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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.

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).

The purer the sand (i.e., the higher the silica concentration and less iron) the better, as it is the other impurities - desired or undesired - that give glass its color.

Low iron means more control over the ultimate color (Hunter 1950; Tooley 1953).

So called "natural" colors are those that result "naturally" from the basic ingredients in a glass batch (Mc Kearin & Wilson 1978).

In general, with lesser amounts of iron or less oxidation of that iron, shades of bluish to greenish aqua are achieved.

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.

With higher amounts of iron or higher oxidation of the iron, darker greens will usually occur (Toulouse 1969a; Jones & Sullivan 1989)).

In order to create other colors, the iron needs to be variably neutralized and appropriate colorizing agents or compounds added to achieve the desired color.

Although classification by colour is simple to do, the end result is of little value for the following reasons: colour does not have a direct relation with glass type (the common green, amber, and brown glass colours can occur in soda, potash, and lime glasses; many lead glasses are coloured); colour is not related to the technology of glass object production (i.e., it has nothing to do with whether the glass is free blown, mould blown, pressed, or machine made); colour is only weakly related to the function of the object (almost all colours can be found in all types of objects, an obvious exception being "black" glass which does not occur in tableware).

Given these factors there is little justification for using colour as a means of classification.

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.

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