Page 25 - Curriculum Visions Dynamic Book

The drying properties of lead
When a paint has been applied, it is still liquid, and the faster it can be made to dry, the more attractive it will be. The first phase of the drying occurs when the solvent used to apply the pigment evaporates.
But often a further chemical reaction is needed to make the paint into a hard solid. Many paints – gloss paints, for example – use an oil base. In these cases hardening (“drying”) occurs as the oil polymerises on exposure to air, forming long chains of material that can no longer move about.
These setting processes are speeded up by the use of a catalyst. Several metals, including lead, have been widely used as drying additives.
Lead glaze
catalyst: a substance that speeds up a chemical reaction but itself remains unaltered at the end of the reaction.
ceramic: a material based on clay minerals, which has been heated so that it has chemically hardened.
flux: a material used to make it easier for a liquid to flow.
A flux dissolves metal oxides and so prevents a metal from oxidising while being heated.
pigment: any solid material used to give a liquid a colour.
The shiny, smooth, glass-like finish on many pots and other ceramic objects is called a glaze. Colour and decoration are
also added to the glaze, while the hard smooth finish makes the finished ceramic object much easier to wash and clean.
The glaze is made from powdered glass mixed with a colouring
material. Lead oxide is a common colouring substance. Powdered glass and lead oxide are mixed in water and then coated on the pot or
other object. The glaze is then dried and heated in a kiln to make the glass powder soften and form a continuous coating, and thus fix the glaze in place. At the same time, the glaze reacts
chemically with the ceramic and so
forms a permanent bond.
The lead oxide has two roles. It
adds a white colour to the glaze, and at the same time, it lowers the temperature at which the glaze melts and ensures that it runs more smoothly while in the kiln. This property of a material makes it
an efficient flux.
 Lead is a common component of glaze, the reflective surface coating on pottery.
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