Glaze chemistry and food safety
In my work as a part-time potter, my aim is to make the best work that I can - I’m lucky that my ‘main’ job provides me some space to study, learn and practice ceramics. One of the things I’ve been interested in learning over the pandemic is glaze chemistry - I found a fantastic university-level online course with Ceramic Materials Workshop . I now know how to make high quality glazes with *just* the right visual effects whilst ensuring it's durable and safe for use for many years.
Below are a few key things I learnt from this course which people sometimes ask about at markets and fairs.
How do I know if my glazes are safe for people to eat from?
Ultimately, without sending that specific piece to an industrial testing lab, you can't be 100% sure that some part of the glaze or clay body isn't going to leach a tiny element into your food - especially if it's used for years in the dishwasher, which is a very caustic cleaning method.
Having said that, the levels of anything we use in studio ceramics are so minimal, it's very unlikely to have any adverse affect. A couple of rules I use to be confident of this, are:
never use materials which the body can't process, like cadmium and lead
use a simple and durable liner glaze wherever food might meet the pot
I also calculate my glazes carefully to be as stable and durable as possible - I want to know what’s happening within the kiln. See below for the detail on how this is done!
What’s in a glaze?
A basic glaze is made up of silica, alumina and a flux. Silica and alumina are compounds which have incredibly high melting temperatures. A flux is needed to break their chemical bonds and melt at a lower temperature to create a fully melted glass - the glaze.
Something which complicates this a little is that we get these chemical elements from rocks, which are rarely just the single compound we need. We use:
Ball clay - which is made of silica and alumina
Flint (also called Quartz) - which is silica
The fluxes we need are a combination of alkaline Earth and Alkali Metals. We get these from raw materials like
chalk (also called Whiting) - calcium carbonate,
Spodumene - lithium carbonate + silica + alumina
Potash Feldspar - potassium oxide + silica + alumina
Talc - magnesium oxide + silica
I use an incredible spreadsheet from Ceramic Materials Workshop where I can plug in the materials and check on the suitability of the glaze given the materials.
The spreadsheet plots the alumina and silica on a map by Stull from 1912 (see below), showing whether it's likely to be glossy, matt or over/under fired.
The sheet is also highlights an important piece of research done by Matt Katz of Ceramic Materials Workshop on glaze durability. If the materials in a glaze don't have the chance to fully melt or the fluxes are not correctly balanced, the chemical bonds aren't stable, and leaching or a break down in the strength of the glaze may occur. His research indicates:
For the glaze chemical reactions to happen efficiently and completely, the balance of alkaline metal to alkaline earth should be 0.2:0.7
This creates the strongest and most durable glaze when tested.
One more element of complexity comes from our modern practice of firing in electric kilns at lower temperatures - cone 6 - rather than cone 10 in a wood firing or gas kiln. We need something to encourage the melt to happen even lower - but simply adding more fluxes would throw the glaze chemistry off balance. Matt found that boron is the solution to this. Boron enables the melt to happen at a lower temperature but doesn't seem to impact the reactions happening with the silica, alumina and the fluxes. We add a little boron to a glaze to lower the melting temperature when moving from a cone 10 glaze to a cone 6 one.
I hope this hasn’t been too chemistry-heavy for you - hopefully some of you potters might have a better idea of what I’m on about! Thanks for reading.