OK, for those of you who like to know how things are made, here is a brief lesson in making glass, the material itself.

First, the raw ingredients are placed in a pot. These include:

• silica sand (usually washed to remove extraneous material)
• soda (originally ash from plants, now potash which is mined) to make the silica melt at a lower temperature
• lime (to make the soda glass insoluble) from lime stone, and later flint (from which we get the term flint glass)
• any colorants needed.

Cullet, recycled glass, can be used in part to make up the silica portion of the mixture. The exact proportions of the ingredients depends on the ultimate purpose for the glass and the manufacturer’s experience.

The pot is moved into the furnace, where the ingredients melt together. The glass is heated until all air bubbles have been removed. This takes place at 2400 degrees Fahrenheit (1315 degrees Celsius). The molten glass, also called metal (presumably because the same process is used in preparing metal for pouring), appears white-hot.

The temperature of the glass is dropped to 2000 degrees Fahrenheit (1100 degrees Celsius) for working. The glass itself has an orange glow. At this point mechanized processes for forming or molding the glass can begin. The glass loses heat to the molds forming it and loses the orange glow, but it is still too hot to touch. Each piece is cooled as it travels conveyor belts.  If cooling is slow enough, the resulting piece is tempered. If not, it moves into a lehr, where the heat rewarms the glass and it is left to cool slowly. This relieves any stress within the glass body so it is more robust.

For hand working, the glass maker works at a temperature between 1600 and 1900 degrees Fahrenheit (870-1040 degrees Celsius). A gather is taken out of the pot using a blow-pipe.  The glass worker forms the preliminary shape either by hand or in a mold. Using his lung power, the glass blower then inflates the vessel to the size desired. One of the great characteristics of glass is that at this temperature the item retains its shape as it is inflated: the item gets longer and larger around, but any surface details stay on the surface, stretching but not going away. As the glass cools during forming, it is returned to the glory hole to be reheated.

When the piece is complete, it is put in the lehr for annealing. The glass pieces are heated to between 800-900 degrees Fahrenheit (430-480 degrees Celsius), and left to cool slowly. The annealing process allows the structure of the atoms to realign themselves, which reduces the internal strain created by working the glass over and over to make the form. Without annealing, the glass may shatter with a light blow or even, eventually, crumble. Glass which sluffs off parts of itself has “glass sickness.”

Once the glass body is complete and cool, it can be further treated with sand-blasting, acid-etching, faceting, polishing and/or fuming to produce the final product.

For the curious: for window glass, the molten glass is directed in a wide, thin ribbon from the pot onto molten tin, where the glass floats away. The glass cools as it gets further from the furnace, and is eventually taken from the tin bath onto rollers, and directed into machines to apply any chemicals needed and into the cutting machines which finally cut the glass into sheets. Glass made this way is called float glass, and its invention in England in the 1950′s revolutionized the window glass industry.

For more details, see the Wikipedia article on glass.

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Enamel paints, basically liquid glass, are used on glass objects for decoration. Enamel paints can be cold applied, where simple drying is all that is necessary, or hot applied, where the glass item must be heated to some extent to fix the painted decoration. Cold enamels are subject to wearing off with use, and must be washed by hand.

Floral enamelled and gold decoration have been added to this cased luster.

Gold is also added by hand to many glass objects. Here the gold emphasizes the circles cut in the white layer to show the pink layer of glass inside of it.

(Photo by the author.)

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Once the shape has been made by the glass blower, a spout can be formed by pulling the lip. Additional glass is gathered from the melting pot, and formed into a handle.

Clear handles added after shaping of the blue vase.

This handle is applied before the object is annealed.

Another form of decoration is called threading. A gather of glass is pulled out until it is a narrow stringer. This stringer is then wound around the object, usually at the neck.

Blue glass threads around the neck of a blue vase.

(Photos by the author.)

Look for this additions to glass vessels. They indicate that the object was made by hand and not molded, although handles and threads can be added to molded glass objects after they are removed from the mold and before annealing.

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Cased glass involves two or more layers off different colors of glass, melded into one body and formed into shape. The layers are cut into to show each color.

Green glass cased in white, cut to show.

This technique has been in use for centuries. The objects so formed are frequently also embellished with enamel paints and gold, as in this case. Altogether, these efforts produce beautiful objects.

