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Introduction of glass blowing and lampworking
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C2 Glassblowing
The revolutionary discovery that glass could be insufflated 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 (about 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 molds: Part-size molds pattern the gather, which is then removed and blown to the desired size. Full-size molds 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 color by dipping it into molten glass of contrasting color. To make cased glass, a gather is placed within, and fused to, one or more layers of differently colored 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.
In 1903 a fully automatic blowing machine was perfected, thereby making mechanical glassblowing possible.
C3 Pressing
Some pressing was involved in the production of ancient cast wares to ensure that the glass had full contact with the mold. 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 U.S., that led to the development of fully mechanical pressing. In this process, a gather of glass is dropped into a mold, and a plunger then squeezes the glass between itself and the outer mold and forms the final shape. Both the mold and the plunger may be patterned to impart decorative design to the object being made.
C4 Drawing
Molten glass can be drawn directly from the furnace to make tubing, sheets, fibers, 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 center of the cylinder.
C5 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.
D Lampworking
Lampworking consists of the reworking of preformed and annealed glass, generally to produce scientific laboratory equipment and decorative toys and figures. Rods and cylinders are reheated by air-gas or oxygen-gas flames and refashioned by hand or machine.
E 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 introducing new stresses. Stresses can be added intentionally to impart strength to a glass article. Because glass breaks as a result of tensile stresses that originate across an infinitesimal surface scratch, compressing the surface increases the amount of tensile stress that can be endured before breakage occurs. A method called thermal tempering introduces surface compression by heating the glass almost to the softening point and then cooling it rapidly with an air blast or by plunging it into a liquid bath. The surface hardens quickly; the subsequent contraction of the slower-cooling interior portions of the glass pulls the surface into compression. Surface compressions approaching 2460 kg per sq cm (35,000 lb per sq in) can be obtained in thick pieces by this method. Chemical strengthening methods also have been developed in which, through an ion-exchange process, the composition or structure of the glass surface is altered and surface compression introduced. Strengths exceeding 7000 kg per sq cm (100,000 lb per sq in) can be attained by chemical strengthening.
F Decoration
After annealing, a glass object may be embellished in a number of ways. Some of them are as follows:
In cutting, to produce cut glass, facets, grooves, and depressions are ground into the surface with rotating disks of various materials, sizes, and shapes and a stream of water with an abrasive. The steps are marking the pattern, rough cutting, smoothing, and polishing.
Designs are engraved by means of a diamond point or a metal needle, or with rotating wheels, generally of copper.
In the etching process intaglio decoration is achieved with acid, the results varying from a rough to mat finish.
In sandblasting, fine grains of sand, crushed flint, or powdered iron are projected at high speed onto the glass surface, leaving a design in mat finish.
In cold painting, lacquer colors or oil paints are applied to glass but are not affixed by firing.
In enamel painting, enamel colors are painted and then fused onto the surface in a low-temperature firing.
In gilding, gold leaf, gold paint, or gold dust is applied to glassware and sometimes left unfired; low-temperature firing, however, is necessary for permanency.
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