The screw is the least glamorous part of any old fixture, and antique screws are one of the most useful clues for dating it. The way a screw was made records the industrial revolution in miniature, and because screws rarely get replaced, they often carry a house's age more honestly than anything else.
The early screw was nearly handmade. Threads were cut by hand or on primitive machines, so early screws have blunt, uneven tips and irregular threads, and their slots are often off-center, hand-filed rather than machined. As screw-making machines improved through the nineteenth century, threads grew uniform, tips grew pointed and consistent, and slots grew centered and clean. The screw became a true interchangeable part. The Phillips head draws the sharpest line of all. The Phillips screw, with its cross-shaped recess, didn't exist before the 1930s, so any Phillips screw in a piece of hardware dates that installation to 1936 or later, with no exceptions. For a restorer, this matters in both directions: reading the screws helps date a piece, and using the right screws keeps a restoration honest.
Salvager's Note — dating by screw: Phillips head = 1936 or later, always. Off-center, hand-filed slot = early, pre-standardization. Uniform machine slot, pointed tip = later nineteenth century on. A modern Phillips screw in an 1890s lock is a small anachronism a careful eye will catch.
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