Raw Material.

Silica is the main ingredient in the making of glass. It is found in sand or in quartz, to which different percentages of sodium carbonate and calcium carbonate are added. Silica is rarely found in pure state; commonly it is found combined with other substances useful for crystallization such as iron and copper sulfates, lead and tin oxides and even several salts. When talking about clays it is inevitable to mention complex aluminum silicates. To be more specific on the subject, “an aluminum silicate is a compound made up by silica and aluminum”. When the raw material, that is silica sand, has a minimum amount of impurities, will produce, as final result, glass of major transparency and purity, thus, if found combined with more minerals, glass will acquire a cloudy appearance. Silica sands can be classified in two groups: primary and secondary. The first are known as residual sands that have been formed from their mother rocks. Within this category we find granite, pegmatite and feldspars. Due to the fact that water or wind has not shifted them, they were not mixed with other clays and kept a high level of purity. Secondary clays are those that, due to the mechanical action of water, wind, earthquakes and glaciers, they were shifted from their place of origin. The latter are less pure but are also the most common. Soda, an essential element in glassmaking, has been obtained from the ashes of marine algae, while potash has been extracted from the ashes of tree leaves. The Egyptians used la rochette, which are basically the ashes obtained from the plant known as alkali. On the other hand, the Spaniards obtained soda from barilla; the French from salicor or marine crest and the Germans from fir ashes. The most primitive form of glass had a greenish colour that prevailed until glassmakers learned to purify and discolour the material in order to obtain a totally transparent quality. Glass natural colour is greenish or cloudy, according to the impurity in the mix (especially iron impurities). To make it colourless, the materials have to be purified and then discoloured by physical procedures. Thus, and following the principle of complementary colours, green is blot out by red. Lead crystal is obtained by using minium (Pb2 O3) or lead as fusing agent. This type of glass was fabricated in England since the XVII century and was patented by George Ravencroft. This material can be easily fused, it is soft, very shiny and sonorous, with a high heat resistance index. It is known as lead crystal because of its similarity to rock crystal. Metal oxides give glass diverse colouring: iron produces bluish-green or yellow, according to its valence; copper gives bluish-green or red; cobalt, blue; gold, purplish-red; manganese, purple or violet; sulfur and antimony, yellow; and tin gives opaque white. What is Glass?. Glass composition and properties Raw Material Silica Structural Unit

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