When the city of Venice
first came into being, artisans
took the fine sand found originally on the small islets that make up the city and began to use them to make elaborate works of glass (decent ceramics were hard to come by, so Venice used glass for containers, lamps, etc.). Eventually, most of the major glassworks congregated on the island of Murano
in order to maintain trade secrets as well as simplify supply, and to protect the main city from the inevitable fire hazard
s caused by combustion-fired glass furnace
The Venetian maestros of glasswork produce all manner of works from cheesy to magnificent. They produce sculptures of great complexity as well as functional items such as stemware, lamps, and chandeliers. One particularly attractive style is 'layered glass,' where objects such as vases or lamps are constructed of layers of glass of either different color or different shades.
In order to produce glass that is free of defects from impurities in the silicate used to make it, soda ash must be added to compensate for these impurities. Soda ash, however, lends a yellowish tone to the glass if too much is present. Although the foundries used to use local sand, they presently purchase sand from the Fontainebleau* region of France which is a very pure grade.
Colors are produced by adding trace amounts of minerals to the glass. Green is produced by adding copper oxide. Blue is produced by adding cobalt. Red is produced by adding gold; yellow by adding cadmium (I believe) and white through addition of manganese. The multiple layers are produced by dipping the glass blob in various different furnace cauldrons containing the different colors, while allowing cooling time in between layers. This temperature differential prevents the glass layers from mixing and preserves the sharp color gradations.
Small sculpture, especially, is extremely impressive to watch being made. The glass only remains pliable for perhaps thirty seconds to a minute after being withdrawn from the final dipping; and with small objects, reheating is problematic as it can cause layer mixing or collapse of already-done work. As a result, the maestro typically will sculpt the finished object - in the example I saw, a rearing horse - from the shapeless glass in the space of maybe thirty seconds, so that as it cools it cools into its final shape. Apparently it takes an apprenticeship of around fifteen years to produce expertise of this level.
If you are ever in Venice, be sure to vist Murano to tour one or more glass factories!
*Thanks to wharfinger for catching a typo on the spelling of Fontainebleau!