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Glassmaking: history and techniques

Cup with Blue, White, and Yellow Canes, Greek, 100-1 B.C.E. mosaic, glass, 1-1/2" high x 4-1/16 inches in diameter (The J. Paul Getty Museum,  2003.246)
Glass is magical stuff. When you look at a glass object, you might never guess that it was once a hot, flowing liquid that could be inflated or molded or swirled into almost any shape.
According to the Getty's Art and Architecture Thesaurus, glass is:
an amorphous, inorganic substance made by fusing silica (silicon dioxide) with a basic oxide; generally transparent but often translucent or opaque. Its characteristic properties are its hardness and rigidity at ordinary temperatures, its capacity for plastic working at elevated temperatures, and its resistance to weathering and to most chemicals except hydrofluoric acid. Used for both utilitarian and decorative purposes, it can be formed into various shapes, colored or decorated. Glass originated as a glaze in Mesopotamia in about 3500 BCE and the first objects made wholly of glass date to about 2500 BC.

Precious containers

Blue Perfume Flask with a White Trail, Roman,  1–100 C.E., glass, 4-3/4 inches high 
(The J. Paul Getty Museum,  2003.246)
Glass had both practical and decorative uses in antiquity, just as it does today. The cup illlustrated above was used nearly 2,000 years ago for eating and drinking.
The ancient Greeks, Romans, and Etruscans stored ink, food, cosmetics, and perfumed oil in glass containers.
They used glass tableware and played with glass game pieces. They looked into glass mirrors, lit the night with glass lamps, and gazed through glass windows. The wealthy decorated their homes with glass mosaics, inlays, and statuettes.
Perfume was precious and merited fine containers such as this one.  A glassmaker inflated this vessel and then wound a white trail around the blue glass body to create an elegant pattern.

Core-Formed Glass

White Perfume Flask with Purple Zigzags, Greek, 600–300 B.C.E., core-formed glass, 5 3/16 inches high (The J. Paul Getty Museum, 2004.5)
Core forming was one of the earliest glassmaking techniques. Glassmakers shaped the body of the vessel around a core of ceramic-like material, wound colored trails of hot glass around it, and added handles and a rim. They then let the vessel cool and removed the core. Most early core-formed containers were small flasks for perfumed oil, such as this one, which is only about five inches tall. The coloration of the container shown here indicates that it was meant to imitate marble.

Amber Bowl with White Ridges, Roman, 1–100 C.E., cast glass 4-1/16 inches in diameter (The J. Paul Getty Museum, 2003.475)

Cast Glass

Casting is a technique of pouring hot glass into a mold. After the glass cools, glassmakers use various grinding and cutting techniques to refine the vessel's form and decoration. Decorative patterns are sometimes cut into the sides with a cutting wheel.
Bowls were the most common cast vessels. Pendants, inlays, and other small objects were also created using this technique.
The bowl above has two layers of different colors. Much of the white overlayer was removed through grinding, leaving ridges of white over the amber underlayer.
Bowl with Blue and White Canes, Greek or Roman, 100–1 B.C.E., cast glass, 2-1/16 inches high x 4 inches in diameter (The J. Paul Getty Museum,  2004.24)

Mosaic Glass

Mosaic glass vessels are among the most colorful ancient containers. They were formed by fusing numerous slices or ribbons (lengths) of cane in molds until they melted together into a swirl of colors, as seen on this blue and white bowl.
Multicolored canes and figural compositions for plaques and beads were made by layering different colors and manipulating them into designs. Roman mosaic glass later inspired Venetian glassmakers to create millefiori (Italian for "thousand flowers").
Lidded Container with Amber and White Marbling, Roman, A.D. 1–100 C.E. Roman, marbled glass, 2 inches high x 2-1/4 inches in diameter (The J. Paul Getty Museum, 2003.256)
This pyxis, a lidded cosmetics or jewelry box, is made from marbled glass, a variant of mosaic glass.  The Roman glassmaker created a swirling pattern (similar to agate) by melting multiple colors of glass together
Free-Blown Glass
In the environs of Jerusalem, in about 50 B.C.E., glassmakers discovered they could inflate glass into a bubble at the end of a tube. This new glassblowing technique allowed glassmakers to produce vessels so quickly and cheaply that glass containers began to replace clay ones for household use.
Blue Splashware Cup, Roman, 1–100 C.E, free-blown glass, 4 13/16 inches high x 5-11/16 inches in diameter (The J. Paul Getty Museum, 2003.290)
Blown-glass vessels were decorated using a variety of techniques—pinching, pressing, pulling, painting, applying trails (threads of glass), and rolling in colored glass chips before reinflating (to create splashware). Glass with cut decoration was made to imitate hard-to-cut rock crystal and is often colorless. Painted glass is very rare, and the pigment has often worn away.
The splashware cup above was a luxury item that was probably used in a wealthy home. It was decorated by rolling hot blue glass into glass chips of other colors.

Mold-Blown Glass

Lotus Bud Beaker, Roman, Eastern Mediterranean, 1st century C.E., mold-blown glass, 8-3/8 inches (The J. Paul Getty Museum, 2003.320)
In about 25 B.C.E. glassmakers began to create decorative vessels, such as this cup, by blowing glass into a mold with incised designs. Mold fragments of stone, clay, bronze, and plaster have survived from antiquity. The earliest makers of molds for glassware may have been ceramists familiar with constructing molds for clay vessels.
Mold designs often incorporated decorative elements, such as palm fronds, columns, or lotus buds (as on this vessel), that disguise the joins of different sections.

Decorative Techniques

Glassmakers decorate vessels using a variety of tools and techniques. They use pincers and tongs to pinch, pull, and push hot glass into different shapes and patterns. They wind glass around a vessel to create snakelike patterns and add handles, rims, and feet.
To create a special kind of decorative glass known as splashware, glassmakers roll a vessel in multicolored glass chips, reheat the vessel, and then further inflate it to stretch the chips across the surface to look like splashes of color.
Glassmakers also ornament glass vessels after they cool by painting them or using a lathe and cutting wheel to form decorative patterns. Painted glass vessels are rare, and none have survived with their pigments intact.

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