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The essential part of a hammer is the head, a compact solid mass that is able to deliver the blow to the intended target without itself deforming. The opposite or back side has a cross, diagonal or straight pein (peen).
As the impact between steel hammer heads and the objects being hit can, and does, create sparks, which in some industries such as underground coal mining with methane gas, or in other hazardous environments containing flammable gases and vapors, can be dangerous and risk igniting the gases. In these environments you should use extreme caution.