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Many colorizing compounds work in different ways depending on whether the glass pot environment is oxidizing or reducing (Tooley 1953; Kendrick 1968; Toulouse 1969).However, discussing the simple addition of chemical additives makes any discussion of glass making and glass coloring too simplistic.For instance, cobalt oxide added in proper quantities to a properly prepared glass batch results in a distinctly intense blue as shown in the bottle to the left.
This is just informational because the actual chemistry is of little utility and glass colors only contribute a little to the process of dating or typing historic bottles.
It is, however, part of the overall "story" of bottles covered by this website.
Having quoted this, color is still an important descriptive element for the recordation and classification of bottles.
Although color is one of the more obvious and relatively easy to describe attributes of a historic bottle, it is unfortunately of limited utility in classifying a bottle as to age or type.
One of the better discussions on this is from The Parks Canada Glass Glossary by Jones & Sullivan (1989), and is quoted below:"Because colour is a universal attribute of glass and is convenient for mending and establishing minimal vessel counts, it has been latched onto by some archaeologists as a classification device.
For example, if one has a colorless ("clear") bottle which was de-colorized with selenium and/or arsenic which gives the thick parts of the glass a subtle "straw" tint, it very likely dates no earlier than World War I (1914-1918) and infrequent in bottles after the 1940s or early 1950s (Kendrick 1963; Lockhart pers. There are also some colors which where very rarely used for one type of bottle (i.e., cobalt blue for cylinder liquor bottles is very uncommon though do exist) but quite common in others (e.g., cobalt blue for poison bottles or Civil War/Antebellum era soda water bottles).