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Many people are confused about the safety of glazes, and rightfully so. It is a complex issue with many variables. So we will attempt to clarify this without causing more confusion.
The two materials that are proven toxic are lead and cadmium. Lead is used to make glazes flow better at low temperatures. Cadmium is used primarily to create bright orange and red colors. There are other materials which may be toxic, but there is not enough evidence that they are unsafe at this time, so they are not regulated. Many of these materials are safe in low doses (for example, nickel, barium, selenium and cobalt), but toxic in high doses. So reducing leaching as much as possible is always a good idea.
Commercial glaze manufacturers label their glazes using ASTM D-4236. All their glazes are either AP Non-toxic, which means non-toxic in liquid or dry form, or CL Cautions Required, which means it has proper labeling of ingredients for health and safety. In this sense, non-toxic only refers to lead and cadmium.
All glazes sold in K-12 schools must be AP Non-toxic. This is to reduce the risk of harm if a child drinks the glaze. You don't want a lead-based glaze in the classroom for example. You will see the AP Non-toxic label on the glaze bottle; a circle with an AP inside.
Remember, all glazes in DRY form are unsafe for breathing, and you should use a good mask whenever dealing with dry glazes. There are chemicals such as manganese which are known to be a health hazard when breathed in dry form, but are not believed to be a problem after being fired. And even clay particles with no toxicity get trapped inside lungs and thus are bad for potters to breathe.
This is what most potters are interested in. Can you use a certain glaze on a piece which will contain food and beverages?
Toxicity is one aspect of this. If there is no lead or cadmium in your glaze (including no Frits which contain lead), and your kiln is not contaminated with lead, then you pass one toxicity test (for lead and cadmium). (If you have fired leaded glazes before, your kiln brick may have absorbed lead and could be depositing it on current firings. You can get an inexpensive kit at the hardware store to test your kiln for lead release. And of course you should never fire dinnerware in a kiln with other leaded glazes.)
There are some glazes that have lead or cadmium and still say they are dinnerware safe. They have been fired and tested, and found to pass the test for lead and cadmium release. (A small amount of leaching is allowed by law.) There are also some glazes where the cadmium is encapsulated in other glaze ingredients which traps it when fired. The only caution here is that your firing conditions will be different, so it is possible that your pieces could leach when the test pieces did not. For this reason, it is best to have a sample tested anytime you use glazes which contain lead or cadmium. Later on I will tell you how to do that.
Any time you begin to layer glazes, you are pretty much on your own. Any testing that the manufacturer did will not be applicable. If you don't use any glazes with lead or cadmium as ingredients, you are pretty safe (with the caveats above.) Otherwise, test!
Many potters believe that you should never use these ingredients in dinnerware period. Who knows what may happen to the glaze after years of use, after going through the dishwasher 30 times, after the glazes craze, after they are microwaved and frozen and bombarded with acidic food. It is always possible that a piece will leach lead or cadmium at some point in the future. So to be safe, just avoid them.
Then of course there are the ingredients which are not regulated, but may be toxic especially in high amounts. Unfortunately there is no simple answer to this problem. If you are producing dinnerware then it is advised that you have sample pieces tested for the various ingredients which might be leaching. (See instructions below for how to do this.)
There are other issues that determine whether a piece appropriate for dinnerware use.
Resistance to abrasion (does it scratch easily with silverware?). You can test this yourself. It is usually a problem more with matt glazes than shiny.
Ability to handle acidic foods. To test for acid resistance, slightly squeeze a slice of lemon to get the juices flowing, and leave the whole slice on the glaze overnight. See if the color changes. If it does, there is some leaching going on. A customer could run into this same color change, and there may be chemicals leaking out. Another way to test this is to put a sample in vinegar for 3 days. Finally, there is lab testing, described later.
Ability to withstand alkaline dishwashing detergents. To test this, mix 50 grams of soda ash in 1 liter of water in a stainless steel pan. Bring to boil. Reduce heat to a simmer. Place samples in the pan, cover, and simmer for 6 hours. Compare the color and surface gloss to a similar but untested sample. (This test came from Mastering Cone 6 Glazes see below for more details).
Ability to withstand thermal shock. This does not mean that you can place a ceramic pan over a flame, or directly into a hot oven. It is very difficult to make pieces that can go directly over flames, and not something an individual should attempt. Ceramic casseroles, etc. should be put into the oven at room temperature, and brought up to temperature slowly. However, your customers might not know this, and even if you tell them, they probably won't remember. To test for thermal shock, place a test pot in the freezer for several hours. Then submerge the piece in a pot of boiling water. (Alternately, put the pot in the sink and pour the boiling water into it). Repeat this 3 times, looking for minute crazing on the glaze. It is also a good idea to do example what a customer would do. Take a completed piece out of the refrigerator, and put it into an already heated oven. Make sure the piece does not crack.
Ability to go from the dishwasher to the microwave. Metal overglazes should never go in the microwave, so it is a good idea to keep them off mugs and other dinnerware items Other than that, the problem with microwaves is if there is any water trapped inside the clay, it will expand in the microwave and cause the piece to crack. Low fire clays are porous by nature, and always problematic in the microwave. If your glaze is fit very tightly (can withstand the thermal shock test with no crazing), then the glaze may prevent water from getting into the clay, and this will be ok in the microwave. High fire clays should be fired to vitrification to keep water out. See our tip for more information about vitrification.
To test microwave safety, take a piece (such as a mug or bowl) and immerse in a pan of water. Bring the water to a boil, then simmer for a few hours. This will allow the piece to absorb water. Then put the piece in the microwave. (The piece should be empty, and you should also put a separate mug of water in the microwave to protect the microwave.) Heat the microwave on high in 10 second increments. After each 10 seconds, carefully touch the piece to see if it is hot. If it has absorbed water, it will heat up. This tells you the piece is not dishwasher safe. You can stop the test when the water in the second mug is boiling.
To have your pieces tested for leaching of lead or other substances, make a small cup, fire and glaze it your normal way. Two labs are Alfred Analytical Laboratory in NY (607-478-8074), and Brandywine Science Center in PA (610-444-9850). The cost is approximately $20-$30 depending on whether you want to test 1 or 2 metals for leaching.
This might all seem very complicated. So in the end one must use their best judgment to determine when and what to test. In my opinion, if you make pots as a hobby, not in large quantities, and not pieces that will be used every day for many years, then for dinnerware I would just stay away from glazes with lead, cadmium, and barium as ingredients. If you want to use glazes with those ingredients, have a sample tested. If you do volume production, I would test leaching on all potentially toxic ingredients. And for anyone, I would try the home tests described above. What you learn may surprise you, and is a good next step in your growth as a potter.
One should always assume that people will use your pots for things other than you intended. They may drink or eat out of things that are obviously not dinnerware (such as vases.)
While there are many books that discuss crazing and how to do the thermal shock test, few books discuss these other aspects of glaze safety. In fact, many books publish glaze recipes which do not meet some or all of these tests. One exception is the recent book by John Hesselberth and Ron Roy called "Mastering Cone 6 Glazes." John and Ron have spent much time studying durability of glazes. They go into these issues in much more detail than I have here, including the chemistry behind it, and how to make your own glazes which are safe and durable. It is a particularly good book if you are firing in the Cone 5-6 range, but also an excellent book for understanding glazes in general. They also discuss glaze mixing, application, formulation, and troubleshooting, in addition to durability and testing. John and Ron note that they have tested many glazes published in popular books, and even some commercial glazes, and found that they often fail one or more of the tests above.