Cobalt oxide turns glass blue. Cobalt and nickel neutralize other metals and make glass colorless. Iron produces an aquamarine cast but adds strength. Reducing the amount of iron creates glass that is less reflective and therefore appropriate for retail applications. Phosphorus makes glass opalescent.
Many companies have their own formulas for achieving different effects.
The mechanical properties of glass can be altered by heat strengthening or tempering. By reheating and then quickly chilling the glass, it is made anywhere from two to six times stronger than untreated glass and able to stand up to greater temperature extremes. The drawback to tempered glass is that when it does break, the fragments are small and, consequently, dangerous. Glass can also be chemically treated for strength, which makes it suitable for paving or flooring. Safety glass is made by sandwiching glass between layers of plastic.