This blog will explore how a lack of glaze knowledge can cause concern for potters/ceramicists
when glazing for domestic purposes. It will encourage the questioning of practice and hopefully inspire the desire to further educate on the craft.
As a potter, hobby potter or pottery studio educator making your own glazes, how often have
you questioned the make up of a glaze?
For example, when you find a glaze recipe, test it, like it and decide to use it, how many of the following questions have you asked yourself?
- Is it safe?
- Is it fit for purpose?
- Is it balanced? What is the alumina: silica and flux: silica ratios?
- Does it contain toxic metal oxides?
- What quantity of metal oxides does it contain and is that a safe level?
- Where did the recipe come from and is that a reliable source?
These questions are just the tip of the iceberg and whilst chemically an unbalanced glaze may not contain dangerous materials, adding metal oxides to it will create a different scenario. The glaze imbalance may encourage leaching of these oxides. An absolute NO for utilitarian ware glazing.
Many glazes are known to be non-functional. If you have sourced your glaze recipe from the many that are available on the internet, you may want to question the number of times this recipe has been circulated. The glaze may have originally been designed purely for decorative purposes and labelled accordingly. However, upon its 5th re-circulation by its 5th recipient on the internet, this essential piece of information may have been left out and to an untrained eye, that recipient may decide to glaze utilitarian ware with it. That ware may then be sold at pottery faire and used for that purpose. Not an ideal outcome.
A sure way to avoid such outcomes is, if as a potter, you are making wares for utilitarian purposes and glazing them as such, you educate yourself on glaze balance:
- Know/calculate the flux: silica and alumina: silica ratios of the glaze. For example, having recipes with >50% feldspar can be concerning in stoneware glazes. The chances are this glaze is unbalanced and the high feldspar content will encourage leaching.
- Be aware of why each material is there.
- Question why a recipe contains expensive alternatives to cheaper materials. For example, why use tin oxide when cheaper zircon materials are just as good.
- Question expensive stains. They can often contain a potent concoction of metal oxides.
- Know that excessive metal oxide colorants can saturate the melt and yield an unstable glaze.
An unbalanced glaze can also create weak utilitarian ware and create ‘Cutlery Marking’, a
term used to describe glazes that are soft and easily marked with metal cutlery.
Beware of the thought that higher temperature glazes are harder. Create an unbalanced, high
temperature glaze and it will most likely be soft. A Chemically balanced and well designed low temperature fired glaze will result in considerable hardness, rendering it suitable for domestic purpose.
The most common cause of glaze softness is an imbalance in the alumina: silica and flux:
silica ratios. This can also lead to a poor fitting glaze. A good ‘glaze fit’ will allow the glaze to expand slightly less than the clay body. I like to think of it as a ‘tight hug’. A chemically balanced glaze will produce a glaze with flexible expansion, suitable to avoid crazing on most common clay bodies.
So, lets all be aware of what purpose we are making pottery for and ensure that, not only are the designs fit for purpose, but the glaze is too.