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Are Your Glazes Food Safe?

We live in a 'no fault' generation. People tend to blame others for things. For example, it is fashionable to blame alcoholism and criminal behavior on our genes now. However, if your pottery is leaching harmful compounds into food a drink, there is no one to blame but you. To be honest, you probably like to take the credit for your designs. Then you should also take responsibility. If you can calculate the oxide formula of your glaze and do a couple of simple tests you will be in the best good position to defend your use of them.

There is definitely a school-of-thought in both industrial and hobby ceramics that the 'leaching glaze' issue is totally overblown. Thus, if there is pottery leaching harmful compounds into food and drink, it is being produced both by companies and people that do not know any better and who have made a conscious choice to ignore the issue.

The US FDA (Food and Drug Administration) tests ceramic ware extensively for leaching metals often with distressing results. After a meeting and lunch with their representative at the American Ceramic Society convention I was very encouraged by their position on this matter. While they are ready to take action against offenders, they are much more anxious to see initiatives by companies and potters to learn how to recognize leach-likely processes, materials, and formulations and learn how to fix the problems.

There are lots of things we don't know about leaching in glazes. However what follows are some things we do know.

Glazes are glass and we tend to think of them as timeless, indestructible. However all glass leaches to some extent when it comes into contact with water or acids, especially if the contact occurs over a period of time or the acid is hot. This is evident by a change in the gloss and texture of the glass surface over time. Glaze color can change also. As a demonstration try 33% CaO, 42% B2O3, 6% Al2O3 & 18% SiO2 at 950C. It should fizz and dissolve in vinegar within minutes even though it fires to a clear and apparently hard surface.

If a glaze is made from harmless materials like silica, dolomite, kaolin, feldspar, whiting, ball clay, etc. leaching is only a functional and aesthetic issue. But if the glaze employs metallic colorants (other than iron) or other minerals containing lithium, barium, lead, chrome, etc. then safety and legal liability becomes a concern.

The likelihood of leaching is not just a matter of whether the ingredients used to make a glaze are dangerous. The issue is complex, involving the ways in which the materials are prepared and fired and the formulations that are used. It is possible to use toxic materials safely and it is possible to compromise an otherwise safe glaze by unbalanced mixtures.

If a customer claims injury from leaching of your ware you have to demonstrate that there was no reason for you to have been concerned about the hazard and that you were diligent in researching the subject. If you don't know how to appraise a glaze's safety then play it safe. For example, there are ways to use barium safely but if you don't know them then don't use barium on food surfaces.

Is the glaze mature?

If a glaze is not properly melted one cannot expect it to be resistant to leaching. While a simple visual inspection of a glossy glaze is usually sufficient to judge the degree of melt, it is especially difficult to tell if a matte glaze is properly developed. This is because glazes can be matte because a network of surface crystals have developed, because the glaze has a surface texture imparted by incomplete mixing of oxides during the melt, because high alumina affects the reflectivity or index of refraction of the glass, or simply because the glaze is not melted.

If a glaze is not properly melted then it will be leachable. The simplest way to tell if this is the case is to fire the glaze at one and two cones higher and lower than your production temperature. Line up the samples and it should be obvious.

Is it balanced?

Glazes are leachable if they contain inadequate glass former and alumina. 'Flux saturated' glazes are very common because they develop interesting fired surfaces associated with non-homogeneous melts of more and less fluid components. However, for a glaze to be fluid, it needs to have lots of flux. If it has lots of flux then it is very likely that the silica and alumina are lacking. A glaze with high feldspar (50%+) is a classic example (feldspar by itself is leachable). If inadequate glass and intermediate oxides are available, coloring oxides that might otherwise be securely held in the glass structure may be available for leaching. Calculating the formula of your glaze (i.e. with INSIGHT) and comparing it with limit charts can be a good way to tell if adjustment is needed. Sometimes it is necessary to compromise a little of the visual character to produce a product more resistant to acid attack.

Another simple thing you can check for is material amounts that do not seem normal. It is common, for example, to see 5% talc in a glaze, but 30% is definitely not normal. Likewise more than 5% lithium carbonate or zinc is strange and needs explaining. Watch also for high amounts of Gerstley borate or boron frit (more than 10%) in high fire glazes, this is not normal. In addition, every glaze should have as much silica and kaolin as it can tolerate. High and middle temperature glazes with little or no flint or kaolin need an explanation.

Firing Temperature

Higher temperature glazes contain less flux and more silica and alumina. Since silica and alumina are so closely related to glaze stability it follows that high temperature glazes are intrinsicly more stable. However it is fairly easy to make unstable high temperature glazes also (fluid flux saturates are an example, they often contain little alumina). At high temperatures experimenters can mix a far greater range of materials and get a good melt because temperature is on their side. Thus it is easy to get 'sloppy' and adopt the attitude that if a glaze looks good it is also safe. Some potters even assume that any glaze is safe as long as it is fired at high temperature. It is common to see popular high fire recipes with very high metal oxide and barium contents. Matte glazes often contain abnormally large magnesia contents. These are destabilizing factors that temperature cannot counteract. With a knowledge of glaze chemistry you can create stable glazes at low fire, without it you can make unstable ones at high temperatures.

Does it contain known carcinogens or poisons?

