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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.
In fact, this color is known as "cobalt blue" in the glass manufacturing world (Scholes 1952).
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 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.
Thus, some information can sometimes be gleaned from knowing what color is or is Simply put, people observe or interpret colors (or in Canada - colours) differently.
Broken glass (aka "cullet") on hand from misblown, broken or returned bottles was also often added (Toulouse 1969).
From this point in the glass producing process, the final color of the glass is a matter of both controlling off-coloring impurities and achieving the desired color.
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).