An antique doorknob carries the history of how metal got worked in America, and how it was made tells you roughly when. Three broad methods made most of the hardware you'll find, and they came in that order: forged, cast, stamped. Each leaves marks, and the marks date the piece.
Forged, then cast
Forging came first and by hand. A blacksmith heated iron or steel and shaped it under a hammer, so early hardware carries the slight irregularity of hand-work, no two quite identical. This is the hardware of the earliest buildings, uneven, individual, and increasingly rare. Casting took over as foundries grew. Molten bronze or brass poured into a mold produces crisp, repeatable, detailed pieces, which is why the ornate hardware of the Victorian era is mostly cast: deep, sharp pattern, real weight, the same design turned out again and again.
Then stamped
Stamping came with mass production and cost-cutting. Thin sheet metal pressed into shape between dies makes hardware fast and cheap, but light, and the pattern is shallow because it's pressed rather than poured. Stamped hardware dominates the cheaper end of later production. Knowing the method places the piece and tells you what it is: a heavy, sharply detailed cast knob is a different object, and a different value, than a light stamped one that copies its look.
Salvager's Note: Weigh it in your hand. Cast hardware is heavy and solid with deep, crisp pattern; stamped is light and hollow with shallow pressed detail. That single heft test separates a quality Victorian casting from its cheaper imitation.