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Art glass is an item that is made, usually for decoration but also for purpose, from glass that has been worked into a form that is considered pleasing to the eye. Such techniques include stained glass windows, leaded lights (also called leadlights), glass that has been placed into a kiln so that it will mould into a shape, glassblowing, sandblasted glass, and copper-foil glasswork. Art glass has grown in popularity in recent years with many artists becoming famous for their work; and, as a result, more colleges are offering courses in glass work. Many amateurs now undertake making art glass a hobby.
Stained glass, such as the windows that are seen in churches, are windows that contain an element of painting in them. The window is designed. After the glass has been cut to shape, paint that contains ground glass is applied, so that, when it is fired in a kiln, the paint fuses onto the glass surface. Following this process, the sections of glass are placed together and held in place with lead came that is then soldered at the joints. Leadlights and stained glass are manufactured in the same way, but leadlights do not contain any sections of glass that have been painted.
Glassblowing is one of the most used technique for creating "art glass" and is still favoured by most of today's studio glass artists. This is because of the artist's intimacy with the material, and an almost infinite opportunity for creativity and variation at almost every stage of the process. Glassblowing can be used to create a multitude of shapes and can incorporate color through a wide range of techniques. Coloured glass can be gathered out of a crucible, clear glass can be rolled in powdered colored glass to coat the outside of a bubble, it can be rolled in chips of glass, it can be stretched into rods and incorporated through caneworking, or it can be layered, cut and fused into tiles, and incorporated into a bubble of glass for intricate patterns through murrine. "Blown glass" refers only to individually hand-made items but can include the use of moulds for shaping, ribbing, and spiking to produce decorative bubbles. Glass blown articles must be made of compatible glass or the stress in the piece will cause a failure.
Kiln formed glass is usually referred to as warm glass, and can be either made up from a single piece of glass that is slumped into or over a mould or different colours and sheets of glass fused together. The process of hot glass is highly scientific in that the types of glass and temperatures that they must be fired at is quite complicated operation to undertake correctly. Art glass that is kiln formed usually take the form of dishes, plates or tiles. Glass that is fused in a kiln must be of the same co-efficient of expansion (CoE). If glass that does not have the same CoE is used for fusing, the differing rates of contraction will cause minute stress fractures to form and, over time, these fractures will cause a piece to crack. The use of polarizing filters to inspect the work will determine if stress fractures are present.
Glass can be decorated by sandblasting the surface of a piece in order to remove a layer of glass, thereby making a design stand out. Items that are sandblasted are usually thick slabs of glass into which a design has been carved by means of high pressure sandblasting. This technique provides a three-dimensional effect but is not suitable for toughened glass as the process could shatter it.
The technique of using copperfoil is mainly used in the construction of smaller pieces such as Tiffany style lamps, and it was, in fact, frequently used by Louis Comfort Tiffany. It consists of wrapping cut sections of glass in a self-adhesive tape that is made out of thin copper foil. This technique requires a great deal of dexterity and is also very time-consuming. After the sections have been foiled, they are soldered together in order to form the item.
Most antique art glass was made in factories, particularly in the UK, the United States, and Bohemia, where items were made to a standard, or "pattern". This would seem contrary to the idea that art glass is distinctive and shows individual skill. However, the importance of decoration – in the Victorian era in particular – meant that much of the artistry lay with the decorator. Any assumption today that factory-made items were necessarily made by machine is incorrect. Up to about 1940, most of the processes involved in making decorative art glass were performed by hand.
Manufacturers got around the problem of an inherent similarity in their products in various ways. First, they would frequently change designs according to demand. This was especially so in the export-dependent factories of Bohemia where salesmen would report sales trends back to the factory during each trip. Second, the decoration for mid- and lower-market items, often done by contracted "piece" workers,  was often a variation on a theme. Such was the skill of these subcontractors that a reasonable standard of quality and a high rate of output were generally maintained. Finally, a high degree of differentiation could be gained from the multiplication of shapes, colours, and decorative designs, yielding many different combinations. Concurrently, from the same factories came distinctive, artistic items produced in more limited quantities for the upper-market consumer. These were decorated in-house where decorators could work closely with designers and management in order to produce a piece that was profitable.
Many items that are now considered art glass were originally intended for use. Often that use has ceased to be relevant, but even if not, in the Victorian era and for some decades beyond useful items were often decorated to a such a high degree that we can now appreciate them for their artistic or design merits.
Some art glass retains its original purpose but has come to be appreciated more for its art than for its use. Collectors of antique perfume bottles, for example, tend to display their items empty. As items of packaging, these bottles would originally have been used and thus would not ordinarily have been considered art glass. However, because of fashion trends, then as now, producers supplied goods in beautiful packaging. Lalique's Art Nouveau and Art Deco designs and Joseph Hoffman's Art Deco designs have come to be considered art glass due to their stylish and highly original decorative designs.
A major shift in the definition of what constituted "art glass" came with the 1977 publication of the book Glass - Art Nouveau to Art Deco by Victor Arwas. Following the book's publication there was a growing recognition that moulded, mass-produced glass with little or no decoration but high artistic and fabrication quality such as that produced by Lalique should be considered art glass.
Up-market refined glassware, usually lead crystal, is highly decorated and is revered for its high quality of workmanship, the purity of the metal (molten glass mixture), and the decorative techniques used, most often cutting and gilding. Both techniques continue to be used in the decoration of many pieces made from lead crystal, and nowadays these pieces are regarded as art glass.
Cut glass is most often produced by hand, but automation is now becoming more common. Some designs show artistic flair, but most tend to be regular, geometric, and repetitious. Occasionally, the design can be considered a "pattern" to be replicated as exactly as possible, with the main purpose being to accentuate the refractive qualities, or "sparkle", of the crystal – certainly an aesthetic consideration, but not generally considered artistic.
A clear exception could be made for the highly distinctive cut crystal designs which were produced in limited quantities by designers of note. Examples are the designs of Keith Murray for Steven & Williams and those of Clyne Farquharson for John Walsh Walsh. A relatively new term is coming into use for this genre: "Art Cut" 
Detail of sculpture by artist David Patchen.