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Page 1 of 3 Glass (art), an amorphous substance made primarily of silica fused with alkaline at a high temperature. The silica is generally obtained from sand, quartz or flint, and the alkali is generally soda-ash (obtained from seaweed) or potash (obtained from brushwood). To these fundamental materials, other ingredients are added to obtain different effects. The addition of lead, for example, produces glass of a distinctive clarity and brilliance. Glass can also be coloured by the addition of sulphides or metallic oxides.
Glass was first made before 2000 BC and has been used for a range of decorative as well as practical purposes. Its use as jewellery, vessels, and ornamental objects, and as window glass, will be considered here. For the industrial applications of glass, see Glass (industry).
Shaping
In its molten state, glass can be shaped by casting, blowing, pressing, drawing, and rolling.
Casting
In casting, known since ancient times, molten glass is simply poured into a mould and allowed to cool and solidify.
Glassblowing
The revolutionary discovery that glass could be blown and expanded to any shape was made in the third quarter of the 1st century BC, in the Middle East along the Phoenician coast.
Glassblowing soon spread and became the standard way of shaping glass vessels until the 19th century.
The necessary tool is a hollow iron pipe about 1.2 m (4 ft) long with a mouthpiece at one end.
The glassblower, or gaffer, collects a small amount of molten glass, called a gather, on the end of the blowpipe and rolls it against a paddle or metal plate to shape its exterior (marvering) and to cool it slightly.
The gaffer then blows into the pipe, expanding the gather into a bubble, or parison.
By constantly reheating at the furnace opening, by blowing and marvering, the gaffer controls the form and thickness.
Simple hand tools such as shears, tongs (pucellas), and paddles are used to refine the form, often while the glassblower sits in the special "glassmaker's chair", one with extended arms to support the blowpipe.
Blown glass can also be shaped with moulds: part-size moulds pattern the gather, which is then removed and blown to the desired size.
Full-size moulds into which the gather is entirely blown impart size, shape, and decoration.
Additional gathers may be applied and manipulated to form stems, handles, and feet, or they may be trailed on and tooled for decoration.
A shaped bubble can be "flashed" with colour by dipping it into molten glass of contrasting colour.
To make cased glass, a gather is placed within, and fused to, one or more layers of differently coloured glass. For finish work and fire polishing at the mouth of the furnace, the gather is transferred to a solid iron rod called a pontil, applied opposite the blowpipe, which is then removed.
When the pontil is cracked off it leaves a "pontil mark" that may be later ground or polished away.
Pressing
Some pressing was involved in the production of ancient cast wares to ensure that the glass had full contact with the mould. Islamic artisans used simple handpresses to form glass weights and seals.
European manufacturers rediscovered the technique in the late 18th century, using it to make decanter stoppers and the bases of stemmed tableware.
In the 1820s patents were taken out, particularly in the United States, that led to the development of fully mechanical pressing.
In this process, a gather of glass is dropped into a mould, and a plunger then squeezes the glass between itself and the outer mould and forms the final shape.
Both the mould and the plunger may be patterned to impart decorative design to the object being made.
Drawing
Molten glass can be drawn directly from the furnace to make tubing, sheets, fibres, and rods of glass that must have a uniform cross section. Tubing is made by drawing out a cylindrical mass of semifluid glass while a jet of air is blown down the centre of the cylinder.
Rolling
Sheet glass, and plate glass in particular, was originally produced by pouring molten glass on a flat surface and, with a roller, smoothing it out prior to polishing both its surfaces. Later it came to be made by continuous rolling between double rollers.
Lampworking
Lampworking consists of the reworking of preformed and annealed glass, generally to produce decorative toys and figures. Rods and cylinders are reheated by air-gas or oxygen-gas flames and refashioned by hand or machine.
Annealing
After being formed, glass objects are annealed to relieve stresses built up within the glass as it cools (see Annealing). In an oven called a lehr, the glass is reheated to a temperature high enough to relieve internal stresses and then slowly cooled to avoid creating new stresses.
Decoration
After annealing, glass can be embellished in various ways, the most common being engraving, etching, and painting. In wheel-engraving, the design is produced by holding the glass against rotating discs of various sizes. In diamond-point engraving, a freehand technique, the surface is hammered with a diamond-pointed stylus. Both diamond-point and wheel-engraving are suitable for producing pictorial designs and inscriptions. Stipple engraving, whereby the glass is pricked with tiny dots, produces a design that appears almost to be breathed onto the glass; it is a delicate technique particularly associated with 17th-century Dutch glassware. Designs on glass can also be produced by etching the surface with acid, or by sandblasting; this produces a frosted effect and is particularly suitable for large items such as window glass.
Glass can also be painted with enamels, which are then fused onto the surface in a low-temperature kiln. In gilding, gold leaf, gold paint, or gold dust is applied to the glass and sometimes left unfired; low-temperatire firing, however, is necessary to render it permanent.
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