wont allow your kiln to get too hot, too fast, unless it malfunctions. IF you were to leave the lid open on a computer controlled kiln, the heat escapes from the top, the pyrometer cant tell the temperature properly, and it may leave the circuit on your elements closed, building very hot zones around your pots which may blow up because they got too hot too quick.

  Some folks like to leave the top spy hole open until they've gotten above 300* or so, when the physical water in your clay has been smoked off, and some even go longer until the clay has completely gone through the burnoff stage. The idea being that there is enough oxygen entering the kiln to allow proper combustion of the the organic materials, and enough ventilation to allow those materials to escape. Personally, since leaving college Ive never owned an electric kiln which was so tight sealing that there wasnt enough oxygen entering my kiln through all the little cracks/gaps. If you were bisque firing in a gas kiln this would be a different story.

   To sum it up; if your pots ain't dry, then go slow, if they are dry, then go faster. Leave the lid closed on a computer controlled kiln. Leave your plugs in the kiln during the firing unless your kiln is super tight, and then maybe leave one peep out until youve gained color in your kiln.

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