Glass – A Collector’s Guide to the History and Craft
In the modern world glass is a ubiquitous and largely unremarkable product. Made by heating a mixture of silica and either potash or soda, the prized properties of glass include translucency and strength. Modern manufacturing methods allow high quality glass to be made and shaped cheaply and easily, finding use in everything from the decorative arts to housewares to industry. However throughout much of history glass has been a precious commodity; as valuable as gemstones, sought by the wealthy and powerful, and the secrets of its manufacture closely guarded.
Prior to the industrial revolution, all glass had to be made and worked by hand, at extremely high temperatures. It was difficult, time consuming, and dangerous work. One wrong move could leave the artisan doused in deadly molten glass, injuring them and wasting their precious materials.
Although glass as we know and use it is chiefly a man-made material, it does occur naturally. To make glass in its most primitive form, all that's really required are minerals and heat. Minerals such as obsidian are forms of glass formed by the high temperatures found in volcanoes. Sometimes, glass is formed during storms when lightning strikes sand or even pavement. Although science has a broader definition, the material commonly referred to as glass is made of silica, either from sand itself or pulverized silicate minerals like sandstone. Silica produces strong, clear glass, but it has a relatively high melting point that makes it difficult to work with. The higher a mineral's melting point, the hotter it has to be to get it to a state that allows it to be worked and shaped. Therefore, other substances, like potash, are added to the silica to decrease its melting point. Lime, from limestone, is also added to help homogenize the mixture, and render it insoluble. Other ingredients can be added at this time too, which will further alter the properties of the glass. Different ions will create colored glass, while certain metals will affect how the glass sparkles and refracts light.
All of these ingredients are combined and heated to about 2500°F for several hours, sometimes over the course of a day. Once the separate glass ingredients have all melted together and combined completely, then the glass must be allowed to cool. It generally isn't allowed to cool completely, however, since the glass itself must still be several hundred degrees to be able to be worked. Glass will harden rapidly as it cools, so it is imperative that the artisan work the glass quickly at this stage, so they can create their design before it has a chance to harden.
The first glass was produced around 3500 years ago in the Middle East. However, according to Pliny, merchants first discovered glass around 5000 BC when they rested hot, heavy cooking pots on top of blocks of nitrate placed by their fires. When the heat of the fire, the nitrate, and the sand below combined, a strange, thick liquid formed, and oozed out. When glass first began to be produced deliberately, it was used mainly as a glazing and waterproofing agent for pottery. The earliest fragments of all-glass objects date back to the 16th century BC, and were found near what used to be Mesopotamia. In the 15th century, Pharaoh Thoutmosis III took three glass makers captive, and brought them to Egypt from Asia. Eventually, Alexandria would become a major glass making cultural center. From there, it is believed that glass making spread to Italy, and areas like Murano which are still famous for their glass.
Few advances were made in the art of glass making until about 20 BC, when Syrian artisans discovered the techniques needed to blow glass. The Romans later discovered that glass could be blown inside molds to produce different shapes and patterns. Remarkably, modern glass blowing has changed very little from these early techniques.
By about 1000 AD, regional differences in glassmaking began to appear. Whereas much of world created soda glass, Alpine areas began to switch to using potash obtained from burning the area's many trees. Other areas that were less heavily forested, like the Mediterranean, continued to produce soda glass. As a result, glass made north of the Alps began to differ from that produced by countries such as Italy. From the 13th century to the 16th century, Italy would remain the dominant figure in many sectors of the glass making industry.
Around the 1300s, France created (and jealously guarded) the secrets to making “crown glass,” a method of creating thin sheets of glass in a circular form. France guarded the secret to making crown glass so jealously, it wouldn't be made in England until over three hundred years later.
In a quest to match the quality of Italian and French manufacture, and reduce England’s dependence on foreign sources, English glass maker George Ravenscroft was commissioned by the Worshipful Company of Glass Sellers in 1674 to produce a domestic alternative to Venetian crystal based on reconstituting quartz (which is pure silica) and potash (obtained from trees). Substituting lead oxide for a portion of the potash, Ravenscroft invented lead crystal (or flint glass), a glass that was stronger than the Venetian glass that could be faceted and engraved, and refracted light brilliantly. France made significant advances in producing optically sound plate glass in the late 1600s. Prior to this, instruments like mirrors were often made of polished metal, or inferior glass that displayed warped, distorted reflections.
