Metal leaching from ceramic glazes: Lab report example and a better way

This lab is certified by the US Department of Environmental Protection (DEP) for drinking water and wastewater analysis. They also provide pottery glaze leaching analyses (an acid solution is kept in contact with the glaze and then analysed for trace levels of specific metals). Each suspected metal to be tested for entails a separate charge ($30-60 in this case, could be less for you). That means that testing one glaze for several metals could cost $200. How to make sense of these numbers? Google the term: "heavy metals drinking water standards", and click "Images" to find charts with lots of data. Searching pages for this term will find books having detailed sections on each of the metals. Typically you are only interested in one metal in a specific glaze (often cobalt or manganese). There are ways to sleep better (about the likelihood your glazes are leaching metals) if you cannot do this: Do a simple GLLE test. And avoid online trafficking in hazardous recipes. Better to find a quality base glaze (matte and transparent) that works well on your clay body. Then add colorants, opacifiers and variegators; but doing so in a conservative manner. And use a liner glaze on food surfaces.

Related Pictures

Glaze recipes online waiting for a victim to try them!

A pile of printed recipes to try, but few are likely to work

You found some recipes. Their photos looked great, you bought $500 of materials to try them, but none worked! Why? Consider these recipes. Many have 50+% feldspar/Cornwall/nepheline (with little dolomite or talc to counteract their high thermal expansion, they will craze). Many are high in Gerstley Borate (it will turn the slurry into a bucket of jelly, cause crawling). Others waste high percentages of expensive tin, lithium and cobalt in crappy base recipes. Metal carbonates in some encourage blistering. Some melt too much and run onto the kiln shelf. Some contain almost no clay (they will settle like a rock in the bucket). A better way? Find, or develop, fritted, stable base transparent glossy and matte base recipes that fit your body, have good slurry properties, resist leaching and cutlery marking. Identify the mechanisms (colorants, opacifiers and variegators) in a recipe you want to try and transplant these into your own base (or mix of bases). And use stains for color (instead of metal oxides).

Want bright orange? Use a stain in your own base transparent recipe.

Fired glaze tiles showing an orange promise, what actually came out and a better way

Orange is a very difficult color in ceramics. Inclusion stains are the only reliable method, they universally used in industry. But you could ignore that and try a bunch of recipes online. When they are presented on flashy web pages they can look tantalizing. But beware! Are the exotic materials you need to buy worth it. Will it actually fire orange? Will it craze or run or blister or leach or cutlery mark or crawl or settle like a rock in the bucket? It is much better to put an orange encapsulated stain into a transparent glaze you already know works on your clay. Then just experiment with percentage to get the color you want. Or, how about trying a premixed orange at low fire? Ware can be amazingly functional at low temperatures (e.g. cone 03-02) and bright colours labelled for cone 06 mostly work fine in that range.

Links

URLs	https://www.dep.pa.gov/Citizens/My-Water/PrivateWells/Pages/Water-Testing.aspx USA DEP water testing info page
URLs	https://www.who.int/water_sanitation_health/publications/2011/9789241548151_ch12.pdf Chemical contaminants in drinking water
URLs	http://bsclab.com Brandywine Scientific Lab Website
Glossary	Leaching Ceramic glazes can leach heavy metals into food and drink. This subject is not complex, there are many things anyone can do to deal with this issue
Glossary	Liner Glaze Liner-glazing is a way to assure that your ware has a durable and leach resistant surface. It also signals to customers that you care about this.
Articles	Are Your Glazes Food Safe or are They Leachable? Many potters do not think about leaching, but times are changing. What is the chemistry of stability? There are simple ways to check for leaching, and fix crazing.
Articles	Attack on Glass: Corrosion Attack Mechanisms Max Richens outlines the various mechanisms by which acids and bases can dissolve glass and glazes. He provides some information on stabilizing glazes against attack.
Articles	Having Your Glaze Tested for Toxic Metal Release Having Your Glaze Tested for Metal Release
Articles	Concentrate on One Good Glaze It is better to understand and have control of one good base glaze than be at the mercy of dozens of imported recipes that do not work. There is a lot more to being a good glaze than fired appearance.
Tests	Glaze Leaching Test Simple tests to evaluate the stability of a ceramic or pottery glaze against leaching metals in food or drink.

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