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Firing Moist Greenware Is Expensive


If the mirror fogs, the greenware is still releasing moisture.

The most expensive way to dry greenware is to heat it in a kiln. The moisture in the clay rusts the kiln, wears out elements faster, wastes electricity, and can cause the ware to explode.

John R. Hohenshelt, who ran Paragon for many years, told me about a customer he met at a trade show who complained that her Paragon kiln fired slowly. Her face flushed with emotion as she spoke. John listened quietly, trying to think of solutions for her.

“And furthermore,” she said, “the kiln drips ugly black water around the case and makes a mess of my floor.” When she mentioned water, John knew the answer: She was using the kiln to dry the greenware on low heat.

During firing, moisture from the greenware turns to steam. As the heated air in the kiln expands, it escapes by pushing its way into the pores of the firebricks. When the moisture reaches the cooler stainless kiln case and galvanized steel base plate, it condenses, causing the water to drip around the kiln. This happens at the beginning of the firing. When the case and base plate become hotter than 212 deg F., the moisture no longer condenses on those surfaces.

The firebricks in a typical 8-sided kiln can absorb approximately 50 pounds of moisture from wet greenware. This reduces the insulating capacity of the firebrick. It also takes a tremendous amount of electrical power to convert water to steam during firing. This slows the kiln to a crawl.

John told the customer to fire her kiln empty overnight on low heat to burn off the moisture in the firebricks. "Load only bone-dry greenware in your kiln from now on,” he advised. “Not only will moist greenware reduce the firing capacity of your kiln, but it will also rust it out.” A few weeks later, she phoned to say the kiln was firing beautifully.

The following suggestions will help you determine when the greenware is dry and what to do if it won't dry completely:

1. Give the greenware enough time to dry--in most areas at least two days. Drying time depends on humidity and the thickness of the clay. In areas of low humidity, such as Tucson, blowing a fan on the greenware can dry it so fast that it has to be turned to avoid cracking from shrinkage. In humid areas, such as New Orleans, the greenware might not ever dry fully.

2. Touch the greenware to the inside of your wrist or to your cheek. If it feels warm, it is usually dry. Dry longer if it feels cool. Note, however, that in humid areas, even damp greenware can feel warm. Greenware feels cool due to evaporation. In high humidity, even damp greenware can feel warm when the moisture in it stops evaporating.

3. If you live in a humid area and the greenware is still moist after an extended drying time, load it into the kiln. Prop the lid about an inch using the kiln's lid prop. (If your kiln does not have a lid prop, use a scrap of firebrick.) Fire to 200° F. slowly. Maintain 200° F. until the greenware is completely dry.

Electronic kilns: Use the Preheat feature in Cone-Fire mode, or program a preheat segment in Ramp-Hold mode.

Manual fire kilns: Turn the bottom switch on low; leave the other switches off. (You may need to vary this switch setting for your kiln.)

If you have a downdraft kiln vent, keep the lid closed and leave the vent on during preheat. It will help prevent the kiln from rusting, because the vent will draw the moisture from the kiln.

The Dead Man Test: Checking for Dryness with a Mirror

(The term “dead man’s test” came from the days of the old west, when a mirror was held under the nose of a presumably dead person to verify that they were actually dead.)

Hold a mirror above the lid or top peephole where hot air from the kiln will move across the mirror's surface. If the mirror fogs, the greenware is still releasing moisture. Keep the lid propped and maintain 200 deg. F. until the mirror no longer fogs. (If you are firing with a downdraft kiln vent, first turn off the vent. Then perform the mirror test.)

For this test to work, the mirror must be at room temperature. The mirror fogs when moisture in the hot air condenses on the cooler mirror. If you hold the mirror too long near the kiln, the mirror will heat up and will no longer fog when moisture hits it. So hold it at the lid for only several seconds at a time.

READER RESPONSE

The Kiln Pointer for last week was testing glass for compatibility.

Sue Williams of Gravois Mills, Missouri wrote, “I have been reading your email articles faithfully all summer. Your article for testing glass fusing compatibility came on the exact day I really needed the information.”

Bohart Brinson of Dallas, Texas wrote, “For polarizing sheets to check glass compatibility, you can also use polarized UV sunglasses and just break them in half. Much cheaper. I got this tip from Richard LaLonde at a frit fusing workshop I attended at his studio on Widbey Island.”

On Friday, September 8 next week, I will visit the Nancy Weisser and Vitrium studios in the Washington, DC area. On Sunday, September 10, I will visit the Rainbow Art Glass Open House in Farmingdale, New Jersey. I would enjoy meeting you if you are in the area.

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 ahoward@paragonweb.com / www.paragonweb.com

PRIVACY NOTICE: Under no circumstance do we share or sell your email address.

Copyright 2006, by Paragon Industries, L.P.



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