by Ron Fox
Drinking vessels have been made from many different materials throughout the centuries. In this article we will examine those made of glass. Chemically, glass is a silicate. It is made from sand melted in a furnace with the aid of an alkaline flux. Glass is considered a liquid in solid form.
The properties of glass change if you change its components. Soda ash was used in all glass made from the 15th through the 18th centuries and was obtained from the ash of seaweed from the Mediterranean. This gave the glass a soft nature, and it hardened slowly. Potash, obtained from the ash of deciduous trees, had been used as early as the 11th century. It gave the glass a very hard nature, and it hardened quickly. Because it made the glass more malleable, soda ash gave the glass blowers more time to work the glass. In the 18th century, lead oxide was substituted as the flux. Lead oxide melted easily, was soft, and gave the glass a brilliant shine. For the complex cutting done in the late 19th century, this was the most suitable flux.
The natural color of glass is actually light green or light brown, due to impurities in the ingredients. To give glass a clear look, it had to be de-colored. This was accomplished by adding manganese dioxide. Manganese dioxide has a reddish-violet color which, when used in correct proportions, cancels out the light green. Other chemicals were also used which achieved the same result.
There are three methods used to shape glass: freehand blown, mold blown, and pressed.
Freehand Blown Glass
With this shaping method the desired glass mix was put into a clay pot within a furnace. When heated to 1100°F (about 600°C), the ingredients melted or liquefied. The glass worker used a long, hollow iron rod, which he laid into the clay pot through a hole or small door in the furnace. A ball of glass would form and be “gathered” on the end of this rod. The rod with the glass ball was then removed from the pot and the worker would blow into the opposite end of this rod, or blowpipe, by mouth. The taffy-like ball of glass would begin to inflate. The glass worker would have to keep reheating the glass to keep it in this pliable state. He also had to keep spinning the blowpipe to create a centrifugal force, neutralizing the gravity which would otherwise pull the glass “balloon” out of shape. He would shape the glass using watersoaked wood paddles of various shapes and sizes. More blowing enlarged the piece to the desired size.
At this point, a solid rod, or pontil, was attached to the opposite end of the glass. The blowpipe was then cut free. This enabled the worker to continue to hold the hot glass and shape the opening from which it was blown. Not all blown glass had to be transferred to a pontil rod.
The piece of glass was next put into an annealing kiln for reheating. It was then allowed to cool in an even, gradual manner. This process removed all stress within the piece. If the process was not done slowly enough, the glass would crack or shatter.
Mold-Blown Glass
Whether a wood or metal mold was going to be used, the process of making mold-blown glass always started out like that of freehand blown glass, with the mixture melting in the furnace. If a wood mold was going to be used, the glass was gathered at the end of the blowpipe, then blown into a wooden mold which would give it the pre-designed shape. While the glass was being blown, the rod would be spun continually to eliminate mold marks from the two-part water-soaked wood mold. The shaping paddles and wood molds would remain in the water when not in use.
If relief decorations or indentations were desired on the body, instead of a wood mold, the glass was blown into a carefully designed metal mold. This enabled the glass worker to achieve the desired designs without cutting or grinding. It is easy to distinguish a metal-blown piece from one that was freehand-blown, as the metal-blown interior will be indented where the exterior is extended, and vice versa. The interior pattern is the exact opposite of the outside design.
The use of the pontil rod to shape and widen the opening was the same in both methods. Even the cooling process was the same. But, because the mold-blown glass used carefully-shaped molds, the glass was always even and cylindrical. A piece of glass that is freehand blown will be uneven and not perfectly round. Close examination can reveal which process was used.
Pressed Glass
This is the process by which molten glass was poured, or pressed, into pre-designed molds. Pressed glass steins were shaped using a cylindrical plunger to create the hollow chamber within the body. Steel mold-blown and pressed stein bodies are similar. There are two major differences to look for:
1. Pressed steins are all in one piece. Body and handle are one. Steel mold-blown steins have a handle which was applied separately to the body. When examined, you can see that there are two pieces.
2. The relief decorations or indentations on the exterior of a pressed stein have no effect on its interior. The interior is cylindrical and void of any design. Steel mold-blown glass has an interior design mirroring the exterior.
Pressed glass was the process manufacturers used to cut down on artistic work and time, while achieving a design that would appeal to the buyer.
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*Reprinted from The Beer Stein Journal, November 1994, by permission from Gary Kirsner Auctions.