Marble (toy) conventions. Additionally, many glass artisans produce art marbles for the collectors' market only, with some selling for thousands of dollars. Marbles are made using many techniques. They can be categorized into two general types: hand-made and machine-made. Marbles were originally made by hand. Stone or ivory marbles can be fashioned by grinding. Clay, pottery, ceramic, or porcelain marbles can be made by rolling the material into a ball, and then letting dry, or firing, and then can be left natural, painted, or glazed. Clay marbles, also known as "crock marbles" or "commies" ("common"), are made of slightly porous clay, traditionally
Elgin Marbles until the mid-20th century and have suffered irreparable damage by previous cleaning methods employed by British Museum staff. As early as 1838, scientist Michael Faraday was asked to provide a solution to the problem of the deteriorating surface of the marbles. The outcome is described in the following excerpt from the letter he sent to Henry Milman, a commissioner for the National Gallery. The marbles generally were very dirty ... from a deposit of dust and soot. ... I found the body of the marble beneath the surface white. ... The application of water, applied by a sponge or soft