Glass makers in Egypt and Rome experimented by mixing different substances into the glass while it was still melted. An Arabian chemist by the name of Gerber compiled a definitive list in the eighth century. He notated the formulas of which metal oxides made certain colors when mixed with glass. Today, several different methods have been developed to add color to glass.
Metal, in the form of oxide or sulfide, is added to glass melt in powder form. Different metals turn glass different colors. One of the most commonly used is Cobalt Oxide, which makes the cobalt blue glass that is often used to make household decor. Green glass is also often used for beer bottles and dishes. Different shades of green are achieved by mixing Iron Oxide, Chromic Oxide or Uranium Oxide. Reds are the result of mixing in Selenium Oxide or Gold Chloride. Yellow glass is formed by using either Cadmium Sulfide or Lead Compounds. Other colors of glass can be made as well.
Staining is the technique of creating an item made of clear glass and then applying a metal oxide to it. The glass is then heated again, adhering the metal compound to the glass. Once the glass cools, it may be etched or engraved upon, scratching away the layer of colored metal oxide. A pattern of the etching remains on the glass below, resulting in a pattern that appears as clear glass, with a layer of color all around it.
Overlay and Enamel Overlay:
Overlaying is the method of dipping an item of glass made of one color into the glass melt of another color. The glass blower will blow a ball of glass and as it just begins to set, he dips the end of his blowing pipe into the heated pot of another color of glass. This creates layers of different colors on the finished product. Those layers can be carved and cut away, revealing patterns of the colors below. This type of overlay can be done with a clear layer of enamel as well. The enamel is layered on once the piece is finished. The enamel layer makes the piece look like it has a clear glass overlay.
A lustre on glass is achieved through by applying a thin layer of metal oxide in vapor form. The end result is to give a tint of iridescent color to a clear glass piece, as opposed to creating fully colored glass. The metal oxide is sprayed or brushed onto the finished glass piece. Alternatively, the oxide may be placed in a container in the kiln while the glass is firing and the gas vapors from the oxide tint the glass.