What Is the Process of Making Colored Glass?
- The art of glass making dates back 6,000 years. The Egyptians learned to create glass by mixing sand with sediment from the Nile River, then heating and cooling the mixture. The sediment was rich in calcium oxide and sodium oxide. The copper oxide in the sediment gave the glass its blue color. The Egyptians and Romans continued to experiment with ways to achieve different colors in glass. In the eighth century, Gerber, an Arab chemist, discovered that different metals produced different colors of glass. The demand for beautiful glass, especially by churches and mosques, fueled further experimentation and the development of today's modern glass-making industry.
- Sand is still the basic ingredient in glass making. According to the Purdue University's Chemistry Department, "more than 90 percent of the sand consumed each year is by the glass industry. Glass makers combine sand with modifiers and heat the mix until it achieves a molten state. They add metals and minerals while it is still molten, then cool the mixture quickly to produce glass.
- Some metals, in the form of powdered oxide or alternate compounds, create specific colors in glass. Cobalt oxide achieves a blue-violet color; selenium oxide a red color; sulfur a yellow-amber color and chromic oxide an emerald green color. Adding carbon oxides to molten glass changes the glass color to amber brown. Some metal compounds achieve certain colors or neutralize color. For example, manganese dioxide is used to create purple glass and to prevent impurities from coloring glass. Minerals also play a key role in the making of colored glass. Nonmetallic elements such as selenium, phosphorus, tellurium and sulfur change the color or depth of color achieved when mixed with the metal compounds.
- Along with the additives, certain other factors affect the finished outcome of colored glass. These factors include the temperature of the molten glass and the oven, the amount of colorant used, the type or combination of colorants used and the number of times the glass is melted during the working process. Even the composition of the oxide added to the molten glass can make a difference in the final color. For example, one composition of iron oxide will create yellow glass while a different form of iron oxide turns the glass blue.
