Glass

Virtually all the basic techniques of glass-making in use today were known in antiquity, although on a technical level, furnaces, chemistry and mass-production methods have been improved.

Glass is made by melting together silica in the form of sand with soda ash and limestone in a furnace at temperatures of 1700oC. Other materials can be added to produce different colours or properties.

Glass is chemically more like a liquid, but at room temperature it is so viscous or 'sticky' it appears solid. At higher temperatures glass gradually becomes softer and more like a liquid. It is this latter property which allows glass to be poured, blown, pressed and moulded for such a variety of shapes and uses. 

Depending on the end use, the composition of the glass and the rate at which it is allowed to cool will vary, as these two factors are crucial in obtaining the properties the glassmaker is seeking to achieve.

In this section:

Cast Glass

Cast glass uses a mould to form the molten material in a kiln and may pre-date blown glass. There are examples of Ancient Egyptian experiments with this technique using "cold core casting" in which the glass was applied to the outside of a mould.

See also Sand Cast Glass and Kiln-Formed Glass.

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Crackle Glass

This ornamental technique gives glass a deliberately rough and fractured appearance. The hot glass is plunged into cold water during blowing, creating a network of cracks and fissures that continue to grow during successive blowings and reheats. A sensitive touch is required to maintain the crackled effect otherwise known as "ice glass". 

Another technique called "overshot" is used for similar effect. The still workable glass is roled in a bed of crushed glass laid out on a slab of marble or metal; the crackled effect remains on the outside while the inner surface remains smooth. The Venetians of the 17th C used this process to create very elegant pieces which have inspired glassmakers throughout Europe.

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Engraved Glass

In glass engraving a tool is used to abrade the surface in order to leave a mark. Many tools are available, ranging from a simple diamond-point hand tool to complex machinery. 

Point engraving involves working with a hand-held diamond or tungsten carbide point. The tool can be used to draw lines, or by tapping the surface of the glass lightly, tiny white dots.

Drill engraving uses a tool with a rotating bur in an electric drill. This tool can create surface effects similar to those of point engraving but can also cut into the glass more deeply. 

Copper wheel engraving creates a very precise mark and is the tool used for traditional cut crystal. A belt-driven lathe operates a range of inter-changeable copper wheels. The wheels vary in width, diameter and profile for making different types of cut. A slurry mixed from carborundum grit, oil and paraffin forming is applied to the turning wheel and the glass is held against it to make the cut. Coarse grit enables rapid and large scale cutting, fine grit more polished, delicate work. Stone and diamond impregnated wheels are now commonly used.

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Etched Glass

Etching is the technique of creating designs on the surface of glass using acidic, caustic, or abrasive substances to remove the surface after the glass has been formed. 

Acid etching consists of immersing cold glass into a bath of hydrofluoric acid to remove material, leaving a frosted surface. Surfaces protected by a coat of bitumen resist the acid and remain transparent. 

Sandblasting uses compressed air to project sand through a nozzle to cut more or less deeply into the surface of the glass. The surfaces to remain smooth are protected by a stencil. 

Etching is most commonly used on the back of the glass for viewing from the front.

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Fused Glass

Pieces of glass are fired in a kiln at a range of high temperatures from 593ºC to 816ºC, hot enough to melt the glass and fuse the pieces together. Although developed by the Romans, there is evidence that the Egyptians were familiar with rudimentary techniques ca. 2000 BC. Fusing was the main method of making small glass objects for approximately 2,000 years, until the development of the glass blowpipe.

Most contemporary fusing methods involve stacking, or layering thin sheets of glass, often using different colors to create patterns or simple images. The stack is then placed inside the kiln and heated until the separate pieces begin to bond together. The longer the kiln is held at the maximum temperature the more thoroughly the stack will fuse, eventually softening and rounding the edges of the original shape. While fused glass techniques are generally used to create glass art, glass tiles and jewelry, the slumping process used with moulds allows the creation of larger, functional pieces like dishes, bowls and plates. Producing functional pieces generally requires 2 or more separate firings; one to fuse the glass and a second to shape it.

