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Raku - When Is It Safe For The Second Piece?


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Hi,

I have done raku before. Many, many years ago. 

 

I am getting set to give it a try again in my own studio and I had a question that the manufacturer of the clay I am using (Laguna EM345) was unable to answer.  The question I posed was how long should I approximately wait (in time or temperature) after my first piece is removed to let the raku kiln cool down before I put my second piece in to keep it from cracking.  I will be using a pyrometer to measure temperature (if I feel uncomfortable just watching the glaze), and should be able to use it to gage the kilns overall temperature after the first piece is removed.

 

Perhaps it is an unfair question to the manufacturer as they would want to potentially push away from and liability as raku can be somewhat unpredictable.  I did feel however that they should have a general idea what the shock temperature thresholds are for the clay the make.

 

Any guidance on how long most people wait before you put your second piece in?  Do most people start to pre-heat the work so the shock will not be so dramatic?

 

Thanks

 

Mal~

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I used to sit the next pieces on top of the kiln to pre heat and they went in as soon as the others came out. They were very hot and had to be placed in with tongs or gloves. I never waited.

Hey, if the shock of the heat gets them then they would not survive the shock of the cold either.

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I usually put the new ones in, as soon as I finish taking the old ones out.  I don't even preheat on the lid, and I don't have issues.

 

Much like Chris said, any body that works for Raku, should be able to tolerate quick heating and cooling.

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we usually cool our kiln down for  a half hour or so, but its  mainly because its  a pretty big  kiln and its hard to load raw glazed stuff into a hot kiln without getting toasted or messing up the raw glaze.  Sometimes we let items  sit in the hot but unlit kiln for awhile before lighting. rakuku

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have watched this done twice and it seems that the potter in charge sets the pots outside the kiln, (one of the fiber ones inside a wire cage fired with a weed burner) on bricks, starts heating the kiln and at some point when everything is warm, puts the pots on a centered shelf with tongs and lowers the kiln down over the pots and onto the bricks.  this is done with gloves.  it took about 20 minutes.  at that point, more pots were placed on the outside bricks (using long tongs) to get warm.  about 20 minutes later, the first batch were lifted out and put into trash cans filled with combustibles. once they were in the cans, the waiting pots were put on the shelf, kiln lowered, and another batch put onto the outside bricks.

 

by the time that batch was fired, the original pots in the combustibles were put into a bucket of warmish water to clean them.  this was a demo at Highwater clay in st pete florida.  the woman in charge was very careful to let everyone know what to do to help.  very safe, very efficiently done.  did some and used one as my first avatar.  granted, all the pots were thrown and fired at highwater by employees so they were pretty uniform.  odd shapes would have slowed down the process.  the ease with which this was done made me want one of the kilns.  so simple.  wire attatched to a metal trash can lid so the diameter was just right to lift and move safely with welders gloves.

 

i did a series of photos of this but they were in the camera that was stolen.  sigh........................

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There are two concerns when loading pots into a hot raku kiln. First, the pots must be dry. If they go into the hot kiln while still damp, you can get steam explosions or cracks. It is very common during raku workshops to glaze the pieces on the same day as firing, so you have to get them to dry quickly. We place glazed pots on top of the kiln to dry, and it works like a charm. The second concern is placing pots directly onto a really hot shelf. Even though the kiln may have cooled down to 500F or less, the shelf in the kiln has not, and setting a pot on it can cause the pot to crack. I have a couple dozen pieces of soft brick chunks, and every time we load the kiln we put each pot on one of the brick chunks so they don't touch the shelf. At the end of the firing we pull the brick pieces and replace them with new ones. By the time we finish a load, the hot bricks have cooled and can go back in for another load.

 

As for the kiln temperature, I like to wait until it gets down to 500F or so before we load it up again. My kiln is made of soft brick, so it retains a fair amount of heat between loads. Once we close the door the temp will go up to 800F or more without even turning on the burner.

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