The transition from scribal reproduction to mechanical typography in 15th-century Europe was predicated not upon the press itself—which was a modification of the existing agricultural screw press—but upon the development of a specific metallurgical alloy for movable type. The material substrate of the Gutenberg press consisted of a ternary alloy of lead ($\text{Pb}$), tin ($\text{Sn}$), and antimony ($\text{Sb}$).
The selection of these metals was a precise engineering solution to the problem of dimensional stability. Lead served as the primary medium due to its low melting point and abundance. However, pure lead is too soft to withstand the pressure of the press and shrinks upon cooling, which would result in blurred or distorted characters. The addition of tin lowered the melting point further, facilitating faster casting and improving the flow of the molten metal into the hand-mould. The hand-mould was the critical instrument of this process: a two-part adjustable device that allowed the founder to cast characters of varying widths while maintaining a constant height, ensuring that every piece of type sat flush on the press bed.
The critical component, however, was antimony. Antimony is one of the few substances that expands upon solidification. By alloying antimony with lead and tin, Gutenberg created a metal that expanded slightly as it cooled, locking the alloy firmly against the walls of the matrix and ensuring the sharp, precise edges required for legible typography. Without this expansion, the "face" of the letter would pull away from the mould, leaving the printed page a blur of imprecise ink.
The extraction and casting of this substrate involved significant labor and toxicity. Lead and antimony were typically recovered from galena ($\text{PbS}$), a lead sulfide mineral. The smelting process released sulfur dioxide and lead vapors, which, in the confined space of a casting shop, led to chronic plumbism (lead poisoning) among the craftsmen. The biological cost was severe: lead accumulates in the soft tissues and bones, eventually attacking the peripheral nervous system. This manifested as "wrist-drop"—a paralysis of the extensor muscles of the hand—and a progressive cognitive decline characterized by irritability and memory loss. The antimony, while providing the necessary structural expansion, added to the toxicity of the fumes, affecting the central nervous system and respiratory health of the workers.
Furthermore, the ink required for this substrate was a departure from the water-based inks used by scribes. To adhere to the metallic type without beading or smearing, Gutenberg developed an oil-based varnish—a mixture of linseed oil and soot (lampblack). This created a viscous, tacky medium that could be transferred from the metal face to vellum or paper with high contrast and permanence. The physical reality of the early printing press was thus a synthesis of mining, metallurgy, and chemistry, where the birth of mass-produced thought was inextricably linked to the inhalation of heavy-metal fumes and the slow decay of the founder's body.