To create certain effects, lime, alumina, or lead oxide may be added to the mix. For example, crystal contains a minimum of twenty-four percent lead oxide. The high lead oxide count imparts both clarity and brilliance to the glass. Glass with a high lead content is also referred to as flint glass. A wide variety of materials could be used to change the color of bell glass as well.
Copper produces a red.
Some very unusual materials were once used to color glass including buffalo bones and twenty-dollar gold pieces. While gold pieces created a beautiful purple-ruby color, the United States government frowned upon the practice and gold chloride was substituted for the currency. Once the batch was created and fired, the hot glass was formed into bell shapes using a variety of techniques. Glass bells could be hand blown, mold blown, pressed, or machine made. Glass blown into molds could feature decorations imparted onto the glass body by designs within the mold itself.