Antimony is a hard metal used as an alloy with tin and lead for various purposes where great hardness and durability are needed. Printers' type metal, which must be firm enough to bear the pressure of a heavy printing press, is composed of lead and antimony. Antimony is also used as a medicine, while its oxide is valuable for the purpose of coloring glass. Sometimes found in the native state, its most important ore is the sulphide, stibnite (Sb2S3). The pure metal is obtained by strongly heating the ore with iron filings in a crucible, or by roasting the sulphide in a furnace until it is converted into the oxide (Sb204). It is then mixed with powdered coal and heated to a redness. As a result the carbon dioxide escapes and antimony remains.