1359 Cartwright St., Granville Island Vancouver, B.C. V6aH 3R7 (G04) 683-9629 Potters Guild of British Columbia NEWSLETTER OCTOBER 1989 ISSN 6319 812x CHLORINE AND SALT GLAZE FIRING I need to respond to an article in the June Potters’ Guild newsletter which left the reader with the false impression that salt firing harms the ozone layer. Although erroneous In specific example, the intent of the article's author is a good one: that we should stop for a minute and take a look at what we do In our lives and our work and consider what effect it has on the world around us. In traditional saltglazing (as it has been done over the last 900 years or $0), common salt is thrown into the kiln toward the peak of the firing. The high tempera- ture causes the salt to disassociate into sodium and chlorine . The sodium then combines with the vitre- ous Clay forming a sodium alumino-silicate glaze on the surface of the ware, The chlorine vapor leaves the kiln along with carbon and water vapor. Chlorine is a very heavy gas - heavier than air, When the gas cools it settles to the ground and dissipates (within fifteen minutes, according to Charlies Hendricks, Professor of Engineering at the University of Illinois). Ovone is a ight gas which rises to the highest levels of the atmosphere to form a protective shield, reflecting back much of the sun's ultra violet rays. Chlorine gas in its natural state cannot come in contact with the ozone layer. Chioro-fluoro-carbons (C.F.C,s) are a commercially manufactured family of gases which chemically com- bine the heavy chlorine with the much lighter fluoro- carbon. C.F.C.s are so insidious because when re- leased into the atmosphere, they rise, the fluoro- carbon carrying the chlorine all the way up to the ozone layer where the chlorine will then destroy ozone. But, the chlorine cannot get there on Its own. Chlorine gas does pose other problems, In high cancentralions and in confined spaces it can be toxic, Salt glazing should only be done in well-ventilated areas and not in an urban setting or where neighbors would be bothered. The first effects of exposure to an over-concentration of chlorine is a reddening of the eyes and an irritation of the mucous membranes (I've not yet experienced that around a salt kiln, though I do at the swimming pool. Yes, that's chlorine gas you smell at the pool), Chlorine is also released when you have a beach fire or burn beach logs or even cremate a human body (we contain chlorine as does the ocean around us). The effects of chlorine from salt glazing are more short term and localized, but that ts not true for other things close to home for us, Qur cars emit several different noxious gases which accurmulate in the atmosphere, contributing to long-term problems such as the “green house effect”, and global warming. Also of note is that firing fuel-burning kilns in heavy reduction releases carbon dioxide and monoxide, adding to this problem. Several things can be done to minimize chlorine release. One is not to use salt. There are other sources of sodium, such as soda ash (sodium carbonate) or sodium bicarbonate. These materials don't volatize as readily as salt and tend to cake up in the fire box, but I have found that if added in smaller quantities and more frequently, they work well. The addition of a small amount of borax helps. The effect is very handsome, though not exactly the same as salt, For my “standard” salt glaze at Cone 9, I use about 2/3 soda ash and 1/3 salt and a litle borax. Five or six years ago, | was getting some very interesting effects at Cone 01 using just soda ash and borax. In low temperature salt /sagar firings, very small amounts of salt seem to produce the most beautiful surfaces. I shudder when | think of the “old days” salt glazing back in Illinois in the GO's, The kilns were crude. At Cone 9 you could see between all the bricks and at night it looked like a Jack-o-lantern. We just kept dumping In the salt till the pots finally looked right. The bricks around the salt ports were dissolving, and flowed in great undulations down the inner walls of the kiln. Today we are a little smarter. A salt kiln should be tightly built. The inner walls , the kiln shelves, and the posts should be coated with a kiln wash of 2/3 hydrate of alumina and 1/3 kaolin. This will protect the bricks and lengthen the life of the kiln . as well as greatly reduce the amount of salt needed. This wash repels soda vapor so that only the pots are receptive. After introducing the salt or soda materials, the damper Continued on Page 2...