A few weeks ago I sent a kiln pointer on the downdraft kiln vent. (I have copied some of my message below.)
Tony Rodriguez, a kiln technician in San Antonio, Texas, kindly sent the following pointers:
"The lid air intake holes in top-loading kilns should be drilled close to the walls of the kiln to prevent thermo-shocking the ware. Position the holes in the lid so that they are 1/2" to 1" at most from the edge of the firebrick walls. This will allow the elements in the walls to heat the colder air that comes in through the vent holes.
"You say that you can mix glazed ware and greenware on the same shelf in the kiln. This is misleading as not all glazes are fired to cone 04--only specialty glazes such as metallics. There must be a two-cone difference between bisque and glaze firing for good body fit. Otherwise the glaze can separate from the bisque and peel off."
Most top-loading kilns with the down-draft vent have 1/4” air intake holes drilled in the lid. Try not to place ware directly under these vent intake holes. The room temperature air coming in through the holes can cause small areas of glaze imperfection such as crazing. The vent intake holes should be drilled toward the outer edges of the lid so that they won't interfere with ware placed toward the central area of the top shelf.
You can place ware near the vent exit holes, however. In most top-loading kilns, the exit holes are drilled in the kiln bottom. The movement of air will not interfere with ware because the exit air is hot.
With the vent, you can mix glazed ware and greenware on the same shelf in the kiln. Because the atmosphere is oxygenated and replenished, glazes are not as affected by surrounding ware.
PARAGON KILN SEMINAR
On October 7 - 8, 2005, we are holding a 1-1/2 day Basic Kiln Maintenance Seminar at the Paragon factory in Mesquite, the rodeo capital of Texas. This is about 30 minutes east of Dallas.
The seminar fee is $95. To register, please call 800-876-4328 or email firstname.lastname@example.org. If you are flying and don't want to rent a car while you are here, ask the receptionist about Paragon’s airport pickup schedule.
With best wishes,
Arnold Howard Paragon Industries, L.P. - Better Designed Kilns 2011 South Town East Blvd. Mesquite, TX 75149-1122 Voice: 972-288-7557 & 800-876-4328 / Fax: 972-222-0646 email@example.com / www.paragonweb.com