Crystal Clear
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Crystal Clear W e use clear, transparent glass for many things, from window glass and fine crystal goblets to thermometer tubes. Glassmaking is an ancient art, but colorless glass is a relatively récent development. Since the beginning of civilization, people hâve known about two types of narurally occurring glass. When lightning strikes sand, the heat can fuse the silica grains into long slender glass tubes that are known as "petrified lightning," or fulgurites. The heat and pressure of a volcanic éruption can fuse sand and rock into a black glass called "obsidian." As early as about 3000 B.C., some artisans discovered how to make their own forms of glass as glazes on ceramic vessels. The art of glass manufacture reached great heights during the Roman empire, but many of the techniques were lost during the Dark Ages. Simple fused-silica glass can be made by melting sand alone. This makes finequality glass, but it is very difficult to manufacture because the melting point of sand is 1723°C, a température higher than most furnaces can reach. Adding a flux of soda ash (sodium carbonate), though, reduces the melting point to only 850 °C, a température much more easily attainable to early societies. Unfortunately the resulting glasses are relatively water soluble, making them impractical for many uses. The addition of a smaïl amount of calcium carbonate, from limestone, to the mixture makes the glass insoluble again and suitable for many purposes. Thèse "soda-lime" or "crown" glasses are the most common types produced even today, accounting for about 90% of current glass manufacture. The basic ingrédients of glass—sand, ash, and lime—were readily available to early societies, but many natural impurities affected the transparency and color of the resulting material. The contaminants varied from région to région, allowing each local glassmaker to claim a characteristic product. Crude manufacturing techniques also left streaks and bubbles in the glass, further decreasing its clarity. Glassmaking underwent a tremendous résurgence in Venice in the 13th century, possibly because of revived contact with the Eastern Roman Empire. Not until later, though, did Venetian artisans rediscover how to make transparent and colorless glass. They discovered how to eliminate unwanted colors and contaminants from a glass melt by adding counter-colorizers.
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The resulting grayish glass, somewhat less transparent than the colored material, proved acceptable if the finished article remained thin—the loss of transparency was less noticeable than the unwanted tint. This cristallo, named because of its resemblance to natural rock crystal, proved to be the greatest export product of the Venetian glassmaking industry. Cristallo was an extremely ductile material, which cooled quickly and could be blown very thin. The glassmakers' need to work with great speed and dexterity to shape the glass properly influenced the style and shape of Venetian glass objects. For the first half of the 16th century, cristallo glasses remained rather simple, but soo
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