Bottle Colors Page

Colorless (aka "Clear")

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, e.g., "clear green"; the latter term can describe milk glass which is discussed separately 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 (i.e., 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 which made colorless glass easier and cheaper to produce using 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" glass 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, gray, or grayish blue.  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).

Colorless glass actually does have more utility in dating and typing than most other colors, though still of limited application.  Some of the better dating reliability is for bottles with manganese dioxide decolorized glass.  Upon exposure to sunlight, this glass will turn a light pink or lavender to moderately dark amethyst or even a deep purple depending on the amount of manganese in the glass mix and amount of ultraviolet (UV) light.  This is called "sun-purpled" or "sun colored amethyst " (SCA) glass.  The picture to the right shows a Johnson's Chill and Fever Tonic (Savannah, GA.) manufactured ca. 1900-1915.   This bottle began its life as colorless glass and has "turned" a much darker than average color of amethyst most likely due to the application of artificial UV light or other artificial source (like sterilization equipment), i.e., an "irradiated bottle" (Kendrick 1968; Lockhart 2006b).  The light lavender tint produced by manganese offsets the green tint of the iron impurities in sand creating a largely colorless glass.  (For an interesting article on the artificial irradiation of historic bottles, see the late Dr. Cecil Munsey's article available at this link:  http://www.sha.org/bottle/pdffiles/munseybottleirradiation.pdf )

By the 15th century, the Venetians apparently discovered that manganese could be used to decolorize glass.  Manganese became known as "glassmakers soap" due to the ability to "cleanse" or neutralize the effects of other impurities in the sand, particularly iron (Hunter 1950).   Manganese dioxide induced colorless glass was most commonly used from the 1880s to about the end of World War 1.  At that time manganese dioxide use was greatly reduced for a variety of reasons, although in part because it did not work as well as other chemical decolorizers (see next paragraph) in the open, continuous glass tanks used by the increasingly dominant bottle making machines - both semi-automatic and automatic.  It is often noted in the literature that the reason for the switch from manganese dioxide to other decolorants was due to the cut-off of imports (usually from the Caucasian Mountain region of Russia) to the U. S. due to predation by German submarines during WW1.  However, it is more complicated than that with other reasons being wartime allocation of the now scarcer manganese to the more important need for producing steel and the dramatic increase in the application of chemical knowledge to glass manufacturing which lead to the eventual realization that glass batch mixes with selenium as the primary decolorizer simply worked better (Peter Schulz, unpublished manuscript 2014).  It should be noted that occasional manganese dioxide decolorized bottles may date as early as the 1820s or as late as the 1930s (McKearin & Wilson 1978; Giarde 1989; Jones & Sullivan 1989; Lockhart 2006a & b), although the large majority of bottles decolorized this way were made between about 1890 and 1920 (empirical observations). 

Colorless glass which was decolorized with selenium or arsenic (or typically a combination of the two in conjunction with cobalt oxide) results in a very faint "straw" or amber tint to the thickest portions of the glass (Scholes 1952; Tooley 1953; Lockhart 2006b).  The picture to the left shows this color evident in the thick portion of a milk bottle (underneath the line pointing out the valve mark) that dates between 1925 and 1930 based on the makers mark for the Pacific Coast Glass Company (Toulouse 1971).  Click Cloverdale Dairy Co. to see the entire bottle which was used by a dairy in eastern Nevada.  This colorless "color" can be very diagnostic of a machine-made bottle made from about 1912-1915 to typically no later than the 1950s (Giarde 1989; Lockhart 2006b; empirical observations).  The straw tinted colorless glass in bottles does show up frequently in later (1900-1920) mouth-blown bottles although such can be found occasionally in bottles from the mid-19th century.  (Click French mustard bottle to view an 1870s era bottle with a faint straw cast - evident at the heel - to the otherwise colorless glass.)  Selenium was the best decolorizer for glass made in open glass tanks (versus the earlier closed pots) which was used with most all automatic bottle machines.  Like the colorless manganese dioxide glass, selenium decolorized glass will react slightly to UV light which produces or enhances the straw tint (Scholes 1952; Lockhart 2006a & b).  Such glass can also be irradiated to produce a medium yellowish brown color which looks abnormal for glass color (empirical observations).

Diagnostic/Dating Utility:  Both of the above colorless glass tints can be useful diagnostic tools for an archaeologist who may be dealing with bottle fragments.  One can be quite confident that if the fragment is colorless with a slight straw tint, it very likely is from a machine-made bottle, unlikely to date from much prior to World War 1 (i.e., mid-1910s), and could date as late as the mid-20th century (or even later).  Conversely, a colorless fragment with a slight amethyst tint is quite likely to date to or prior to World War 1 (1915-1920) and is more likely than not to be from a mouth-blown bottle.  Bottles with a grayish tint seem to date between 1915 and 1925, although numerous examples outside that range have been noted by the author (Giarde 1989; empirical observations). 

Generally speaking, bottles of colorless glass were relatively uncommon prior to the 1870s but became quite common after the wide spread use of automatic bottle machines in the mid to late 1910s (Kendrick 1968; Toulouse 1969a; Fike 1987; U. of U. 1992).  Nothing is absolute in these date range estimates, but they are believed to have reasonably high reliability; other contextual information or evidence should be used also.  Be aware that non-glass bottle products (e.g., tableware) does not follow these general dating rules as colorless tableware (plates, cups, vases, etc.) can go back much further than was common for utilitarian bottles (McKearin & McKearin 1941; empirical observations).  Keep this in mind if trying to identify glass fragments which may be from bottles or other non-container glassware.

As a side note, crown top soda bottles were generally not decolorized with manganese after 1914, giving a good ending date for such "colorless" bottles with an amethyst (manganese dioxide decolorized) cast to the glass as the beginning of World War 1; most of these type bottles would be mouth-blown (Lockhart 2006a & b).  Bill Lockhart's Historical Archaeology journal article from 2006 is available on this website at the link below. This is by far the best reference on the subject!

Lockhart, Bill.  2006b. The Color Purple: Dating Solarized Amethyst Container Glass.  Historical Archaeology 40(2):45-56.   http://www.sha.org/bottle/pdffiles/TheColorPurpleLockhart2006.pdf