Are Glaze Combinations Food Safe?

Food safety is a joint partnership, whether you're a manufacturer, a retailer, or consumer and pottery is no exception. We all play a role in keeping our bodies safe.

The conversation around glaze combinations and food safety is easily the most debated topic in ceramics. For those that aren't immersed in the pottery and ceramic world, there is a school of thought that combining different glazes, especially on the interior part of the piece, creates a new chemical reaction that is not tested for food safety.

To break this down a bit, we first have to identify the chemicals that would make a glaze unsafe for food/beverages, and those two chemicals are lead and cadmium. Both chemicals would be incredibly difficult to find to even add into glazes and are not used in commercial glazes.

You keep referencing commercial glazes, what does that mean?

There are essentially two ways to glaze your pots. One is by purchasing a pre-made glaze (commercial glazes) and the other is by making them yourself by combining chemicals and water.

Every glaze has a specific chemistry and a specific point in time where the chemical reaction takes place to melt and form glass, which is typically referred to as "cones". Glazes are mostly made up of silica, alumina, and flux. If those chemicals are balanced properly, it should form a continuous and stable glaze. The same holds true for clay. Clay changes from a soft, malleable piece of clay and forms into a hardened structure. The chemicals in each type of glaze and clay finish their chemical reaction at certain temperatures (cones).

Vitrification

Many potters who use electric kilns, use clay that finishes that chemical reaction at cone 6, which is 2,232 F or 1,222 C. Ideally that end result will create binding and form a glassy network, and that is what we call vitrification. Vitrification essentially means the chemical reaction has finished, and water absorption is minimal or at 0. When we have a water-resistant mug, bowl, or plate, we reduce the chances of unwanted bacteria forming over time, and the piece won't seep out water or coffee when you forget it on your coffee table overnight. 

If your clay never reaches the correct temperature to form the glassy network, water will inevitably seep in over time, causing a breeding ground for bacteria. For me, the correct firing of the clay and glaze is much more important than the glazes that form on the top layer of the piece. 

Glaze Combinations

According to many experts, adding one or two glazes on top of each other does not create a new chemical or chemical reaction, but rather two or three glazes melting on top of each other. By this train of thought, if all three glazes are food safe, they will remain food safe when used together, if fired correctly and to the correct temperature. It's essential to make sure your clay and glaze finish their chemical reactions and reach vitrification at the same time. If you use a glaze that vitrifies at cone 6, but use a cone 8-10 clay, the clay will not be vitrified. Visually your piece might look solid, but the clay is underfired. 

There are home tests you can do to get a snapshot of that one specific piece, such as leaving boiling water in a cup overnight, boiling an unglazed piece of clay and letting it soak, among others, but I won't get into that in this blog. 

Many times I will limit the number of glazes on the inside of the pot, because of two reasons:

1. For folks that are fearful of glaze combinations and food safety. I am not a chemist or a glaze expert, so I cannot guarantee safety to the extent someone in that field can, but I am good at collecting information and making best judgements. At the end of the day, I like to sell my work, and if someone feels safer with only one glaze on the inside, I want to offer that.
2. It's cheaper. Most times people will only be looking at the outside of the mug, so by reducing the layers on the inside, I save money in the making process.

At the end of the day, I'll keep doing my part to make sure pieces are fully vitrified, and the glazes I make or buy are stable, and I encourage you to do additional research into glaze safety. Much like our food consumption, we all have to weigh the pros and cons of what is safe to fuel our body. 

Dishwasher Use

One suggestion I always make is to run your pieces through the dishwasher every now and then, for all handmade mugs. There are been tests that show dishwashers can improve, or I should say, eliminate bacteria growth on handmade pottery. Especially for my pieces where I stamp names, there are small crevices which can be hard to clean. A run through a hot cycle should keep your mug fresh, clean, and bacteria free.