I often get questions about pottery glazes: exactly what are they, what are they made of, are they safe, how long should they last, and quite a few others. In this and several coming articles, I will be discussing these questions. The more you know about pottery and glazes, the better informed your purchasing decision will be and the happier you will be as a result.
So to the first question: What exactly are glazes?
In the simplest terms, glazes are suspensions of minerals and metallic oxides that are used to coat ceramic forms. When applied to those forms, the water soaks into the clay form leaving an even coating of the minerals and other materials on the surface. When fired to the correct temperature, the glaze materials melt to form a glass coating on the form.
Sounds pretty simple, right? Tomes have been written on glazes. New recipes are created every day. The vast majority of these are discarded as failures. Only a few of those that succeed will see regular use. Even fewer will become favorites destined to be used for many, many years. A fraction of a percent of them will become heirloom glazes recognized throughout history and in many cultures. The art and subtle science of glaze making are complicated, and as time goes on, the preference for mere beauty is equaled by a need to safety and permanence.
But enough marketing talk, what are they made of?
At its simplest, a glaze contains three things; a flux to promote melting, a glass maker to form glass and an intermediary to harden and stabilize the glaze. The most common fluxes are sodium, potassium, boron, calcium, magnesium, lithium, zinc, barium, strontium and lead (pretty much in that order). Each of these has its own specific set of properties and behaviors; advantages and disadvantages that need to be kept in mind. Usually a glaze will contain more than one primarily because they do not exist in pure form naturally. They must be “sourced” from various minerals. The exact chemical composition of the various minerals used in ceramic glazes helps to determine which material to use.
The most common glass maker is silica. Silica is also found in virtually all minerals including clay. It can also be found in a relatively pure form by powdering quartz. Boron is a secondary glass maker in addition to being a flux.
The most common intermediary material is alumina, and while it can be had in pure form, it is most often obtained from clay – the pure form being extremely expensive. The intermediary hardens and strengthens a glaze. The ratio of alumina to silica in a glaze will decide how shiny or matte the surface of the glaze will be. The more silica, the shinier the glaze will tend to be – as long as there is sufficient flux to maintain the melt.
Ideally, a glaze recipe will have as few ingredients as possible. The more materials in a glaze, the more issues that will tend to arise as various chemicals in the composition react with each other. Keeping things simple helps to avoid trouble.
Glazes have to be carefully formulated if they are to be used for dinnerware and everyday use. Formulas that are not properly balanced and hardened sufficiently may be prone to leaching; the gradual loss of glaze material into the food or liquid in contact with the pot. The leaching of lead – once the most popular flux in ceramics manufacturing – came to the fore a few decades ago when people were poisoned by food that had been stored in lead glazed pottery. That pottery was not manufactured in the USA (there has never been a case of lead poisoning from domestically produced pottery to the best of my knowledge), but the fact that lead is toxic and accumulates over time in the body has made its use in pottery manufacturing impossible. It is not even possible to obtain lead from ceramic suppliers any longer.
More on glazes in my next article!