Lemon and salt to clean copper

Does it work?

Established practitioner practice

Recorded at practitioner-practice level rather than higher: it is long-established domestic and trade practice with a mechanism that is uncontroversial in its parts, and no source read here tests the specific combination. That is a real evidential position rather than a hedge — the catalogue's whole argument about traditional technique is that trade knowledge is evidence of a kind, and that saying which kind is more useful than either dismissing it or inflating it.

This is a verdict on whether the technique is effective. Whether it is safe on a particular surface is a separate question, answered below.

Where not to use it

Why it works

Citric acid from the lemon reacts with the copper oxides, sulphides and carbonates that make up tarnish, converting them into soluble copper salts, while it also chelates copper ions and holds them in solution. The salt crystals provide mild mechanical abrasion which breaks up the film and exposes fresh surface to the acid. The two together are a chemical and a mechanical treatment applied simultaneously, which is exactly what a commercial metal polish is. The mechanism is well established in its parts; what has not been tested in any source read here is the specific combination against a commercial product, or what residue it leaves.

Background

This is a real traditional technique with a coherent mechanism, and reading it as chemistry rather than as folklore makes both its usefulness and its limits obvious. It is two things at once: citric acid, which attacks the copper oxide and carbonate corrosion products, and undissolved salt crystals, which are a mild abrasive. That combination does remove tarnish from bare copper and brass, which is why it has survived. Three limits follow directly. It is abrasive, so it removes a little metal along with the tarnish, and everything the conservation guidance says about polishing frequency applies — on a plated piece the plating is finite. It is acidic and it is chloride-bearing, which is a poor combination to leave on a metal: an incompletely rinsed surface corrodes faster afterwards than it did before, which is why traditionally the piece was washed and dried at once. And it is entirely wrong for a lacquered piece, where it abrades and clouds the coating without reaching the metal. The conservation position for copper alloys is that treatment depends on whether the surface is bare, lacquered or deliberately patinated, and identifying which comes first.

Conditions

• Bare, unlacquered copper or brass only
• Requires washing and thorough drying immediately afterwards

What it does not do

• It removes metal as well as tarnish, so it is not a treatment to repeat often on a plated or engraved piece.
• It leaves an acidic, chloride-bearing residue which accelerates corrosion if the piece is not washed and dried at once.
• It does nothing useful on a lacquered surface except damage the lacquer.

What to use instead

• Metal polish — A very fine abrasive with a mild chemical assist. Every use removes metal, which is why polishing a plated or antique item is a one-way decision.
• Neutral cleaning — The default that four separate trades converge on independently: a neutral detergent, warm water, a soft tool and a thorough rinse.
• Lacquered metal — Brass, copper or steel under a clear film applied to stop it tarnishing. Every metal polish sold for the metal underneath attacks the film instead.