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 Marbles were originally made by hand.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 "the Greeks were guilty of excessive cleaning of the marbles before they were brought to Britain." During the international symposium on the cleaning of the marbles, organised by the British Museum in 1999, curator Ian Jenkins, deputy keeper of Greek and Roman antiquities, remarked that "The British Museum is not infallible, it is not the Pope. Its history has been a series of good intentions marred by the occasional cock-up, and the 1930s cleaning was such a cock-up". Nonetheless, he claimed that the prime cause for the damage inflicted upon the marbles was the 2000-year-long weathering on the Acropolis.