Dinnerware Safe

In pottery, the terms dinnerware safe and food safe, are not the same. Perhaps the best way to understand the former is to consider what the opposite might be.

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Dinnerware safety of glazes means something different in industry than it does in pottery or hobby ceramics. For the former, it is about conservative formulations that work well in high-production machinery and kilns, that produce glassy smooth and easy-to-clean surfaces. It is about commercial testing labs with well-defined procedures to flag leaching, crazing, pitting, staining, etc. In pottery, suppliers use the term more loosely. No potters attach "Dinnerware Safe" stickers to their ware.

Dinnerware safe is not the same as the official term Foodsafe: The latter is regulated by the FDA and regulates the release of lead or cadmium from glazes. The regulation does not require that glazes be lead or cadmium-free, just that they do not leach them. In pottery, we tend to think of food safety as, at a minimum, not containing lead. But we do use stains containing all sorts of heavy metals. And the commercial glazes we buy are full of them.

While there is no industry standard or official symbol that signifies the dinnerware safety of a bottle of commercial glaze sold at ceramic suppliers, some manufacturers have designed their own symbol and labelling. On their websites they explain what these mean. Typically, the term refers to chemical durability and freedom from surface micro-textures that could trap bacteria or make it difficult to clean. The term is sufficiently clear and companies test it with enough confidence that they even promise to back any customer that is targeted by litigation. That being said, it is difficult to imagine that they could indemnify users given all the ways their products can be misused and all the issues surrounding their interactions when layered.

These commercial glazes don't leach on their own, but when layered like this pronounced leaching, the white, has occurred with lemon juice overnight

So this all means that it is up to you, the potter, to test your ware. The term "dinnerware safe" embodies the ability of a glaze to withstand exposure to acidic and base liquids is a basic expectation (often referred to as surface durability). Coffee, acidic fruits and coke can degrade surfaces - we can be sure glaze manufacturers test for these exact liquids. That means you can do it also (e.g. the GLLE test).

Added reason for doing your own testing comes with the realization that the "Non-Toxic" seal on jars of glaze, which refers to ASTM D-4236, does not likely mean what you think.

The term Dinnerware Safe can also be understood by considering its opposite. For example, to make a glaze that leaches heavy metal one would use a high percentage (e.g. 20%+) of heavy metal oxide (or even stain), make it highly melt fluid (having inadequate SiO₂ glass former and Al₂O₃ stabilizer) and cool it slowly in the firing to crystallize out the metals in concentration. Further destabilization could be accomplished by layering all manner of different products, especially ones containing copper, to produce interactions.

Potters who make their own glazes are the ones who can speak with greatest assurance about their ware. Glazes that do not contain any potentially toxic ingredients are no danger, even if they leach (from a liability standpoint). This is because potters know the recipes of the glazes they mix. As noted above, practical testing for leaching is easy to do. And testing for crazing and surface textures that resist cleaning is equally easy.

Related Information

Feeling good about the glazes we use on functional surfaces

This picture has its own page with more detail, click here to see it.

The chemistry of the popular cone 6 Val's Turquoise glaze surprised a customer recently (when she was comparing the Gerstley Borate and Gillespie Borate versions). Here is what she said initially about using this on food surfaces:

"I’ve noticed the alumina is below the suggested limits. I have read that too low alumina can cause a glaze to be more prone to leaching, especially with the addition of copper carb. I’ve soaked samples in vinegar and lemon juice. I’ve also run pieces through the dishwasher several times with no visible changes."

Her concerns about this chemistry anomaly are valid. This glaze has no clay, that is very strange, clay sources vital Al₂O₃ that imparts durability. The little Al₂O₃ there is being sourced by lots of feldspar, that is a red light for crazing when little MgO is present (from talc or dolomite). Of course, even if this glaze does leach, the copper is not going to hurt anyone. However, if another heavy metal pigment were added the Cu could destabilize it enough to make toxicity a concern.

Her final comment was this: "I never want a customer coming back to me with a faulty pot. I just want to do everything within my power to make the best ware that I can - which includes a durable, reliable and functional glaze."

Links

URLs	https://www.astm.org/d4236-94r21.html ASTM D-4236 - Standard Practice for Labeling Art Materials for Chronic Health Hazards - It is not what you think! A standard that "applies exclusively to art materials packaged in sizes intended for individual users of any age or those participating in a small group". It "concerns those chronic health hazards known to be associated with a product or product component(s) when it is present in a physical form, volume, or concentration that in the opinion of a toxicologist has the potential to produce a chronic adverse health effect". The word "toxicity" is not mentioned on the page nor any methods for determining such. Furthermore, the labelling refers to hazards to which the potter is exposed in applying the glaze to the ware, long term, in small hobby quantities. IT DOES NOT ADDRESS leaching hazards the ware presents to users of the pottery. Even then, the standard states that "it is the RESPONSIBILITY OF THE USER ... to establish appropriate safety, health, and environmental practices ... based upon knowledge that exists in the scientific and medical communities". It also admits that "since knowledge about chronic health hazards is incomplete and warnings cannot cover all uses of any product, it is not possible for precautionary labelling to ensure completely safe use of an art product." It is interesting that one manufacturer displays this warning on pages relating to dipping glazes and accessory products (which are used by manufacturers): "Safety Warning: Tableware producers must test all finished ware to establish dinnerware status, due to possible variations in firing temperature and contamination." This warning does not appear on brushing glazes, even the reactive metal-saturated ones that potters and hobbyists use!
Glossary	Food Safe Be skeptical of claims of food safety from potters who cannot explain or demonstrate why. Investigate the basis of manufacturer claims and labelling and the actual use to which their products are put.
Tests	Glaze Leaching Test Simple tests to evaluate the stability of a ceramic or pottery glaze against leaching metals in food or drink.
Tests	300F:Ice Water Crazing Test Ceramic glazes that do not fit the body often do not craze until later. This progressively stresses the fit until failure point, thus giving it a score

By Tony Hansen
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