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 Marbles were originally made by hand.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 that can injure the surface of the marble. Yet another effort to clean the marbles occurred in 1937–38. This time the incentive was provided by the construction of a new Gallery to house the collection. The Pentelic marble mined from Mount Pentelicus north of Athens, from which the sculptures are made, naturally acquires a tan colour similar to honey when exposed to air; this colouring is often known as the marble's "patina" but Lord Duveen, who financed the whole undertaking, acting under the misconception that the marbles were originally white probably arranged for the team of masons working in the