Monday, December 10, 2012

December 3rd Hint of the Week

Mixing or layering glazes.

When you layer two glazes, you create a new, third glaze. Sometimes the components of the glazes you layer mix together nicely, and give you good results. Sometimes, something in one glaze may not play well with the ingredients of the other glaze . . . resulting in something unexpected either good or bad. Once you layer two glazes, the chemistry has completely changed, and the safety labels don't apply any more. So layering clear or a food safe glaze over a non-food safe glaze does not necessarily make it food safe. Glazes are not stiff like underglazes, and cannot be covered over by another glaze. When you layer, they mix. Unless you know and analyze the recipes (chemistry) for both glazes or have the combination tested by a lab, you do not know what the food safety maybe.

The two materials that are proven toxic at this time and the only two regulated by law in the USA are lead and cadmium. Lead is an active flux used to make glazes flow better at low temperatures. Cadmium is used primarily to create bright orange and red colors. The generally toxic glaze materials that are prohibited in many community studios and guilds are of course lead and cadmium, but often barium too. However, there are materials, other than lead, cadmium and barium which may be toxic, but there is not enough evidence that they are unsafe at this time, so they are not regulated. Many of these materials are safe in low doses (for example, nickel, barium, selenium and cobalt), but toxic in high doses. So reducing leaching as much as possible is always a good idea. Any time you begin to layer glazes, you are pretty much on your own. Any testing that the manufacturer did will not be applicable. If you don't use any glazes with lead, cadmium or barium as ingredients, you are pretty safe. We have erred on the side of caution at the guild and all of our glazes that have proven to be or suspected of being toxic are labeled not food safe. A few that have not been tested, but may be a problem are labeled “May not be food safe.” Frog Pond Pottery has information on some glazes that have been tested. (The information used here is taken from Article(s) in Ceramics Arts Daily, the Ceramics Community Forum and the Big Ceramic Store.) If you are interested in what ingredients are in our glazes, ask Sarah, Steve P., Doug, Jim, Gene or others who have been making and mixing our glazes, to show you the recipes.

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