Date a fairly medium dating
Bottle colors also warrant coverage here simply because they are of fascinating interest to people.
As implied in the quote above, there are some time related trends in color that can be of utility for dating. The specific "diagnostic utility" of a given color is noted in the descriptions below.
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.
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).
Having quoted this, color is still an important descriptive element for the recordation and classification of bottles.
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).
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.