How the Downdraft Vent Affects Glazes
Penny Hosler of Sequim, Washington asked a question about a recent Kiln Pointer I sent out.
I had said that if you place low-fire greenware and glazed ware in the same load, the glazed pieces should be in the bottom of the kiln and the greenware should be on shelves above.
I also stated, “Vent the lid with the lid prop for about an hour. Red glazes should be placed in the top of the kiln for extra venting. Separate clear glazes from colors. Load clear glazes in the bottom of the kiln and colors above them.”
Penny said, “I have an EnviroVent under the kiln floor, so I have always assumed that the greenware should be at the bottom so the gases are sucked out first and don't contaminate my glazed pieces above them. Does having the heat and oxygen traveling downwards make a difference?”
The Skutt EnviroVent that Penny mentioned is a down-draft kiln vent. The Orton KilnVent is similar to the Skutt. This type of vent draws air from the bottom of the kiln through small holes drilled into the firebrick bottom. Makeup air enters the kiln through holes drilled into the lid. The air removed from the kiln is vented outside through an aluminum dryer duct.
My Kiln Pointer recommending that glazed ware and greenware be loaded separately in the kiln was for kilns without the down-draft vent. 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.
I asked Penny about her experience with the down-draft vent. She replied, “I've had EnviroVents for about eight years with no problems whatsoever. The heating and cooling are very even - no hot spots, no difference in glaze appearance from one side of the pot to the other, and they really make a difference when I'm firing thick pieces. My thick, carved tiles used to crack if they weren't precisely in the middle of the shelf, but with the vents running it doesn't seem to matter where I put them.”
Thanks, Penny, for responding to the Kiln Pointer.
With best wishes,
Arnold Howard Paragon Industries, Inc. firstname.lastname@example.org