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 (for green)
- Cobalt (for blue)
- Nickel (for violet/brown)
- Selenium metal (for 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 (for red) and cobalt (for blue) in proper proportions to yield a gray glass that appears colorless, hence the term “decolorization.”
- Amber is the most common colored glass, and is produced by adding together iron, sulfur, and carbon.
- Amber is a “reduced” glass because of the relatively high level of carbon used. 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 Glass is made by adding non-toxic Chrome Oxide (Cr+3); the higher the concentration, the darker the color.
- Green glass can be either oxidized, such as Emerald Green or Georgia green, or reduced, as with Dead Leaf green.
- Reduced green glass offers slight ultraviolet protection.
- 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 glasses are nearly always oxidized glasses. 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.