Skutt Ceramics Article: Fighting Corrosion

Fighting Corrosion
FOR IMPROVED FIRING RESULTS AND LONGER KILN LIFE

What Causes Corrosion
When clay bodies and glazes are fired in a kiln they emit different amounts and types of gases and vapors. Venting allows these gases and vapors to escape before they can "react" with the kiln our your ware.
The type and magnitude of the reaction depends on the mineral and moisture content of the clay, slip, glaze and environment.
Areas along the coast or near any body of salt water tend to have the most problems because of the high amounts of moisture and salt in the air. The moist salt air can be absorbed into the clay, slip, glaze or by the ware itself while air drying.
When the salt in the ware reaches a certain temperature, between 800 and 1400 degrees F, it is transformed into hydrogen chloride gas and released into the kiln's atmosphere. The hydrogen chloride gas then combines with any water vapor or condensation to form hydrochloric acid which can cause corrosion to occur in the kiln sitter and on the bands, elements and handles.
This is just one example of how minerals and moisture can combine to attack the components of your kiln as well as the finish quality of your ware. Sulfur is another mineral which is commonly found in many clay bodies which will also wreak havoc on your kiln in the form of sulfuric acid.

What You Can Do To Fight It
There are 3 things you can do to help prevent corrosion:
1) Reduce the mineral content.
2) Reduce the moisture.
3) Proper ventilation.

Reducing The Mineral Content
Some clays and glazes have higher concentrations of corrosion causing minerals than others and your distributor may be able to recommend ones with lower concentrations of these minerals. Another source of corrosion causing minerals or elements could be the water used to mix slip and glazes. Extremely hard water may be suspect.

Reducing The Moisture
The most important thing you can do to help prevent corrosion is to avoid firing any pieces that are not completely dried. Electric kilns are not designed be dryers. All pieces should be bone dry before they are placed in the kiln. simple test is to keep an unfired piece around that you know has had sufficient time to dry. Pieces you place in your kiln to fire should feel no cooler than one you have saved.
If you live in a real humid environment try to locate your kiln in an area where it will be protected from the humid air. If this is not possible try to cover the kiln when it is not being used and wipe off any condensation on the kiln jacket.

Venting
Venting can be done with the use of venting equipment or manually by removing peep hole plugs and regulating the air that passes through the lid. most effective and convenient way to vent is to use down draft ventilation equipment.
These vents are mounted underneath the kiln and work by pulling air through small holes in the lid. The air travels downward through the kiln chamber sweeping all the gases and vapors out the slab and through the vent where it is directed outside.
If there is excessive corrosive vapor in the kiln it can cause corrosion vent itself. If it is not possible to reduce the mineral and moisture content to level the venting equipment can handle it will be necessary to ventilate the kiln manually.
If your kiln is equipped with a lid shutoff safety system, the kiln must vented through a hole in the lid and by using the top and bottom peep holes. is done by removing only the top and bottom peep plugs until the kiln reaches 1400 degrees or all the vapors have been emitted. At this point you replace bottom peep plug only. This stops the sweeping movement of outside air and allows the kiln to reach the desired temperature. If you wait too long to replace the bottom peep plug the kiln will not reach temperature uniformly throughout kiln. If you don't leave it out long enough, harmful vapors may be trapped in kiln chamber.

For more information about electric kilns, contact Skutt Ceramic Products, 6441 S.E. Johnson Creek Blvd., Portland, OR 97206-9552; (503) 774-6000; fax (503) 774-7833; e-mail skutt@skutt.com.