Modern marbles are made from a combination of sand, soda lime, silica, and several other ingredients added for pigment or decoration. These other additives range from aluminum hydrate to zinc oxide. The primary component, sand, is essentially loose, granular particles of disintegrated rock. Soda lime is the chemical term for the mixture of calcium hydroxide and sodium or potassium hydroxide. It is a drying agent and absorbs carbon dioxide. Another compound used in marble manufacturing is silica, a white or colorless crystalline found in agate, flint, quartz, and other rocks. Some marbles are also made from cullet, or scrap glass.
•Sand, soda lime, and crushed cullet are fed into a large, furnace-driven tank. In the tank, the mixture is heated to 2300°F (1260°C) to melt the raw materials. This can take as long as 28 hours.
•Next, the molten mixture moves out of the tank through an opening into another vat known as the flow tank. There an opening in the tank injects molten colored glass. This hot, pigmented glass gives the marbles their distinct appearance. A green marble has been injected with glass containing iron oxide; cobalt results in a blue marble; and manganese will yield a purple one. The use of uranium oxide gives marbles an eerie, greenish-yellow cast. The speed and force of the injection determines the final design of the marble. A grooved feeder device, patented by the Akro Agate Company, was able to produce multicolored marbles known as corkscrews.
Cutting and cooling
•Next, the still-molten glass is released from the flow tank as globs of glass. Automatic cutting devices slice the mixture into equal parts. The globs travel down metal ramps that simultaneously cool them and perfect their spherical shape. Next, they travel down a second metal slide and are sorted by hand. These grooved rollers were the invention of Horace Hill, a former employee of Christensen and Son and later founder of Akro Agate. This device produced marbles much more quickly and reduced the labor necessary by nearly two-thirds. Marbles with flaws are sent back to another area of the factory for re-melting. The marbles cool off in 5-gallon (19 1) containers that house 5,000 marbles at a time.