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Re: [teacherartexchange] reglazing mugs (kiln vapors)

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From: Marvin Bartel (marvinpb_at_TeacherArtExchange)
Date: Sat Mar 17 2007 - 11:25:58 PDT


Mary H. wrote:

>Reading about glazing and fumes--is it normal to have some fumes--bad smelling--when doing a glaze firing? I was firing a load of clay bowls that were glazed with concept underglazes--at cone 05. I have a Skutt kiln with the envirovent system. The smell was really bad. ???
>
Mary H.

I stand corrected. Not all firing odors are toxic. I was referring to aromatic hydrocarbons used as organic solvents (in china paints and lusters). Some other chemicals can also produce toxic vapors - some without any noticeable odor. I think the companies that sell school supplies are careful not to use hazardous materials (or at least include warnings if they do), but we need be vigilant. I am not familiar with "concept" underglaze and could not say if the underglaze you used has anything that is both volatile and toxic in it. Perhaps you should ask the company that sells the product to tell you the source of the firing odor. Tell them at what firing temperature the odor is worst, and try to describe nature of the smell. Also be sure you do not get the same smell from the clay without the underglaze on it. Many clay bodies contain enough sulphur to make a significant haze in the room toward the end of the firing if not well exhausted. You can even taste it. This
 may not be as harmful as some fumes, but I certainly would not want to breath much of it. I have seen glass window panes above kilns that are etched by the kiln acid fumes so badly that they could be used for bathroom privacy glass.

The envirovent systems and other vent systems need to be routinely checked before and during each firing and repaired if any odors are noticed.

1. Look and be sure the openings are not clogged where the system draws fumes from the kiln. Have a piece of coat hanger wire handy to check before loading the kiln to see that the hole(s) is (are) open in the kiln bottom.

2. Be sure the lid and the kiln seams are nice and snug. If you see gaps on an old kiln, seal it with some kiln fiber insulation or at least stuff some clay in the openings. You make wadding with clay and sand that can be used to seal a hot kiln.

3. Be sure peep holes and thermocouple holes are tightly stoppered. Check under the kiln during firing to see if it is sealed tight against the kiln when the kiln is hot. Heat warps the kiln and can leave a gap. Some newer vents are spring loaded to keep pushing them up against the kiln bottom.

To test an under the kiln vent system, close the kiln and close all the peep holes. Close the top peep hole with some clay so it leave a half inch hole. Hold a burning match near this hole. If the flame is drawn toward the hole, it tells you that the vent fan is creating a negative pressure in the kiln and it will be drawing fumes out. If this works on a cold kiln and not on a hot kiln, the seal my be bad at the bottom between the kiln and the vent when the kiln warps from heat.

4. Finally, check the outgoing tubing to be sure there are no leaks at the joints or otherwise. If you find any breaks or leaky joints, seal them with high temp silicone caulking or glue. I was called to one very smelly school in mid winter. The roof vent was covered with snow and the kiln odor was blowing into the space above the ceiling tile and passing on to the hallway and adjoining rooms in the building.

Many teachers have the kiln finish off at night so the air is clear by morning. If it goes off about two hours before you come in the morning, the residue should be clear if you have any kind of exhaust working. Always check first thing in the morning to be sure it went off. I have seen too many things go wrong with automatic kiln controls to tell all the stories here. If firing on Friday night, it means a phone call or a trip in to check it on Saturday morning -- not Monday.

  Marvin (firing kilns for 47 years and still breathing fine)
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>>AVOID MOST OVERGLAZES
>>Pottery is often decorated using overglazes, china paints, and lusters. However, to make them easy to brush on, many of these materials are suspended in TOXIC aromatic hydrocarbons. I use lusters, but only when I can sit outside with the wind from my back to carry the vapors away. If you can smell it, your brain is being effected. It is not safe to use. Firing also releases the toxins.

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