Which glazes are suitable for tableware?

We're often asked in which cases glazes are suitable for making dining and drinking ware, or whether a glaze is "food-safe"! Unfortunately, this isn't an easy question to answer, since, on the one hand, the term "food-safe" isn't a term used in ceramic technology, and, on the other hand, a multitude of regulations may need to be considered.

1) What is the difference between the labelling obligation for the glaze raw material and the “toxicity” of a fired glaze?

To avoid any misunderstandings, the question of the "toxicity" of the fired glaze is an application technology issue that has nothing to do with the labelling requirements for our powdered and liquid products according to the Hazardous Substances Ordinance! To point it out: There are glazes whose raw materials require labelling according to the Hazardous Substances Ordinance, but which can nevertheless be used for tableware without any problems (this is because the hazardous substance binds during firing and is subsequently insoluble). On the other hand, there are glazes that are exempt from labelling and yet are not allowed or should not be used for tableware, e.g., due to poor acid and dishwasher resistance or because they exceed solubility limits (see 3).

2) Which properties make a good glaze for tableware use?

Contrary to the legally binding Hazardous Substances Ordinance, we as retailers are not required to provide information on application properties. Nevertheless, we have decided to test the two "core criteria" such as acid resistance and dishwasher resistance to the best of our ability and to list them under the "Specification" tab (where it makes sense). Acid resistance is usually understood to mean purely "visual abrasion" caused by acidic foods (e.g., fruit and lactic acids). Unfortunately, there is no "standard" for testing this, but rather several different approaches. We decided to treat the fired glaze with vinegar essence (25% acetic acid) for 20 hours. The treated and untreated surfaces are then compared. Unfortunately, there is no "yes-no" logic when evaluating these results. This means that there are glazes where the glaze completely peels off after acid treatment (e.g., low-firing gold glazes), and others where only the surface becomes slightly rougher (when scratched with a fingernail). Since critical end customers may view this phenomenon as a defect, we have designated the glaze as "not acid-resistant" in both cases. Depending on the firing curve and desired product properties (e.g., butter dish), in cases of doubt, the ceramist must retest these values under his own conditions.

Dishwasher resistance can be viewed similarly to acid resistance, meaning that after a certain number of dishwashing cycles, the wash water will accelerate surface wear to a greater or lesser extent. The glaze surface will then usually appear slightly iridescent at first and then gradually fade.

In addition to these two core criteria, there are numerous other criteria that define a good tableware glaze. Upon specific request, we can only provide approximate values, as very different results can be expected due to the dependence of these properties on the firing curve. These additional criteria include, for example, the scratch and abrasion resistance of a glaze. While scratch resistance refers to the damage to the surface with a very hard object (e.g., a ceramic knife), abrasion resistance refers to the visible abrasion of an anodized knife (gray abrasion marks that can often be easily removed with vinegar essence). It is in the nature of things that the smoother a glaze is, the less abrasion can be expected. Especially with stoneware glazes, the ceramist can usually influence the smoothness of the glaze by increasing the firing temperature. Another criterion for a good tableware glaze is sufficient edge impact resistance. In addition to low-stress ceramics (the thermal expansion of the body should be slightly higher than that of the glaze), a customized product geometry is particularly important, which the ceramicist can also influence (e.g., rounded plate edges). A final common criterion to consider is microwave resistance. In many cases, poor microwave resistance is not caused by the glaze, but by a porous body. As a result, the body absorbs moisture from the air or the dishwashing process, which heats up more in the microwave than the food sometimes does – in these cases, it is always helpful to produce a dense body (stoneware). Of course, there are also special cases where certain effect glazes (metallic glazes) exhibit metallic deposits on the glaze surface. This is accompanied by sparking in the microwave! Since almost all such glazes are not acid-resistant, special microwave tests are usually unnecessary!

3) What properties must be tested by the the tableware potter?

Now we come to a point for which the potter himself is solely responsible! Since we don't know the production process in the pottery workshop, every potter who makes pottery available to third parties (whether for a fee or not) must have the heavy metal solubility determined in an accredited laboratory using a fired sample. For example, in the case of lead, it could be that the glaze is lead-free (note: we haven't sold lead glazes for years), but many lead glazes were fired in the kiln in the past! Thus, the fumes can migrate to the new pottery with each firing and contaminate it!

Which heavy metals are to be tested and how high the permissible solubility concentrations are depend on the country of destination and the intended purpose (e.g., tableware with motifs specifically aimed at children is sometimes subject to significantly stricter guidelines in some places than tableware for adults). Currently, only the limit values ??for possible migration of lead and cadmium are legally binding for ceramic items sold in Germany – in Austria, for example, the limit values for zinc, antimony, and barium are also legally binding. As medical research progresses, the introduction of additional solubility limits is also being planned in Germany. There are, in fact, testers (at markets and art exhibitions) who anticipate these future legislative developments (even if they have not yet been implemented into German law) and are also calling for further tests for the apossible migration of cobalt, chromium, etc.!

To avoid legal problems, the tableware maker should have the test carried out on a sample piece for each glaze/tableware series. There are various laboratories – for your country you have to check it by yourself. If the limit values ??have been met, the tableware maker can easily create a declaration of conformity (self-declaration) and present it to any customer or inspector upon request! The tableware maker can then also stamp or paint the "EU tableware logo" shown above onto their tableware. What does such a declaration look like? Just do an internet search for "declaration of conformity porcelain" - you'll quickly find what you're looking for!

However, considering paragraph 2), the EU declaration of conformity or the corresponding tableware logo only tells half the story! The tableware maker could focus solely on solubility according to this paragraph 3 (EC 1935/2004 with test guidelines 84/500/EEC including 2005/31/EEC), but still use glazes that wear off when exposed to acidic foods, are not dishwasher-safe, etc. This would quickly lead to complaints from the tableware user!

In this context, we are often asked how toxic the fired glaze is. Unfortunately, we cannot provide any information on this, as we are not physicians. In this case, please refer to the toxicological expert committees! The declaration of conformity can only provide information on whether or not the legal limits are met. For all substances for which there is currently no legal limit (for ceramics), an assessment is extremely difficult. Anyone who has ever conducted an acid test on a small glaze sample and seen that the acid turns green after one day of treatment (e.g., copper) is usually in a position to form their own opinion on this topic.

4) Which alternatives are possible instead of an external testing?

Unfortunately, this statement cannot be answered with legal certainty. For hobby potters who produce their pottery exclusively for their own use, we recommend using acid-resistant and cadmium-free glazes. If it can also be ruled out that an old kiln was used that may have previously fired lead glazes, then the hobby potter should be on the safe side.

Sometimes, a customers claims a glaze formulation which don't contains heavy metals (this can be required by some local regulations e.g. for schools) This approach may be well-intentioned, but it nevertheless raises the question of how potter are supposed to know whether heavy metals are present in the formulation. While they have the option of explicitly requesting a current safety data sheet (SDS) for the glaze raw material, it only lists heavy metal compounds known to be hazardous. All other heavy metal compounds without hazardous effects, such as tin oxide or cadmium inclusion pigments, are generally not listed there!!

As the above explanations make clear, the entire topic is quite complex and subject to constant changes by lawmakers. While simply labeling a glaze with a single "suitable for tableware" logo would make selecting a suitable glaze much easier for laypeople, such an approach (even if some competitors do so) is unfortunately not compatible with German or European law.