|
InYourFaceFotos.com Thayer Watkins Silicon Valley USA |
|---|
|
|
Humans started producing glass four millenia ago in the Middle East. As with the smelting of metal ores the production of glass came as adjunct to the firing of pottery. The first instances were probably treated as curiosities, transparent stones, and used as jewelery. But after not too long, small useful artifacts such as bowls and bottles were being fashioned from glass.
The first glasses came from combining sand with lime and other ingredients which lowered the melting point. Pure sand, silicon dioxide, has far too high of a melting point to melted in the furnaces of the time with their inefficient bellows.
For millenia the technology of glass was trial and error with discoveries kept as closely guarded secrets. Somehow the practioners of the art learned to produce colored glasses. In the heyday of Venice the production of artistic glass items was the high technology of the period.
After almost four centuries of development there were just two basic types of glasses: the crown glasses and the flint glasses. Crown glasses were soda-lime glasses with low levels of refraction and suitable for mundane purposes such for windows. Flint glasses contained lead and had a high level of refraction and were used for finer purposes such as in decoration. The name flint came from the use of flint stone as the source of SiO2.
The science of glass did not begin its development until about 1800. The systematic experimentation required a number of techniques. Experimenters could try various mixes of ingredients but until there was a method of stirring the batches to achieve adequate homogeneity it was not possible to establish reproducible results. It was Pierre Louis Guinand who developed adequate stirring methods and taught them to Joseph von Fraunhoffer at an optical glass firm in Munich.
The next requirement was methods of measurement of the properties of glass. Fraunhoffer, a very able empiricists as well as theoretician, was able to measure the refraction of glasses. From the refraction measurements he was able to establish quantitatively what had long been known qualitatively, that glasses containing lead (Pb) refacted light more strongly than glasses containing only calcium (Ca from lime).
Fraunhoffer invented the spectroscope. The spectroscope enabled him to determine the refraction of light of different wavelengths. He was then able to determine the effect of four other elements beside Pb and Ca, sodium (Na), potassium (K), aluminum (Al) and iron (Fe), on the refractive properties of glass. The English scientists, Harcourt and Stokes, extended Fraunhoffers work to the elements lithium (Li), berylium (Be), barium (Ba), magnesium (Mg), strontium (Sr), zinc (Zn), cadmium (Cd), boron (B), tin (Sn), phosphorus (P), fluorine (F), arsenic (As), antimony (Sb), tungsten (W), molybdenum (Mo), tantulum (Tl), and vanadium (V). This work required a crucible that would not contaminate the batches. Platinum crucibles and fixtures proved satisfactory for preventing any such contamination. Hydrogen flames were used to get higher temperatures and prevent contamination of the batches from ingredients in the fuel.
From this early work it was discovered that other substances such as boric oxide (B2O3) and phosphorus oxide (P2O5) could take the place of SiO2 in glasses. These special glasses proved valuable as the basis for telescope lenses.
The science of optical glass was created by Otto Schott and Ernst Abbe in the late 19th century in Jena in east-central Germany. Ernst Abbe was at the University of Jena where he worked with Carl Zeiss, another pioneering figure in practical optics. Otto Schott came from a glass-making family who had encouraged him to do basic research in the science of glass. Otto Schott's experimental work came to the attention of Ernst Abbe who induced him to come to Jena. Together they refined the techniques and explored the effects of borates and phosphates on the optical properties of glasses. Otto Schott was particularly interested in the effect of lithium on glasses.
The collaboration of Abbe and Schott culminated in the formation of a firm, the Jena Glass Works of Schott and Associates, which became a world leader in the production of optical glasses. Jena was already the site of the firm of Zeiss, a world leader in optical and precision instruments.
Glasses incorporating borate reduced the problem steming from light of different wavelengths being refracted differently, chromatic aberration. This resulted in improved optical instruments. Ernst Abbe invented a system for eliminating the problem of chromatic abberation in microscopes. The Abbe microscope became the preferred instrument for scientific research and medical investigation around the world.
Before the creation of colored glasses only gems that had the beauty of translucent color. Gems were very rare and beyond possession of most of mankind so the only way for humans to experience that translucent beauty was in the form of colored glasses.
The source of the coloration is now known. Clear light is composed of light of many wavelengths. When this light impinges upon material light some wavelengths may be absorbed; the remaining wavelengths then exhibit coloration. The wavelengths of light which is absorbed is due to some resonance with aspects of the atomic or crystal structure of the material.
Glasses may be colored by their inclusion of ions of chemical elements. The elements iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), chromium (Cr), and manganese (Mn) have certain similarities in the outer shell of electrons in their atomic structures. They have notable varied influences on the coloration of glasses.
Even seemingly clear window glass has a greenish color when viewed from the edge. This is due to small and unavoidable amounts of iron ions in the glass. One cannot however say that element X always produces color Y in glasses because the elements previously listed have a number of different valence states. Iron, for example, has two valence states: ferrous (Fe+2) and ferric (Fe+3). Likewise cobalt and nickel have two valence states. Copper can be in the form of cuprous (Cu+1) or cupric (Cu+2) ions. Manganese, on the other hand, can occur with valencies of 2, 3, 4, 6 or 7 in compounds and each different state may create a different coloration.
The color of a glass may depend upon the nature of the glass as well as the coloration ion. For example, iron ions have the following color influences:
| Ion | Silicate-based Glass | Phosphate-based Glass |
| Fe+2 | deep blue-green | slight greenish blue |
| Fe+3 | yellowish-brown | slightly brownish |
(To be continued.)
|
|