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Glass composition formulas were (and probably still are) closely held glassmaker secrets as the experience of extensive trial and error experimentation in glass making was not readily shared with others.Variations in glass color resulted from a myriad of different causes including the strata of the sand source, the mineral in the soil of the of the trees burned to produce "potash" (an "flux" alternative to soda), and many others known and unknown (Toulouse 1969a).
(Note: Spurgeon's excellent fruit jar color information webpage is located at the following URL: There always has been and will continue to be confusion as to color nomenclature even though many attempts have been made to try to standardize it.Even the same bottle to the same person can vary widely in color depending on differing lighting situations - direct and indirect sunlight out in the field, fluorescent lights in the office, and incandescent lights at home.Adding to the confusion is the jumble of terminology that is used to describe colors and the almost infinite color variations.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.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.
This is done by adding certain types of compounds to the glass batch in certain quantities.
Bottles made from glass with just the basic ingredients (sand, soda & lime) will usually be different shades of green because of the iron impurities in the sand, though other colors can also be attained depending on many factors.
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).
Broken glass (aka "cullet") on hand from misblown, broken or returned bottles was also often added (New York Herald 1910; Toulouse 1969).