(Photo by the author.)

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Cut to clear is a technique where a gather of colored glass is lined with a gather of clear glass.  Heat melds the two layers together and the shape is formed.  After the glass has cooled, the cutter uses copper wheels to cut through the colored glass to show the clear inside.

An example to cut-to-clear.

This technique has been in use for centuries.  As you can see, it produces a beautiful hurricane for candles, and creates beautiful glasses and vases as well.

(Photo by the author.)

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Glass blowing has been a skilled craft for over two thousand years. The ingredients of sand, soda ash, lime and colorants are mixed together in a furnace crucible and heated to 2400° Fahrenheit, at which temperature the ingredients melt together and any bubbles within the batch escapes.  After the temperature is reduced to 2000° Fahrenheit, a “gather” of glass is picked up using the pre-heated end of a blowing pipe. A bubble of air is formed within the glass gather using the force in a person’s lungs. By reheating the glass bubble as needed in the glory hole of the furnace, it can be further inflated to the size needed for the job in hand. Shaping is done using soaked wooden forms and paddles, and metal shears and one-piece molds. During the entire shaping process the glass must be kept in rotation to prevent it from sagging into itself and ruining the vessel. Once the glass piece is the right size and shape, it is transferred to a solid metal rod called a punt. As a part of this process, the glass is cut from the blow pipe and the opening can then be formed into the desired shape: a round even lip for a cup or a bowl, a flared dish, the pouring spout of a pitcher, or the gather can be shaped into a flat round circle. Additional glass embellishment can be made at this time, like an edge of a different color, or a handle added to a pitcher. When all work on the object is completed, the finished shape is cut from the punt into the arms of another glass worker wearing an asbestos apron and gloves, who places the object into the annealing oven where the rate of cooling can be controlled and thermal shock avoided. The glass blower then gets a new gather of glass and the whole process is repeated. To function smoothly, there are usually three or four helpers for each glass blower, and teams that have been together for a while perform the glass dance with few words and the grace of those who have mastered their craft. If you ever get a chance to watch this process, do so. It is fascinating.

Machine-molded glass vessels are made by pressing the correct amount of glass between two forms. The hot glass is extruded from the furnace in a long bar. The appropriate amount of glass is cut from the bar and falls into the heated outer mold. The heated inner mold presses down until the glass is formed into the desired shape. Once the inner mold has retracted, the molded glass form is lifted from the outer mold. Reheating the glass melts it just enough to round the edges and in some cases remove the mold marks. If this is not possible, the mold makes can be removed by grinding or polishing the glass object once it has been annealed.

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The color of glass depends on the mineral or metallic oxides added to the glass batch. These metallic oxides include tin, silver, copper, vanadium, chromium, manganese, cobalt and iron.

Beautiful green glass made into a funky green bottle.

Soda-lime glass has an intrinsic greenish color not visible if the glass is thin. A real green, like in wine bottles, is created by adding chromium oxide. Green beer bottles are created from glass with iron oxide added.

Sample of one green color used for beer bottles.

Manganese is added to soda-lime glass to counter the intrinsic green color, and make the glass a purple if more is added.

A medium purple and medium amber glass.

Cobalt is added to glass to make it blue, suitably enough. This color is the favorite of the blue bottle collectibles owners.

A sample of a cobalt blue bottle.

Copper oxide make glass a turquoise color. Nickel creates colored glass from blue to violet to black.

Dark red is a very attractive color of glass.

Gold is used to create ruby glass. If the glass does not have as much gold added, it is called cranberry glass.

Cranberry glass vase.

Selenium and copper also produce red colors.

Red glass comes in many shades, of which this is one.

Silver produces colors from yellow to orange. The only reason you need to care about how the color is created is in specific cases. If you are collecting ruby glass items, you need to know that it involves gold, and therefore will be more expensive. If you find red glass items that are less expensive, you must suspect that it is colored using some other method than gold, and decide if you really want these other kinds of glass. With other colors of glass, the colorant is less important.

Blue bottle with clear glass stopper.

For more information, go to Wikipedia and search for “glass coloring and color marking.”

The images on this post are from Stock Xchnge, except for the two with the blue velor background, which are by the author.

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