Food safe glazes: how can you tell if your pottery will be food safe?

There has been a lot of chatter recently about food-safe glazes and how to tell if a piece of pottery will be food safe. Generally, a piece of pottery is considered food safe when it meets the following criteria:

1. Surfaces that come into direct contact with food or drink must be fired to their full maturity so the clay particles vitrify enough to form a waterproof surface or be covered by a smooth, glassy glaze without cracks or rough spots that could allow food to penetrate through the glaze to the clay body. Note that even when fired, low-fire clay remains porous enough that fluids may penetrate the surface and soak into the clay. Any surface that comes in contact with food or drink on pieces made from low-fire clays must therefore be covered with a food-safe glaze that has been correctly fired. 
2. Any glaze that comes in contact with food needs to be chemically sound and not leach metal oxides into the food. It is a good best practice to use a transparent or white liner glaze that you know does not contain any harmful elements on the inside of any piece meant to be used with food or drink. Glazes whose surfaces are heavily crystallized or overly melted (runny), brightly colored (without a clear overglaze), or very matte, are the most likely to be unsuitable for food.

Chemical Stability of Glazes

Chemical stability is also very important to food safety. Although a ceramic glaze is considered a glass, it actually can dissolve over time when in contact with liquids if it's not chemically stable. If a glaze contains metal oxide colorants such as barium, lithium, or lead, these compounds can then dissolve into food or drink, especially in the case of acidic foods like lemon juice or coffee.

If the glaze does not contain enough silica there is a good chance a stable glass has not been formed. It is also important to notice high percentages of oxides like BaO, MnO, PbO, or Li2O, which have the potential to leach. High percentages of these oxides or a low percentage of silica would be obvious in the recipe and should warn you that the glaze may not be suitable for food ware.

The absolute limits for all oxides (to prevent leaching) are hard to establish, as glaze chemistry is much more complex than that of regular glass. Still, there is value in being aware of obvious chemistry irregularities like large amounts of metal oxides. In general, any recipe that uses more than 3% of any metal oxide should be tested for leaching before being considered food safe.

Glaze Crazing and Food Safety

Crazing is often seen as an interesting visual element, but from a food safety point of view, crazed ware is unacceptable. Crazing can severely weaken a piece and provide space where water can enter and saturate the clay if it's not completely vitrified. The cracks may also harbor bacteria if not sanitized regularly. Crazing happens when glazes are under tension because the clay and the glaze have different shrinkage rates. Simple awareness of crazing is a strong measure anyone can take to improve the safety of the ware they produce.

Of course, it is worth noting that even though crazing is considered not food-safe, people have been using crazed wares for centuries without experiencing negative health issues. There have even been scientific studies that show that as long as you wash pieces with crazed surfaces with soap they are just as healthy as non-crazed glazes. That said, if you have crazing, it is always a best practice to add a glossy, clear food-safe liner glaze over the food surfaces.

How to test if a glaze is food safe

There are a few easy tests that anyone can perform to test if a glaze is chemically sound. 

A 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 certainly appear if the glaze is under stress. By placing a piece in ice water, then in boiling water repeatedly, you can reveal if these problems will happen over time from thermal shock.

A simple lemon test can also be performed to test if a glaze will leach over time:

1. Squeeze the juice of one lemon and put one slice of the lemon onto a horizontal, glazed surface and leave it out overnight. 
2. The next morning, remove the lemon and rinse the piece. If you see or feel any change in the color of the glaze, this indicates that the acid in the lemon juice was able to leach other materials out of the fired glaze.

If a glaze that fails this test were to be used for functional work, like a cup of coffee, the acid in the coffee could cause similar leaching of unwanted and potentially hazardous materials. If your glaze fails this test, don’t use it on functional work.

It’s important to note that if your glaze passes this test, it doesn’t necessarily mean that materials aren’t leaching out of the glaze at all, just that they aren’t leaching badly. If ever in doubt, use a clear or white liner glaze on all surfaces that will come into contact with food and drink, and save any questionable glazes for the outside of your work.