|Monthly Tech-Tip |
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
Ceramic glazes are not as universally inert and stable as many people think. All are slightly-soluble and will thus leach to some extent, even if minute, into food and liquids they come into contact with. However some glazes are dramatically more soluble than others. Glazes can be leached by acids and bases. Highly susceptible ones (e.g. metallics) will react to overnight leaching tests by changing color (different colors for bases and acids).
It is common for leaching glazes to suffer from more than one risk factor. It is also common to find leaching glazes in use by practitioners and educators who have cultivated a deliberate lack of awareness on this subject. The subject of leaching and glaze safety spans a range of concerns from technical (e.g. glaze chemistry) to simple common sense (e.g. contains high percentages of metal oxides or is not melting well). Obviously, there is more concern if the glaze will be exposed to a hot acidic or caustic liquid for lengthy periods than if the vessel will simply be used to serve cold food. There is a need for common sense about recipes, we embody this by the term “limit recipe”.
At first it might seem logical to send your glazes for testing at a lab. However is this really necessary? Suppose you are using a liner glaze (that you make, without colorants) and it contains no materials that could release barium, lead or lithium. And, suppose it is melting well, not crazing and wears well. Why bother having it tested for leaching?
What if you are using a commercial transparent or white liner glaze, one with an unknown recipe? The chances are very good it too is safe. But what if it is brightly colored? The bottle says it is safe but you know it contains heavy metals (because that is the only way to make bright colors)! How confident can you be it is really safe? It is true that most commercial glazes employ ceramic stains, not raw metal oxides and carbonates. It is true that stains are certainly more stable. But what if a glaze requires 20% or even more stain to achieve the color, how is it possible that it will not leach? You might find all of this worrying enough to avoid these on food surfaces.
Potters have become increasingly aware of potential leaching in ceramic glazes. So have consumers. Assuring that glazes are not leaching metals is about having a comfort zone, a feeling that you are making ware that has a margin of safety. Having a good base transparent and matte recipe and adding your own stains, opacifiers and variegators is a way to achieve this. And what if the only way to get a certain color is by adding 20% stain? You are likely not going to do that! However being able to achieve good color with only 5% stain is evidence of having done your best. Confirmation with some simple leaching tests will give you a good feeling about the ware you are making.
Chemistry, a Way to Get More Insight
Glaze chemistry enables you to look at a glaze as a collection of oxides rather than just a recipe of materials (Insight-live, for example, automatically calculates and displays the oxide formula of glaze recipes). Fired glazes are what they are because of their chemistry more than anything else. Imbalances in the oxide formula can help explain a tendency to leach. Likewise a balanced oxide formula will suggest stability and the ability to host a colorant. The term 'balance' often simply refers to a glaze having oxide quantities in keeping with typical target (or limit) formulas for the temperature range. Balance almost always means there is adequate SiO2 and Al2O3, it almost never means it is flux-saturated and of high melt fluidity. Limit formulas are about what mixes of oxides melt well and form a good glass (not whether they are food safe). That being said, if a glaze melts well and forms a good glass it is less likely to be leachable.
As noted, glaze bases can be inherently safe simply because they are made from nothing but non-hazardous materials. Thus, even if they do not melt sufficiently, it does not matter. The opposite is also the case, glaze bases can be inherently leachable when they are over melting (most often, as already noted, when there is inadequate SiO2 or Al2O3). However, in either case, leaching is almost guaranteed if metal oxides are added. As already noted, stable glaze bases can even be made unstable by additions of excessive percentages of colorant.
While it is true that a recipe containing significant lead, barium or metal oxide, can, by careful engineering of the oxide balance and controlled firing, be made safe against leaching, this is not typically within the reach fo the average potter or small manufacturer. Speed of kiln cooling is another factor: It is common for metal stained glazes to crystallize if cooled slowly, those crystals can be leachable.
The best prevention for leaching on food surfaces is to use non-coloured glazes of known composition. Simple leach-testing (with acids and bases) is also a no-brainer. And enough awareness of chemistry and materials to recognize a glaze that is likely to be stable.