In the 19th century techniques were developed which enabled high-volume production of glass. Whereas prior glass making techniques all relied on glass blowing to an extent, men like Fredrich Siemens paved the way for glass making to be automated, leading to higher-quality, less expensive glass. Siemens is the inventor of the tank furnace, which allowed for much larger volumes of glass to be produced than older methods.
The years 1900-1925 were a time of great advancement in modern glass making. Michael Owens developed an automated bottle blowing machine, which saw very heavy use in the 1920s. Belgian inventor Emile Fourcault, , patented the “Fourcault Process,” a method for producing a continuous sheet of flat glass from a tank. Other inventors gradually worked on and improved the process of producing sheets of glass, leading to stronger, clearer glass, and less waste.
The 1950s saw the invention of “float glass,” A technique pioneered by British glass manufacturer Sir Alastair Pilkington. Float glass is produced when molten glass is poured onto a bed of a denser liquid, usually molten tin or lead. As the glass cools and hardens, it's lifted off of this bed. Because the liquid is perfectly smooth, this glass doesn't require the wasteful, labor-intensive polishing and grinding that other processes did, and could produce glass of varying thickness simply by adjusting the speed at which the molten glass was poured and allowed to spread.
As one studies the development of the glass craft, it is important to consider the challenges earlier artists faced in achieving the necessary 2500°F temperatures. Imagine, a glass worker in the Middle East over 3,500 years ago, without the benefit of an electric furnace and modern heating and insulation technology. All they have to work with are charcoal, dung, coal, and other raw fuel sources, and a bellows to oxygenate the flame and raise its temperature. Needless to say, getting to the high temperatures needed to produce quality glass wouldn't be very easy! As a result of artisans who perfected their glass making technique, certain cultural areas became known for the beauty and quality of their glass. These techniques were very jealously guarded, too. Renaissance Italy threatened any glass makers who attempted to take their trade outside of the country with a death sentence. Later on, France would attempt to seduce Italian glass makers away by offering them citizenship and tax exempt status. Glass makers were highly in demand, and treated as precious commodities wherever the worked.
There are a wide variety of ways to make glass. One of the most familiar is glassblowing, where a blob of molten glass is taken on the end of a long tube, and turned and shaped by the artist's breath until the desired form is achieved. Some items, like glass bottles or jars, can be made by blowing glass into a bottle mold. To make sheets or rods of glass (as are still used for many scientific applications today) molten glass is drawn over a form and allowed to cool. Once cooled, the form is removed, leaving behind the shaped tube or sheet of glass. Pressed glass, much like its name implies, is formed by pressing glass into a metal mold. Pressed glass is useful for controlling the density of the final product, and can be used to create intricately patterned pieces of glass. In fact, since the molds used to form pressed glass very rarely produce perfect, unblemished pieces, elaborate decorations are an excellent way to disguise bubbles, wrinkles, and other imperfections.
Creating colored glass requires its own set of techniques and areas of expertise. Adding small amounts of certain materials to glass can create beautiful colors. Not adding enough won’t produce much of an effect, but adding too much can alter the composition of the glass, ruining an entire batch. Blue glass can be made with the addition of the metal cobalt, brown glass is produced by iron sulfides, and different copper oxides produce green. As beautiful as it is, red and pink are two of the most uncommon colors for antique glass because gold was used in the production of those colors making them very expensive to produce compared to the metals needed for green, blue, or brown glass.
Cutting, engraving, and otherwise adorning glass also requires a high degree of expertise. Aside from the obvious safety precautions needed when dealing with freshly cut glass, it is all too easy to ruin a glass piece with one wrong cut. Engraving glass requires very sharp tools and a steady hand, while etching glass requires the use of caustic chemicals such as hydrofluoric acid. Hydrofluoric acid is a very powerful, very corrosive material that's difficult to store, and can cause very severe burns if it's handled improperly.
During the Middle Ages, however, when stained glass rose to the peak of its popularity as an art form, the art required very expensive glass, colored with very expensive precious materials, cut by highly trained artisans, and assembled by skilled craftsworkers. Only churches and cathedrals could afford this labor intensive, highly skilled art form, and even then they usually required the assistance of a wealthy patron. Gradually, glass achieved the rank of a status symbol. It was outrageously expensive, and much more fragile than other materials, limiting its practicality. If a person passed by a home with glass windows, or a shiny glass mirror, they knew that the occupants had money to spare.
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