David Estey, age 11, of Durham Bridge, N.B., Canada, for his question:

WHAT IS VULCANIZED RUBBER?

Vulcanization gives strength, hardness and elasticity to rubber by treating it with heat and vulcanizing agents such as sulfur. Heat causes the sulfur to combine with the rubber and cure it. This makes the rubber stronger and more durable.

A compound containing one third sulfur and two¬thirds rubber vulcanizes to form hard rubber. Benzoyl peroxide is used instead of sulfur to vulcanize silicone rubber.

Vulcanization can take from just a few minutes to as long as several hours. Small products, such as toys, require about five minutes but large items, such as tires, may take up to an hour. Vulcanization and shaping of molded products can be done at the same time by heating the molds under pressure.