This recipe melts to such a fluid glass because of its high sodium and lithium content coupled with low silica levels. Reactive glazes like this produce interesting visuals but these come at a cost that is more than just the difficulty in firing. Recipes like this often calculate to an extremely high thermal expansion. That means that not only will this form a lake in the bottom of ware when used on the inside, but the food surfaces will craze badly. The low silica will also contribute to leaching of the lithium and any colorants present.
There is a direct relationship between the way ceramic glazes fire and their chemistry. Wrapping your mind around that and overcome your aversion to chemistry is a key to getting control of your glazes. You can fix problems like crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering. Substitute frits or incorporate better, cheaper materials, replace no-longer-available ones (all while maintaining the same chemistry). Adjust melting temperature, gloss, surface character, color. Identify weaknesses in glazes to avoid problems. Create and optimize base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation. Create glazes from scratch and use your own native materials in the highest possible percentage.
This happens. They are glossy, but lack thickness and body. They are also prone to boron blue clouding (micro crystallization that occurs because low alumina melts crystallize much more readily on cooling). Another problem is lack of resistance to wear and to leaching (sufficient Al2O3 in the chemistry is essential to producing a strong and durable glass). This is a good example of the need to see a glaze not just as a recipe but as a chemical formula of oxides. The latter view enables us to compare it with other common recipes and the very low Al2O3 is immediately evident. Another problem: Low clay content (this has only 7.5% kaolin) creates a slurry that is difficult to use and quickly settles hard in the bucket.
The original cone 6 recipe, WCB, fires to a beautiful brilliant deep blue green (shown in column 2 of this Insight-live screen-shot). But it is crazing and settling badly in the bucket. The crazing is because of high KNaO (potassium and sodium from the high feldspar). The settling is because there is almost no clay. Adjustment 1 (column 3) eliminates the feldspar and sources Al2O3 from kaolin and KNaO from Frit 3110. The chemistry of the new chemistry is very close. To make that happen the amounts of other materials had to be juggled (you can click on any material to see what oxides it contributes). But the fired test reveals that this one, although very similar, is melting more (because the frit releases its oxide more readily than feldspar). Adjustment 2 (column 4) proposes a 10-part silica addition (to supply more SiO2). SiO2 is the glass former, the more a glaze will accept, the better. Silica is refractory so the glaze will run less. It will also fire more durable and be more resistant to leaching.
This is a cone 04 terra cotta piece. The coffee stain cannot be removed because the coffee has also leached off the surface gloss. Glazes are glass. Glass is leachable if the chemistry is out-of-balance. So is this glaze poisoning the user? No, it has an insurance policy. It is transparent, made from a mix of two frits (Ferro 3124, 3134) plus kaolin and silica. The recipe contains no heavy metal colorants or pigments and no toxic fluxes like lithium or barium. But the body is red, how can the glaze be white? A white porcelain-like engobe layer was applied at the leather hard stage and it was clear-glazed after bisque. The fix: The predominant frit, 3134, has almost no Al2O3 (the oxide most important in producing durability). So I increased the Al2O3 (doing the chemistry in my Insight-live.com account). I also began firing one cone higher, at cone 03.
This lab is certified by the US Department of Environmental Protection (DEP) for drinking water and waste water analysis. They also provide pottery glaze leaching analyses (an acid solution is kept in contact with the glaze 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 on 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 the 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.
Encapsulated stains can reach their limits in a glaze host at cone six and begin to dissolve and decompose. That is an obvious problem on a food surface. But in a less fluid underglaze they can survive longer. The bright orange color on the left was likely done this way. The transparent over glaze is isolating it from any contact with food or drink. However people are more wary of the risk of glazes leaching heavy metals and having bright colours on food surfaces may not send the right message.
A closeup of a glossy Cone 6 glaze having 4% added copper carbonate. The bottom section has leached in lemon juice after 24 hours. This photo has been adjusted to spread the color gamut to highlight the difference. The leached section is now matte.
Three cone 6 commercial bottled glazes have been layered. The mug was filled with lemon juice over night. The white areas on the blue and rust areas on the brown have leached! Why? Glazes need high melt fluidity to produce reactive surfaces like this. While such are normally subject to leaching, the manufacturers were able to tune the chemistry of each to make them resistant. But the overlaps mingle well (because of the fluidity), they are new chemistries, less stable ones. What is leaching? Cobalt! Not good. What else? We do not know, these recipes are secret. It is much better to make your own transparent or white liner glaze. Not only can you pour-apply it and get very even coverage, but you know the recipe, have control, can adjust to fit your body.
