Colorless (aka "Clear")

Colorless (aka "Clear") Roth Drugstore bottle from Oregon in colorless glass; click to enlarge.This color is the actually the absence of any color. Colorless is preferred over the term "clear" or "white" glass since the former term refers more accurately to the transparency of the glass not its color, i.e., "clear green"; and the latter term implies milk glass which is discussed below (McKearin & Wilson 1978). Colorless glass was a goal of glass manufacturers for centuries and was difficult to produce because it required the use of virtually impurity-free materials. Venetian glass makers produced their crystallo as early as the 15th century and glass makers in 18th century England made what was known as "flint" glass from virtually pure quartz rock (calcined flint) which was simply called "flint" (Hunter 1950). Improved chemistry and glass making methods of the late 19th and early 20th century allowed for process efficiencies that made colorless glass easier and much cheaper to produce with the use of various additives in the glass mixture. The term flint glass was and still is used somewhat erroneously by glassmakers to describe colorless glass that is made with low iron sand. It is, however, not true flint glass. Flint glass is sometimes called lead glass (and vice versa) though true lead glass is made with lead oxide (Dillon 1958; Toulouse 1969a; McKearin & Wilson 1978). Colorless glass was also called "crown" by early glassmakers (Hunter 1950). Colorless glass is not always, or even usually, absolutely colorless. It will usually have very faint tints of pink or "amethystine" (faintly visible in the base of the bottle to the left), amber or "straw", grayish green, grayish blue, or grey. These faint colors are viewed easiest when looking through the thickest portion of the bottle, i.e., sideways through the base. Colorless glass is usually attained by using the purest sand source possible and by adding "decolorizing agents" to the glass batch to offset the residual iron impurities (Dillon 1958). Common decolorizing agents were manganese dioxide, selenium dioxide (usually in conjunction with cobalt oxide), antimony and arsenious (arsenic) oxide which is also used as a stabilizer of selenium in decolorizing glass - or some combination of these compounds 

(Trowbridge 1870; New York Herald 1910; Scholes 1952; Tooley 1953; Lockhart 2006a).