2 posts tagged “glasstops”

Glass Lab

Naturally occurring glass, such as obsidian, has been used since the Stone Age. Obsidian is usually black and has impurities, and is created when magma from a volcano cools. It was the raw material for extremely sharp knives in certain Stone Age cultures because, like flint, it can be fractured to produce sharp blades or arrowheads. There is speculation that obsidian may also have been polished to create early mirrors during this period.

For example, the Pre-Columbian Mesoamericans used obsidian for many objects, including tools and decorative pieces where it was extensively carved. They also made a type of sword with obsidian mounted in wood. It combined the sharp cutting edge of a blade with the ragged cut of a serrated knife, creating a formidable weapon.

Manufactured glass was possibly first made by the Phoenicians. Although they used glass as a glaze for pottery as early as 3000 BC, there is also archaeological evidence to suggest that the first glass objects were made in Mesopotamia. Their glass beads, seals, and architectural decorations date from around 2500 B.C.

The earliest known glass beads from Egypt were made during the New Kingdom, about 2000 - 1500 B.C., and came in a variety of colors. They were highly prized as a trading commodity, especially blue ones which supposedly possessed magical powers. Also used in glass since early Egyptian history is manganese – one of the oldest glass additives – that turned the glass purple.

In addition to glass beads, the Egyptians made small glass jars and bottles using the core-formed method. This process entails winding molten glass threads around a bag of sand tied to a rod. The glass has to be kept in motion until the required shape and thickness is achieved, and the glass is continually reheated to fuse the threads together. The final step is to allow the rod to cool, then to puncture the bag, drain the sand and remove the rod.

By the 5th century B.C. this technology had spread as far as Greece, and by the 1st century B.C. there were many glass centers located all around the Mediterranean. It was at this time at the eastern end of the Mediterranean, that glass blowing, both free-blowing and mould-blowing, was discovered.

Along with the birth of the Roman Empire came the development of many more new glass techniques, and as the Empire spread so did the popularity of glass. Whether through wars or trade, the use of glass objects and the techniques used for making glass were spread as far north as Scandinavia and the British Isles.

This spreading out resulted in glass artists congregating in areas such as Alexandria in Egypt where the historically significant “Portland Vase” was created, the Rhine Valley where Bohemian glass was developed, and Byzantium, where glass designs became very ornate and processes such as enameling and gilding came into being.

Window glass was also quite commonly used in ancient times, and examples found in Egypt from the 1st century B.C. were translucent and very thick. After the fall of the Roman Empire, the Emperor Constantine moved to Byzantium where the use of glass continued.

Glass reached China by the "Silk Road" that essentially came into being from the 1st century B.C., following efforts by China to create routes to the Western world, and India. It was most used during the time of the Byzantine Empire at its west end, and from the time of the Three Kingdoms to the Yuan Dynasty at its east end.

Trade among countries also developed on the sea, between Alexandria in Egypt and Guangzhou in China. There was also what has been called a "Silk Route" across the Indian Ocean. It extended, along ports on the coasts of India and Sri Lanka, all the way to Roman-controlled ports in Egypt and territories on the northeastern coast of the Red Sea.

The Kushan Empire, in the northwestern part of India, was located at the center of these exchanges. They fostered multi-cultural interaction, the proof of which is their 2nd century hoards that were discovered, filled with glass and other treasures from Greece, Rome, China and India.

In Europe, the popularity for glass had waned, but revived in the 7th and 8th centuries. Objects found on an island near Venice attest to this, and make a connection between Roman times and the later importance of that city in the production of glass. In fact, from the 10th through the 14th centuries, Venetian glass was highly prized.

The 11th century saw the emergence in Germany of new ways of making sheet glass by blowing spheres, swinging them around to form cylinders, cutting them into sheets while still hot, and then flattening the sheets. This technique was perfected in 13th century Venice.

Also around this same time another important technical breakthrough was made in Northern Europe when soda glass (meaning it is made with soda ash) was replaced by glass made from a much more readily available material, potash (obtained from wood ashes). From this point on, northern glass differed from that made in the Mediterranean area, where soda remained in common use.

Glass in the 11th century also included the emergence of glass mirrors in Islamic Spain. And although stained glass was introduced 100 years prior, it was not widely used until the 12th century.

From the 14th century, the center of glass making was the island of Murano in Venice. The Venetian Republic, fearing fire and destruction to the city’s mostly wood buildings, had ordered glassmakers to move their foundries to Murano in 1291. Murano's glass makers were soon the island’s most prominent citizens. Many new techniques were developed here and soon Murano’s lucrative export trade included dinnerware, mirrors, and other luxury items.

What made the glass significantly different was the local quartz pebbles used to make it. They were almost pure silica and were ground into a fine, clear sand that was combined with soda ash obtained from the Levant, which the Venetians held the sole monopoly on. This resulted in a superior form of glass – and the Venetians having a trade advantage over other glass producers.

The glass makers were allowed to wear swords, were not prosecuted by the government and were able to marry their daughters into Venice’s most desirable families. But there was a catch – glass makers weren't allowed to leave the Republic. However, many of them decided to take the risk and set up glass furnaces in surrounding cities and even as far as England and the Netherlands.

Murano’s glass makers held onto their monopoly on quality glass for centuries, developing or refining many technologies such as crystalline glass, enameled glass, glass with threads of gold, multicolored glass (“millefiori”), milk glass and imitation gemstones made of glass.

Conversely, the quality of English glass initially left a lot to be desired – in fact, in a court case anything a witness saw through a closed window was not admissible as evidence.

But this changed when, around 1688, a process for casting glass was developed. This led to glass becoming a much improved, and more commonly used, material. France soon perfected the manufacture of plate glass, and by the late 17th century English flint glass with lead oxide became suitable for cut glass designs.

The Crown glass process was used up to the mid-1800s. In this method, the glassblower would spin around 9 pounds of molten glass at the end of a rod until it flattened into a disk approximately 5 feet in diameter. The disk would then be cut into panes. And with the invention of the glass pressing machine in 1827, the mass production of inexpensive glass articles commenced.

In the US, the first glass factory had been established in 1608. The cylinder method of creating flat glass was first used in the 1820s, to commercially produce windows. This and other types of hand-blown sheet glass were replaced in the 20th century by rolled plate glass.