These cone 6 porcelain mugs are hybrid. Three coats of a commercial glaze painted on outside (Amaco PC-30) and my own liner glaze poured in and out on the inside (G2926B). When commercial glazes (made by one company) fit a stoneware or porcelain (made by another company), without crazing or shivering, it is purely an accident! So use them on the outside. But for inside food surfaces make or mix your own. When you know the recipe you can tune the thermal expansion. And the degree of melt. And the application properties. And you can use quality materials to source a balanced chemistry. The place to start understanding your glazes, organize testing and development and document everything is an account at Insight-live.com.
Wrong. It is the one on the right. While the copper looks so much better in that fluid one on the left, that melt mobility comes at a cost: blisters. As a clear glaze it is no glossier than the other one, but it runs into thicker zones at the bottom and they blister. This is because the high mobility coupled with the surface tension blows bubbles as gases of decomposition travel through (in a normal cooling kiln they break low enough that mobility is insufficient to heal them). The fired glass in the one on the left is also not as hard, it will be more leachable, it will also craze more easily and be more susceptible to boron-blue devritrification. But as a green? Yes it is better.
These are four cone 6 glazes of diverse chemistry. They have varying melt fluidities. They are soaked (half way up) in lemon juice over night. None show any evidence of surface changes. All contain 2% copper carbonate. If the copper was increased, especially to the point of going metallic or crystallizing, likely the leaching test would have different results. So, if you use copper sensibly (in moderate amounts), there is a good chance you can make a glaze that resists leaching.
The 80:20 base Alberta slip base becomes oatmeal when over saturated with rutile or titanium (left:6% rutile, 3% titanium; right:4% rutile, 2% titanium right). That oatmeal effect is actually the excess titanium crystallizing out of solution in the melt as the kiln cools. Although the visual effects can be interesting, the micro-crystalline surface is often susceptible to cutlery marking and leaching. This is because the crystals are not as stable or durable as the glass of the glaze.
The SMGs (specific migration limits) on lead and cadmium are being reduced and requirements added for compliance on other metals. Testing must be in accordance with European Standards: EN 1388-1 and EN 1388-2. The migration of elements into a food simulant (4% acetic acid) is measured, by accredited methods according to EN ISO/IEC 17025:2005, using FAAS (flame atomic absorption spectrometry). It is a rapid and generally robust interference-free technique having simple external standardization with matrix-matched solutions. Exposure assessment is performed taking into account actual reference doses introduced by the European Food Safety Authority (EFSA) and the Joint FAO/WHO Expert Committee on Food Additives (JECFA). How can one deal with this? Check the limit recipe, leaching and toxicity topics. For maximum assurance, develop your own base recipe and variegate that.
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.
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.
Having Your Glaze Tested for Toxic Metal Release
Having Your Glaze Tested for Metal Release
Is Your Fired Ware Safe?
Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.
Leaching Cone 6 Glaze Case Study
An example of how we can use INSIGHT software to determine of a glaze is likely to leach
Are You in Control of Your Production Process?
Potters often run operations that are on the edge of control tolerating production and ware problems that industry would not. However ethics will sooner or later demand a better knowledge of process and materials.
What people are saying about leaching and safety in glazes
A way of establishing guideline for each oxide in the chemistry for different ceramic glaze types. Understanding the roles of each oxide and the limits of this approach are a key to effectively using these guidelines.
The vast majority of materials used in ceramics are insoluble. But many still present hazards. And you can add hazards (to you and customers of your ware) by the way you use them. Still, there is a need to be realistic about it.
If you are a potter and want to make restaurant ware, read this. Many of the things you already think you know will mislead you in this type of venture.
Metal oxide powders are used in ceramics to produce color. But a life time is not enough to study the complexities of their use and potential in glazes, engobes, bodies and enamels.
Ceramic glazes vary widely in their resistance to wear and leaching by acids and bases. The principle factors that determine durability are the glaze chemistry and firing temperature.
Water in Ceramics
Water is the most important ceramic material, it is present every body, glaze or engobe and either the enabler or a participant in almost every ceramic process and phenomena.