Glazes
Glaze is like a thin coat of glass on the surface of pottery which makes it waterproof, smooth, and suitable for applying decoration. It is applied by dipping the item into a tub of glaze or by spraying.
Glaze is a glass which has been melted onto the surface of the fired clay body.
After the first biscuit firing, the surface of the pot is still rough and porous. Glaze forms an impervious surface over the ware, preventing liquids from being absorbed into the body of the pot, and making it smooth.
As glazes are available in a wide range of colours, they can be very decorative.
Silica
Silica is the main constituent of glass. It occurs naturally as crystalline quartz, flint and sand. Silica melts at 1700 C then cools to form a glass. Unfortunately pots tend to crumble at 1300 C
Flux
To get around this problem, agents have to be added to the silica to allow it to melt at lower temperatures. These agents are known as fluxes. Examples of fluxes are lead, boron and whiting (powdered chalk).
Alumina
Glazes made of silica and flux alone would be rather runny and would flow off the pot onto the kiln shelf during firing. They need to be stabilised. Alumina is used for this purpose. It is usually derived from feldspar or lithium compounds.
The dangers of dipping
The dipping shop was the place in the potbank where glaze was applied to ware. The pots were quite literally dipped by hand into tubs of glaze and allowed to dry before firing.
Unfortunately the flux used in the glaze was lead. This created serious health hazards. Lead tended to be absorbed into the body (through the skin or by inhaling fumes) and would accumulate in the bones. After the lead reached a certain concentration it would affect the tendons of the lower arm. This led to the potters' 'dropped wrist', and in more severe cases, the 'dropped ankle'. Lead could also affect the central nervous system, causing epilepsy, paralysis and, ultimately, death.
The problem of lead poisoning (plumbism) persisted until the early years of the 20th century. It was the progressive introduction of lead frits which finally overcame this serious health hazard.
What are lead frits?
Lead was dangerous because it was soluble and could be absorbed into the body. Fritting is a method of making lead insoluble. The lead is permanently bonded to silica by heating to a glass. The glass is then fragmented, ground down and added to the glaze mix. After fritting the lead can never become a free active agent and cannot be absorbed either through the skin or by inhaling.