If a glaze recipe contains materials that are 'surrounded in controversy' it might be best to eliminate or reduce these if they are not necessary. Barium for example, is used for the development of certain blue colors and its ability to produce a matte surface. However there is no justification for barium in a glaze having neither of these properties. The matte effects of barium can also be produced with calcia. Other materials that you should be careful about are manganese, copper (it increases glaze solubility), frits that you don't have an analysis for (may contain lead), chrome and lithia. There are others.

How do you apply on-glaze color?

If you apply pure stains or oxides on top of your glaze, there is a real danger than these might be soluble. It is best to mix metallic colors with a glaze (at least 50%). Even then you should be careful about heavy applications because the coloring oxide is saturated. Do not mix on-glaze metallic colors with Gerstley Borate alone (it is soluble), it needs much more alumina and silica to be stable. If you are mixing colors with a borate frit, be sure it is stable. Putting color on the ware and applying a transparent glaze over top is the safest method for food surfaces.

A liner glaze

The best solution is to use a liner glaze with no color on all surfaces that will be exposed to liquids. Liner glazes made from materials like kaolin, flint, feldspar, whiting, wollastonite, dolomite, nepheline syenite, Gerstley borate, talc and zircon opacifiers are completely safe. Frits are also safe as long as you know that they contain oxides common to the above materials (no lead, barium). We have base glaze recipes available for cone 06-04, 6, and 10 and information on how to apply a liner glaze.

Are your glazes crazing?

If your glazes craze and your ware is porous then there is a possibility you are creating a bacterial 'breeding ground'. If the craze lines are of adequate width then who knows what could live in there if the clay body supplies absorbed moisture from below. While there are many factors involved and this could be argued, we must admit that the greater body porosity, the wider the craze lines, and the less the ware is exposed to heat, the more this would be a threat. You can test if your ware is absorbing water by soaking it for a few days and putting an empty vessel in the microwave. If it gets hot (and it is not of a high iron clay or glaze) then it has absorbed water. Weighing a new piece before and after soaking will verify this. Remember also the matter of perception: Crazing dramatically weakens glazed ware. Over time it will lose its 'ring' and users will get the feeling that your ware is not good quality. As a result they may also question its other qualities, such as food safety.

Another factor to consider is this: Glazes craze because they have a high thermal expansion. Often high expansions are associated with low silica and alumina contents. Stability in glazes is almost always linked to the amount of silica and alumina, the more the better. Thus the crazing is often a tip-off that the glaze is potentially leachable.

Simple leaching tests you can do

All glass leaches to some extent when it comes into contact with water or acids, especially if the contact occurs over a period of time or the acid is hot. This is evident by a change the gloss and texture of the glass surface over time. The ability of the glaze to pass two simple tests can be a good assurance that it will give trouble-free service.

Dishwasher Test: Take two identical items and put one in your cupboard and leave one in the dishwasher for two months. Compare the surfaces of the two very closely. If the washed one looks any different, either in color, gloss, or texture then you have a soluble glaze.
Vinegar Test: Fill a glazed container half full of vinegar and leave it for several days. Dry the item and compare the color and gloss of the surface above and below the liquid line. Any difference indicates that the glaze is subject to leaching. There is a way that you can amplify the result: If your glaze is white or transparent add 5% cobalt and do a vinegar leaching test on it (if you are using a colored glaze, add enough of the colorant to get a bright color before doing the test). Excessively unstable glazes can easily turn white with an overnight vinegar leach.
If you want to be a little more scientific you can send the vinegar leachate to a lab to test for the metal in question, then compare the result with its drinking water standards. This type of information is available by using a search engine on the Internet. For example, if you search "manganese drinking water standard" in Altavista you will get 1,300,000 hits! If you search "+manganese +drinking +water +standard" (every word must be found) you'll get 1750. Look around and you'll find that manganese has public drinking water standards of 0.01-0.05 mg/liter. Remember also that to be totally applicable someone would have to drink all their water from the same glazed container, and any metals already in the water, body weight, whether or not the body accumulates it, etc. would have to be taken into account.

These tests are not technical enough to guarantee that ware is going to be absolutely safe but they will certainly expose glazes that are obviously unsafe.

What to do if your glazes are soluble

If you find that your glazes are soluble, use ceramic calculations to increase the amount of silica and alumina and moderate the content of oxides that exist in inordinately large amounts. Test again to see if it worked. There is much information in the literature on this subject and often small changes that do not greatly impact the fired glass have significant effects of solubility.

The bottom line

Make sure your glazes melt well enough to form a good stable glass (enough flux).
Use as much alumina and silica as your glazes will take and still melt well.
Make sure glaze melts are not too runny, this indicates either too little alumina or too much flux, both are leaching hazards.
If it contains an oxide that has a reputation for destabilizing a glaze against leaching (i.e. copper), have it tested.
If any metallic oxide colorants are used, be wary of large percentages.
Be wary of glazes with oxides that are known to have human effects (i.e. barium, lithium, manganese).
Be wary of recipes that 'don't look right' (e.g. 60% feldspar is too much, glazes with no clay are very suspect, glazes that employ frits that you know nothing about).

Having your glaze lab-tested for metal release

Although application of well known principles of ceramic chemistry reduces that risk that a glaze is soluble, the only way to know for sure is to have it tested at a credible lab. John Hesselberth, a potter who has made it his business in recent years to gather and evaluate information on limit formulas and glaze leaching, had this to say: 'You cannot predict leaching from limit formulas; although I believe a glaze within limits is more likely to be durable to leaching than one that is outside of limits--but being in limits is no guarantee.' See our page on toxicology for more information.