Antique lamp bases hide their metal well. Polished and lacquered, a zinc casting can pass for bronze at a yard sale, and a heavy brass column can be dismissed as pot metal at an auction. Because base metal is one of the strongest value signals on any old lamp — the difference between a five-dollar fixture and a collectible — the two-minute identification routine below is worth learning before you buy, not after.
Brass and bronze are copper alloys: neither is magnetic. Spelter — the collector's word for cast zinc alloy — is not strongly magnetic either, so the magnet cannot clear a piece, but it can condemn one: if the magnet sticks, you are holding steel or iron underneath (a plated base, a hidden armature) and the piece is a modern reproduction of the form. A fridge magnet is a start; a rare-earth magnet from a hardware store is better, because it detects weakly alloyed steel that fridge magnets miss.
Zinc is markedly lighter than copper alloys. Two same-size column bases: the one that feels like it is full of sand is bronze or brass; the one that feels like hollow plastic with ambitions is spelter. Handle enough lamps and the test becomes automatic — which is why dealers pick pieces up before they look at them.
Flip the base and find a hidden spot — the underside of the base ring, inside the font, under the felt. Real brass shows a warm yellow right at the metal; bronze shows a deeper brown-red with a matte, chocolate patina. Spelter tells on itself three ways: it chips to a bright silvery-gray (zinc) rather than a yellow, it develops a bumpy, grainy casting surface instead of a fine one, and old lacquer failures leave it with a gray, ashen look rather than a warm glow.
If the underside is lacquered and unbroken, be suspicious: original lamps were not typically lacquered underneath the base.
On a base you own and are willing to mark: a single light file stroke on a hidden spot. Brass files to bright gold-yellow; bronze to a duller reddish-brown; spelter to silver-gray, often with a slightly crystalline sparkle. Photograph before and after. Never file a lamp you are evaluating for someone else, and never file a finished, decorative surface — the underside ring near the maker's mark is the honest spot.
Spelter is not "bad" — it is how most affordable antique lamps were made, and a well-cast spelter base from the right era is still a decorative object worth keeping and using. The reason collectors separate the metals is stability: zinc casts suffer zinc pest (slow, irreversible crumbling in damp storage), bend under their own weight on tall columns, and cannot be safely re-soldered the way brass and bronze can. A brass lamp can be repaired for another century; a cracked spelter arm usually cannot.
So the metal test is really a durability forecast. Buy the spelter lamp to enjoy; buy the brass or bronze lamp to keep.
Every identification page in a lamp directory — maker's marks, glass types, wiring eras — ends at the same place: you, holding the lamp, checking one more thing before paying. Metal is the fastest of those checks and the one most often skipped, because bases sit in the dark under a shade. Flip it over. Two minutes with a magnet and a flashlight settles what the label is hoping you will not ask.