A Kitchen Scale and a Jug Tell You If That Tap Is Really Brass

Where the measurement lies to you

Air is the enemy. A hollow casting, a threaded cavity or a blind hole that keeps a bubble makes the object displace less water than its solid volume and read too dense; a bubble clinging to the outside does the opposite. Tap the piece under water and tilt it before reading, and repeat the measurement twice. Porous materials — untreated wood, plaster, some ceramics — absorb water while you are measuring and give a moving number, so they need a sealed surface or a different method entirely. And a reading that lands on 4.0 g/cm³ should make you suspicious rather than confident: almost no common structural material sits there, titanium being 4.5, so the likely explanation is trapped air or a cavity rather than an exotic alloy.

Density identifies, it does not authenticate

It narrows the field and nothing more. Several materials share a value closely enough that a household measurement cannot separate them — brass, bronze and some bearing alloys cluster between 8.4 and 8.9 — and a plated object gives the density of whatever it is made of underneath, which is the point of the test but also its limit: a heavy core wrapped in a thin skin of anything reads as the core. That last property is exactly what makes the method useless for precious metals, since a gold-plated tungsten piece measures very close to gold. For deciding whether a fitting will corrode in a wet room or take a thread without stripping, the scale and the jug are enough.

Frequently asked questions

How do I measure something that floats?

Hold it under with something whose volume you already know. Measure the level rise with a metal sinker alone, then again with the sinker and the floating object tied together, and the difference is the object's volume. Anything that reads below 1.00 g/cm³ floats in water by definition, which already tells you it is wood, most plastics, or something foamed. If it also absorbs water, wrap it in cling film first and accept the small error the film adds — a measurement with a known small bias beats a measurement that drifts while you watch it.

Does temperature matter?

Barely, for solids and for this purpose. A metal expands by a fraction of a per cent over the range a room ever sees, which shifts the density by less than the reading error of a jug marked in five-millilitre steps. It matters far more for liquids and gases: water is densest a few degrees above freezing rather than at zero, which is why ice floats and why lakes freeze from the top down instead of solidifying through. If you are measuring a liquid rather than a solid, note the temperature; if you are checking whether a fitting is brass, do not think about it.