Page 7 - Curriculum Visions Dynamic Book

Glass contains no pores and so does not let through either gases or liquids.
Glass tends to hold heat rather than conduct it. Cold glass is usually a poor conductor of heat and electricity (although a sheet of thin window glass is nowhere near as good an insulator as the wood or bricks of the walls in which it is set).
Glass will not easily react with other substances and so it can be used as a container for most acids and alkalis. It can also safely be used for holding medicines and foods.
Natural glass
The most common kind of natural glass
is obsidian. Obsidian is usually opaque because of the impurities found in it, but thin splinters can be transparent.
The properties of obsidian attracted Stone Age people, and obsidian sources became Stone Age “factories”: The splinters of glass were used as scrapers, knives,
and arrowheads. These shapes are made
by striking one piece of obsidian against another, a process called knapping.
Obsidian became an important material that ancient peoples traded.
Natural glass can also be created in small quantities by lightning strikes and meteorite impacts – it is called tektites.
(Below) Stone Age people made scrapers, knives, and spearheads from obsidian.
(Left) Glass forms in nature. It is not especially transparent and normally looks black with a glassy sheen. It is called obsidian and is produced as part of volcanic eruptions.
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See Vol. 3: Wood and paper to find out more about wood and insulation.