Color of Glass

The technique for producing colored glass was discovered before the method for producing clear glass was discovered. Egyptian vessels are notably rich in their opaque colors, especially the various shades of blue. Colored glassware was preferred in order to hide the natural greenish brown color that occurs in natural glass. Another reason for using colored glass was that people wanted to imitate the colors of precious stones.
The Romans created colorless glass by adding manganese oxide, and it was greatly prized as luxury good for the wealthy. Quality colored glass was hard to obtain and was thus sometimes as expensive as semi-precious stones. However, after the fall of the Roman Empire, there was little progress in discovering new shades.
Today, numerous kinds of metal oxides are used to create colored glass for various effects. The discovery of chemical elements has made the options of colors even more varied.
How colored glass is made:
There are four stages to creating colored glass.
First, unrefined glass that is naturally green or brown is needed. This can be made by mixing silica and alkali. A related article can be found here.
Second, various metal oxide(s), sulphides, or selenides are added to the batch for creating different colors.
Third, metal colored particles are added.
Fourth, in a process called 'striking,' minute metallic particles are added to the glass, and glass is re-heated. Re-heating must be carefully controlled, because the color of the glass depends on the temperature.
Commonly used coloring agents and resulting colors
The following is the list of metals used to create specific colors. Artists often create their own formula of the right color and right amount of metal to make unique glass products.
Antimony: for yellow
Chromium: for yellow-green
Cobalt: for blue
Copper: for turquoise, green, or red
Gold: for ruby red
Iron: Iron is present naturally in most glassmaking ingredients. This is the element that gives the natural green or brown shades to the glass. If added more, deep emerald green is created. It can also be combined with manganese or antimony to produce yellow, amber, brown, and black.
Manganese: for amethyst, purple, rose-pink
Nickel: for violet (potash-glass), yellow (soda-glass)
Selenium: for pink (soda-glass), amber (lead-glass)
Silver: for yellow
Tin: for opaque white, or opacity in other colors
Titanium: for yellow-brown
Uranium: for greenish yellow
The same color can be made with various metal oxides, but the shade and feel may be different.