In the modern manufacture of natural-rubber articles, the
crude rubber is treated with compounding ingredients in several mixing machines.
The mixture is then applied mechanically to a base or is shaped, and the coated
object or shaped mixture is placed in molds and vulcanized.
The principal sources of crude rubber are the sheets, slabs, or biscuits produced on rubber plantations from the latex of Hevea trees, or, in certain manufacturing operations, the uncoagulated latex. Reclaimed rubber, treated by heating with alkali for 12 to 30 hours, can be used as an adulterant of crude rubber to lower the price of the finished article. The amounts of reclaimed rubber that are used depend on the quality of the article to be manufactured.