Color can distinguish a glass container, shield its contents from unwanted ultraviolet rays or create variety within a brand category.
Color can be obtained by simply adding small quantities of different oxides:
- Chromium > Green
- Cobalt > Blue
- Nickel > Violet/Brown
- Selenium metal > Red
The raw materials used in commercial glass making contain iron oxide as an impurity, which imparts a yellow/green color to the glass.
To offset the yellow/green when making flint, or “colorless” glass, other colors are introduced by adding selenium and cobalt in proportions that yield gray glass that appears colorless, hence the term “decolorization.”
Glass Colors
Amber
Amber is the most common colored glass, produced by combining iron, sulfur, and carbon. It's considered a “reduced” glass due to the relatively high level carbon use.
All commercial container glass formulations contain carbon, but most are “oxidized” glasses. Amber glass absorbs nearly all radiation consisting of wavelengths shorter than 450 nm, offering excellent protection from ultraviolet radiation (critical for products such as beer and certain drugs).
Green
Green Glass is made by adding non-toxic Chrome Oxide (Cr+3); the higher the concentration, the darker the color. It can be either oxidized to make Emerald or Georgia Green, or reduced to make Dead Leaf Green. Reduced green glass offers slight ultraviolet protection.
Blue
Blue glass is created by adding cobalt oxide, a colorant so powerful that only a few parts per million is needed to produce a light blue color such as the shade used for certain bottled waters.
Blue glass is nearly always oxidized. However, a light blue-green glass can be produced using only iron and carbon and omitting the sulfur, making it a reduced blue.
Creating a reduced blue is seldom done because of the degree of difficulty in fining the glass and controlling the color.
Most colored glasses are melted in glass tanks, the same method as flint glasses. Adding colorants to the forehearth, a brick lined canal that delivers glass to the forming machine of a flint glass furnace, produces oxidized colors.