What is glass used for?
First glass made by man
Such naturally formed vitreous rocks are called obsidians and tektites. The Stone Age man already used it as a cutting tool. It is not known exactly when glass was first made by man.
The earliest finds come from the Middle East around 3500 BC. In the second millennium BC, glass production developed independently of one another in the Greek Mycenae, in Egypt, in China and in North Tyrol.
Conjectures suggest that glass was created accidentally when the pottery was burned. For example, if the sand was particularly calcareous, maybe a bit of salt in the mixture and the pottery furnace was too hot, a vitreous coating (glaze) was created on the ceramic.
First vessels made of glass
Around 1500 BC it was finally possible to manufacture glass independently of a ceramic base. It worked like this: First of all, the liquid glass had to cool down from 1400 to around 900 degrees Celsius. Then the glassmaker modeled the viscous glass around a solid core of sand.
He also had a long rod with which he created the shape by turning. It was an art that few had mastered. Thin-walled shapes could of course not yet be produced in this way, because blowing glass was still unknown at the time.
Raw materials and melting process
The Assyrian King Ashurbanipal left the composition of the raw materials for glass production on a clay tablet library around 650 BC: "Take 60 parts of sand, 180 parts of ashes from seaweed, 5 parts of chalk - and you get glass." Sand, lime, soda and potash was later called the recipe.
The chemical components were basically the same. At 1400 degrees Celsius, they melt into glass. When the so-called mixture melts, foam with large bubbles develops, which forms a carpet. Then the mass is "purified". This means that the remaining gases now come out in the form of small bubbles.
In the end, the glass is largely free of inclusions and bubbles. The temperature at which these different phases occur depends on the raw materials added in each case. Today around 60 percent of the around 90 elements found on earth are used in the manufacture of glass.
A technical revolution was the invention of the glassmaker's pipe in Syria around 200 BC. The most elaborate shapes could now be designed with it.
The pipe is an iron pipe one to one and a half meters long with a heat-insulated mouthpiece and a small extension at the other end so that the glass mass can hang.
The Romans improved the kilns so that the good glass quality was just right for the ostentatious Roman emperors: thin-walled, extravagantly colored vessels and intermediate gilding. But with the fall of the Roman Empire around 400 AD, a lot of technical knowledge was lost.
In the 10th century, Venetian glassware became famous: the purest crystal glass with an inimitable shine. The wood that fueled the smelting furnaces came on rafts from the nearby Dolomites.
They wanted to keep the recipe for the everywhere coveted glass a secret. Therefore, the glassworks were moved to Murano, an island off Venice. This had solved the fire hazard problem for Venice.
The island also appeared to be spy-proof. Anyone who betrayed the secrets of glass art should be punished with death.
Despite all the threats, the secret of Venetian glass art could not be kept a secret for long. Quite a few glassmakers from Murano fled and tried their luck beyond the Alps, founded a glassworks and continued to work in the Venetian style.
This also explains the fact that Venetian motifs later appeared in Bohemian glass, for example. The heyday of Venetian glass slowly came to an end.
Wanderglashütten and potash
German glassmakers settled in the wooded areas of the low mountain range. In the Spessart, the Thuringian Forest, the Black Forest, the Bavarian Forest, the Fichtel Mountains, the Bohemian Forest, the Ore Mountains and the Giant Mountains. There they found enough firewood for the smelting furnaces.
The glass was mostly greenish, not colored. That was due to the raw materials, the sand and the potash. Beech and oak trunks were burned and the ashes leached in vessels, the "pots". This resulted in the potash (potassium carbonate).
Once the area was cleared, the melting plant was moved on. It was not until the 17th and 18th centuries that glassmakers settled down. Glass was decorated and refined. It was given heat effects, dyes and precious metal coatings, was cut, sandblasted, etched and deposited with metal.
Glasses come from beryl
It is said that the Roman emperor Nero was very short-sighted. For example, he used a cut emerald as an eye-glass in order to better observe the gladiator fights. But that was more of a one-off.
Grinding lenses from the mineral beryl, on the other hand, was common. The word goggles arose from the name for this mineral rock.
The first glass glasses were made in Venice in the 13th century. It is doubtful whether the first glasses really improved vision. In the 15th and 16th centuries, for example, there was still the phrase "sell someone glasses" or "glasses someone".
What was meant by it was to cheat someone by the grain. A glasses salesman was the one who led someone by the nose. When wives cheated on their husbands around 1500, they did not "hear" him, but instead "shone" him.
The lens of the glasses had streaks, inclusions and bubbles. The chemist Otto Schott was only able to present optically perfect glasses in 1884 because he dared to question old glassmaker's recipes. He experimented with new raw materials and was successful.
Author: Bärbel Heidenreich
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