Placing of Pieces in the Kiln

 Distance from Sides of Kiln

 

"Is there a rule of thumb for interior size of kilns and piece size? (i.e., “allow for X inches between the piece and kiln walls on all sides”).  I’m thinking about how to determine piece size limitations when shopping for a kiln."

I don’t know of a formula, or rule of thumb, to determine the amount of space required between the glass and the kiln walls.

I have only been able to determine the spacing required after I have purchased the kiln.  Each kiln has different characteristics. 

The most obvious is whether the kiln is fired from the side or from the top.  More space is required with side fired kilns.  The radiant heat from the elements tends to heat the edges of the glass before the centre becomes equally hot. This requires more space or baffles between the elements and the glass.

Top fired,  with enough distance to get even distribution of heat

Side fired. Red arrows indicate the important infrared heating.
Blue arrows indicate the less effective ambient heat.

There is less concern about uneven heating with top fired kilns.  But as each kiln is different, you must test the heat distribution around the kiln.  Bullseye Tech Note #1 has a good method.  This will show where the temperature is less than the rest of the shelf.

In general, rectangular kilns are cooler in the corners.  Round kilns do not have the same characteristic, but may still have uneven temperatures, due to the configuration of the elements.  Smaller kilns seem to have more even temperatures than large kilns, which tend to be cooler along the sides.  Kilns with a ring element below the shelf seem to have the most even distribution of temperature.

I had a large kiln 2 metres by 1 metre which had a requirement of 50mm/2” from the edge to even the temperature.  A recently purchased 50cm square kiln has almost perfectly even temperatures across the whole shelf.

[The illustration is taken from the ebook Low Temperature Kilnforming, available from Bullseye and Etsy.]

The required glass distance from the side will depend on side or top elements and size but no formula is available.  Testing for heat distribution is necessary once you have the kiln.