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.

Key phrases linking here: unbalanced chemistry, food safety, food-safety, food safe, food-safe - Learn more

Details

In recent years potters and small manufacturers have become aware (or have been forced to become aware) that ceramics and pottery are not as inert as they once thought. A variety of potential health impacts exist to users of ware they make. These include flaking off of glaze chips (that could be ingested), harbouring of bacteria (in glaze cracks and pores of bodies or engobes), possible fracture or cracking of ware when suddenly heated (e.g. by hot coffee), superheating of ware in microwave ovens (presenting a burn hazard) and leaching of metals or other elements from the glaze surface (by hot, acidic or base liquids).

Perhaps the most public and serious of the issues is leaching. There is need for skepticism. Glaze recipes spread online often lose any non-functional labels they may have had. Potters who produce ware having a transparent, white or iron-colored liner on food surfaces are those most likely to be conscious of this topic. Those producing ware whose food surfaces are brilliantly colored (without a transparent overglaze), very matte, heavily crystallized or overly melted (and thus running) are the most likely to be unaware of this topic.

Although a ceramic glaze is akin to a glass, it actually can dissolve over time when in contact with liquids if its chemistry is not balanced and stable. This is not really a problem if the glaze is a white or transparent and does not contain any elements that are harmful (e.g. feldspars, kaolin, silica, calcium carbonate, talc). But if it contains heavy metal oxide colorants, barium, lithium or lead these present a problem. It is possible that a glaze containing all of these can be stable and will not dissolve enough to be a hazard. But if the glaze does not have a balanced chemistry it certainly can leach metals into food and drink.

What does balanced mean? It means normal. Consider the some obvious questions:

-Every potter knows that 0.5 or 1% cobalt oxide can produce a strong blue in almost any glaze. What if a recipe contains 5% cobalt? This is a red light, the extra cobalt could very well precipitate from the melt during cooling and crystallize rather than form part of the glaze matrix.

-Does the glaze form crystals on its surface during firing? Crystals in glazes are often unstable and leach.

-We expect to see 3% lithium carbonate in a recipe. What if there is 10%? What if there is 30% barium instead of 5%. Or 20% manganese dioxide instead of 5%? These are obvious issues and many a glaze fails this simple appraisal.

-What if there is only 1% cobalt in a glaze base that melts alot (and runs off the ware if applied too thick)? Likely that glaze does not contain enough SiO₂ and Al₂O₃ (the former is the glass and the latter the stabilizer, all functional glazes must have adequate levels of these).

-What if the glaze is a matte and does not even melt properly? Any colorants in it are going to leach.

-What if a glaze contains 60% feldspar instead of the normal 30%? Feldspar is not hazardous, but it is the base of the glaze into which a hazardous metal oxides can be added to get color. If the host glaze into which the colors are added is not balanced they are likely to leach.

A special class of glaze colorants are stains. These powders have been compounded by mixing metal oxides with stabilizers and firing them at temperatures in excess of what a normal glaze would be fired to. The cooled sintered material is then ground into a powder. In theory, these stain particles do not dissolve into the glaze melt so safety issues are mitigated. However some stains produce very bright reds, yellows and oranges, they are very useful because these colors are difficult or impossible to achieve using metal oxides alone. These stains are also much less stable than others and have lower temperature limits. When used above their recommended temperature some of the toxic metals they contain (e.g. cadmium) will dissolve into the glaze melt and become leaching hazards. Cone 6, for example, can be beyond the range of common encapsulated stains.

Balance is also sought in the chemistry of a recipe. Insight-live.com, for example, can display the oxide chemistry of recipes you enter. But caution is needed. Many people say that safe glazes have a specific chemistry profile and one can recognize predisposition to leaching by simply viewing a chemical formula (to see if certain oxides fall outside limits). There is only some truth to this. For example, we might expect to see 4.0 SiO₂ in a middle-temperature glaze, but if a recipe only has 1.5 there is a good chance a stable glass has not been formed. Of course, high percentages of oxides like BaO, MnO, PbO, Li₂O would be potential issues, but the high percentages of their source materials would be obvious in the recipe anyway. The point is that absolute limits for all oxides (to prevent leaching) cannot be established, the chemistry is much more complex than that. Besides, this would drastically narrow the range of acceptable compounds from which to make glazes. Remember, leaching is easy to test (see the links on this page) and anyone can do it. Thus, a better application of chemistry is to give direction for change if a glaze fails a leaching test. Still, there is value in being aware of obvious chemistry irregularities (against common base recipes for the same temperature). You will find articles and videos on this site addressing this.

Crazing is often seen by potters as a visual asset, but from a safety point of view, crazed ware is unacceptable. Even hobby glaze manufacturers regard it as a surface defect (or texture) that can potentially trap bacteria. The crazing severely weakens ware and provides sites where water can enter and saturate the porosity of the body below. In addition to harboring bacteria the cracks provide weakness points from where thermal shock can propagate them deeper into the ware. Simple awareness of crazing is a strong measure anyone can take to improve safety of ware they produce. Crazing happens when glazes are under tension, stretched on to ware. This is fixed by adjusting the recipe to lower the thermal expansion (many articles and videos on this site show how to do this).

Simple methods of physical testing are within the means of everyone. For leaching, a slice of lemon left against the glaze (or some pure lemon juice) overnight will reveal differences in color or surface character if leaching is occurring. Simple visual inspection will reveal crazing or shivering. If a piece of functional ware is used and put in the dishwasher every day crazing will almost certain appear if the glaze is under tension. An ice-water and boiling-water immersion test is even better to reveal if these problems will happen over time.