Table Glass - Glass Composition
The glass that archaeologists can expect to find on colonial and post-colonial archaeological sites is composed of three primary components: silica (normally in the form of sand), soda or potash as a flux, and either lime or lead as a stabilizer (Jones et al. 1985:10). While a visual examination of glass can be used with a limited degree of accuracy, the only way to really determine the composition of glass is through chemical analysis.
Using ultraviolet light to determine glass composition is complex and should be used carefully (Jones et al. 1985:12). Fluorescence is not so much characteristic of the type of glass, but is indicative of refining agents, furnace atmosphere and melting temperature (Newton and Davidson 1989). That being said, a UV light can be very useful in sorting large quantities of glass. A general guide that can be used with caution is shown on the chart.
Click here to see a Quick Reference Table on Glass Composition
- Colorless Leaded Glass - Also known as potash-lead glass. First developed in England in 1676 and by the second half 18th c., was made in abundance in England and Ireland (Jones et al. 1985). Contains at least 20% lead oxide. Leaded glass is relatively soft and takes well to cut decorations (Corning Museum of Glass 2018). Colorless leaded glass is often referred to as “flint glass” by collectors and antique dealers.
- Soda Lime Glass - Historically, the most common form of glass. Glass that uses soda as a flux, either in the form of plant ashes or from table salt converted to soda. Soda-lime glass is relatively lightweight, remains plastic and workable over a wide range of temperatures and lends itself to elaborate manipulative techniques. Venetian glassmakers first discovered how to make soda lime glass colorless in the 13th century. Technological improvements in the glassmaking process at the end of the 18th century and again in 1864 made the use of soda lime glass more common. The post 1864 formula produced a colorless glass that was shiny and took well to press molding. By the end of the 19th century, soda lime glass was replacing colorless leaded glass in tableware forms (Jones et al. 1985).
Note: On this website, any glass that glowed yellow or greenish yellow under short wave UV light was identified as soda lime glass.
- Potash Lime Glass - Glass containing three major compounds: silica (generally 60-75 percent), potash (12-18 percent), and lime (5-12 percent). Slightly denser, but harder and more brilliant, than soda-lime glass. Passes from a molten to a hard state more quickly than soda lime glass and is more difficult to create elaborate forms. Potash lime glass takes well to facet cutting and copper wheel engraving (Corning Museum of Glass 2018).
- Borosilicate Glass - Glass with a flux of boric oxide. First created in 1882. Borosilicate glass withstands sudden changes of temperature, so it is used for laboratory equipment, glass casserole dishes, etc. (Corning Museum of Glass 2018).
Color is usually produced by using metallic oxides, like iron, manganese, copper, cobalt, tin, nickel, silver and chromium.
- Opaque Glass - White opaque glass produced prior to the 1870s, but the colors yellow, ivory, green, blue, turquoise and black were developed in the late 1870s (Jones 2000:147).
- Transparent Glass - Transparent glass allows light to pass through so that objects behind can be distinctly seen. Different transparent colors were developed at different times. Red was developed in late 1820s, and became popular in 1880s (Jones 2000:147). Grass green was first made successfully in 1900 (Jones 2000:147).
- Opalescent Glass - Typified by a milky white edge or a white raised pattern decorating a colored pressed glass vessel. Transparent glass is covered with a layer of colorless glass containing bone ash, arsenic or other minerals. The opalescent effect is produced by re-heating parts of the vessel, which turn white. Many US factories produced opalescent glass between 1880 to 1920.
- Uranium Glass - Uranium glass began to gain popularity in US in the 1830s and continued in regular production until just before WWII. Varied colors from transparent yellow and green, to opaque and opalescent green, white or pink. Contains a small amount of uranium and will glow green under shortwave and longwave UV light.
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