In this blog we will address the issues around food safety glazing and suggest some in house tests you can do to eliminate obvious glazes that are not food safe.
Glazes recommended for utilitarian ware are stable glazes that will not leach toxic materials. Using a liner glaze is the best option. Materials, such as, kaolin, silica, feldspar, whiting, bentonite, wollastonite, dolomite, gerstley borate, rutile and zircon materials are relatively safe.
However, durable glazes pleasing to the eye are often more in demand, so the potter will
add colorants and variegators to that liner base glaze.
Ensuring that you have a stable, balanced glaze, not exceeding the safe amounts of non-toxic colorants will go a long way.
To give further piece of mind you can also perform some simple Food Safe Glaze Tests. Whilst these tests are not technical enough to guarantee food safe ware, they will expose glazes that are obviously not food safe.
Test1: The Lemon Test:
Squeeze the juice of a lemon onto the fired, glazed surface of your piece. Place the squeezed lemon in the juice and leave overnight.
The next morning, remove the lemon and rinse the piece. Any change in colour of the glaze indicates that the acid in the lemon leached some of the glaze materials out of the glaze making it unsuitable for use with food.
Test 2: The Microwave Test:
Fill your fired, glazed piece with water. Microwave for 1 minute. Cracking of the piece or seeing the glaze chip off the piece will indicate that there is a misfit of thermal expansion of the glaze on the clay body.
Microwaving your piece will also inform you if the glaze contains lusters and other metallics that cause sparking in the microwave. These pieces are not suitable microwave ware and should be labelled as such.
Test 3: The Dishwasher Test:
Take two identical fired, glazed pieces. Place one in the cupboard and the other in the dishwasher and leave for 2 months. Then compare the surfaces of each. Any difference to colour or texture will indicate that the glaze is vulnerable to attack by the dishwasher detergent.
Another point worth mentioning is that if, after use, your fired, glaze pieces are crazing then that indicates that your ware has a high thermal expansion. It is porous and absorbing water.
Crazing creates a breeding ground for bacteria as craze lines of adequate width are moist, warm habitats for the microbes. This again will render the piece unsafefor food use.
High thermal expansion is often associated with low silica and alumina content in the glaze. Crazing is a tip off that the glaze is potentially leachable.
So, whilst basic food safety checks and those carried out by the eye on your fired, glaze pieces are not a guarantee that your glaze is absolutely suitable for utilitarian ware, they are a great guide and will definitely eliminate those glazes that you should absolutely not be using for domestic purposes.