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Hand-made Lead Crystal Glass

In 1676 George Ravenscroft discovered that by adding lead oxide to the composition of glass a far more brilliant, highly refractive, sparkling glass could be produced. Besides its appearance lead crystal is much softer than regular glass making it easier to cut.

The maximum lead content is 33%. However, 33% lead crystal requires a lot of skill in forming a shape at the blowing stage. So, lower percentage lead content is sometimes used, although the sparkle is reduced.

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Handblown Glass

Glassblowing technique which was invented by the Phoenicians around 50 B.C. It requires lengthy training and intense concentration. Although glass blowing has reached a high level of sophistication, very little about the technique itself has changed. 

The tip of the blowpipe is first preheated then dipped into molten glass in a furnace. The glass is 'gathered' on to the blowpipe in the same way that honey is picked up on a dipper. The glass is then rolled on a marver, traditionally a slab of marble, but is more commonly a fairly thick flat sheet of steel today. This forms a cool skin on the exterior of the molten glass and shapes it. Then air is blown through the pipe to create a bubble. Then, one can gather over that bubble again to create a larger piece. 

A variety of tools are then used to shape the glass. As the glass cools it begins to stiffen and must be continuously reheated to allow shaping and reshaping. The maker uses a smaller furnace, a "glory hole" for the re-heats.

When the piece is finished, it is placed in an kiln for annealing. This is the process of slowly cooling the glass to room temperature to stabilize its delicate crystalline structure. Rapid temperature changes will cause the glass to crack.
Patterns and color can be applied by rolling the molten glass in powdered color or larger pieces of colored glass called frit. Complex patterns can be created through the use of rods of colored glass known as canes, or with murrine (rods cut in cross-sections to reveal patterns). This refined method of glassmaking was a Venetian Renaissance development. One of the most skilled cane-working techniques is 'reticello', which involves creating two bubbles from cane, each twisted in a different direction and then combining them whilst blowing out the final form.

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Kiln-formed Glass

Kiln casting involves the preparation of a heat resistant mould, often made of plaster. 

A model can be made of wax, wood or metal and after taking a cast of the model, (a process called investment), the model is removed from the mould.

One method of forming a mould is the lost wax method. Using this, a model can be made from wax and after investment the wax can be steamed or burned away in a kiln. The mould is then placed in a kiln with a funnelled opening and filled with solid glass granules or lumps. The kiln is heated to a high temperature, normally 800-1000 C, and as the glass melts it runs, settling into and filling the mould.

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Pate de Verre

Pate de Verre is a form of kiln casting and translated means glass paste. Finely crushed glass is mixed with a binding agent, such as a mixture of gum arabic and water, sometimes with colourants and or enamels. The resulting paste is coated onto the inner surface of a negative mould. The coated mould is fired and the glass is fused creating a hollow object.

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Sand Cast Glass

Sand casting involves pouring molten glass into a pre-formed mould. The sand mould is typically prepared by using a mixture of sand and a small proportion of bentonite ( a water absorbing clay) as a binding agent. This mixture is added to an open topped container. A template, often made from wood, is pressed into the sand to make a clean impression.This impression then forms the mould. The surface of the mould can be covered in coloured sand to give colour to the sand cast glass object. When the mould is complete hot glass is taken from the furnace at about 1200 C and is poured into the mould. During the pouring process, glass or compatible objects may be placed to give the appearance of floating in the solid glass object.

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Stained Glass

Stained glass is a mosaic of coloured and clear glass. Using a drawing as a template, the glass is selected and cut, and each piece of glass is individually coloured using glass paints. The glass is then fired into the surface by heating to approximately 650°C in a furnace. The glass is assembled into panels by bending 'H' section strips of lead around the pieces and soldering at the joins.

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