What is Crystal? The History of Glass Making!
No one knows exactly when or where glass was first made. Glass appears to have been produced as far back as the second millennium BC in Mesopotamia. Glass was a lot less common back then than it is today.
The art of glass making eventually reached Egypt. The Egyptians used a method called core-forming. A shaped core was made of clay and dung, then molten glass was wrapped around it and shaped by rolling it on a smooth surface.
It was much later, around the end of the 1st century BC, that a new method, glass blowing would revolutionize glass production. This art was probably discovered along the Eastern Mediterranean coast, probably in Syria. By blowing through a hollow tube, the experienced glassblower can quickly produce intricate and symmetrical shapes out of the "gather" of molten glass at the end of his tube (rod). Alternatively, he can blow the molten glass into a mold.
The glassblowing innovation, along with the backing of the powerful Roman Empire, made glass products more accessible to the common people. As the size of the Roman Empire increased, the art of glass making spread to many countries.
A flourishing glass industry did not develop in Europe until the end of the 13th Century, when Venice became a major glass making center. They may have picked up their glass making techniques through their contacts with the Near East countries during the Crusades. The Venetians provided the link between the ancient & modern glass making arts. Venetian glass was noted for its brilliance and for its light, imaginative forms.
By the 15th Century, Venice had become the major producer of glassware in Europe. So highly esteemed were the Venetian glass makers, that they were forbidden to leave the Island of Murano, lest their precious trade secrets be imparted to others.
However, it was to be another two centuries later, in 1676, an Englishman named George Ravenscroft searching for a way to improve the luster and clarity of his glassware found that by adding lead oxide to his molten mixture he could not only improve the clarity (crystal clear) of the glass, but dramatically increase the weight; the index of refraction and the ability to cut the material without fracturing. This new glass became known as lead crystal and very quickly became the perfect medium for glass cutters and engravers. Over the past 300 years, their skills, proudly passed from generation to generation, have given lead crystal its world famous reputation.
The presence of lead in crystal softens the glass and makes it more accessible for cutting and engraving. Lead increases the weight of glass and causes the glass to diffract light. Lead crystal is made from a mixture of sand, potash (potassium carbonate) and lead oxide. The components are heated to at least 2,400 degrees celsius until molten and red-orange in color. The temperature is then slowly reduced to the "working" temperature. Glass can contain up to 40% lead, if maximum hardness is desired. On the other hand, crystal can contain less that 24% lead if it has a high proportion of barium oxide, which ensures high quality light diffraction.
The main difference between glass and crystal is the lead content. Glass is composed of sodium and potassium. Crystal is also composed of sodium, but in addition, it contains silicone, and has a lead content of 10-24%. The lead makes the glass heavier and, most importantly, the lead changes the refraction index of the item making it appear brighter and cleaner. The refraction index being higher, the lead crystal is not as brittle as standard glass making it possible to make deeper and more complicate cuts in crystal.
Some Common Glass and Crystal Making Techniques:
Pantograph or Gulloche
The pieces are dipped in wax and then the wax is scraped away in a particular pattern. After this is done, the glass is dipped in a mild acid. Where the wax has been scraped away, the acid burns a pattern in the glass.
Sandblasting
Parts of the piece that are not to be sandblasted are covered and then a small nozzle is used to shoot a fine stream of silica at the piece. This technique is often used in combination with cutting.
Coloring the glass
To make colored glass, different pigments are added to the glass-mass. For example, to produce green, copper oxide is added to the mixture. To produce blue, cobalt oxide is added. It is possible to produce black, brown, red, orange, yellow and purple glass in a myriad of shades.
Cutting
Although it is possible to make some cuts in sodium potassium glass, lead crystal is the material of choice for this technique. Because of the higher refraction index of lead crystal, cuts in the crystal add both to the interest and appearance of the piece, as well as, making it appear very clean and bright.
Machine Engraving
Machine engraving is used on both hand-made and automatic production pieces. This technique is used to ensure uniformity from piece to piece.
Lead crystal, which is mouth blown in the traditional method, may from time to time have a few minor bubbles contained within the piece. This is generally accepted as a by-product of the traditional proccess and does not detract from the piece. On occasion, small barely perceivable lines may be detected within a piece of crystal. These represent slight variations in the density of the molten crystal, which remain after the crystal has cooled. These lines are called cords and are also considered a by-product of the traditional process.
It should be noted that each piece of true hand cut lead crystal is a unique product of the individual artisan or artisans who have produced it. For example, within a set of stemware of the same pattern, there may be very slight variations in pattern size and spacing of the elements. This only serves to embellish the charm and appreciation of such artisanship and is by no means considered a detriment. Absolute product consistency is only achieved by fully mechanized production, not by a craftsperson.
