Dating site colours

Posted by / 01-Feb-2015 23:17

Dating site colours

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.Soda (sodium dioxide) - aka "alkali," "soda-ash," or "potash" in the trade (Trowbridge 1870; Toulouse 1969) - is added to the sand as a "flux" to lower the melting temperature of the silica.Lime (calcium oxide) is added to the batch as a stabilizer since simple glass made from just sand and soda ("water glass") is water soluble making it of little use when formed into a bottle (Tooley 1953; Kendrick 1968; Jones & Sullivan 1989).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.This is done by adding certain types of compounds to the glass batch in certain quantities.

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).Glass which is composed of pure silica (99.9%+) would be colorless glass.However, making glass from pure silica is not practical or commercially viable because of the prohibitive expense of acquiring such in its pure state and the much higher temperatures needed to properly melt.There is a very broad chronology of popularity of various colours over time; however that chronology cannot be applied to individual glass objects with any significant level of meaning..."The majority of common bottle glass is "soda-lime glass" which is primarily composed of silica, soda (aka soda-ash) or potash, and lime - the latter two ingredients often referred to as the "alkalies" (Hunter 1950; Toulouse 1969; Munsey 1970).

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