How to clean silver jewellery

An egg in a box

Conservators at the Canadian Conservation Institute have a simple way of showing how quickly silver tarnishes. They polish a piece of silver, seal it in a clear box with a freshly boiled egg, cut into quarters and still warm, and watch.

Within five to thirty minutes the silver turns yellow. Leave it longer and it moves through red and blue towards black. The egg is giving off hydrogen sulphide, the same gas that slowly darkens a silver chain left on a dressing table, only far more of it.

It is a neat demonstration of two things. Tarnish is a chemical reaction, not dirt. And it starts the moment polishing stops, which is why the gentlest way to clean silver is also the one you can repeat for years without wearing the piece away.

What tarnish is

Tarnish is mainly silver sulphide, formed when silver reacts with sulphur-containing gases in the air, most commonly hydrogen sulphide. On sterling silver, which is 92.5% silver and 7.5% copper, the tarnish also contains copper sulphide. The copper is more reactive than the silver, which is why sterling tarnishes faster than pure silver.

The colours are an optical effect. While the film is very thin, light bouncing off its top surface and off the silver underneath interferes, so as it grows from about 10 to 100 nanometres it shifts from yellow through red-brown to blue. Beyond about 100 nanometres it shows its true colour, which is black.

Two things are worth knowing before you clean anything:

• Every cleaning removes a little silver. Conservators point out that each polish or chemical treatment takes away some of the metal along with the tarnish. Little and gentle is better than rare and aggressive.
• Damp makes it faster. For the same amount of sulphur in the air, silver tarnishes faster as the humidity rises. Conservators advise keeping silver below 50% relative humidity.

Our guide to storing jewellery so it doesn't tarnish covers the prevention side. This one is about getting it off.

Start gently: soap and a cloth

Before tackling tarnish, wash away the grease and grime that sit on top of it. Conservators use a mild detergent in water. At home, a few drops of washing-up liquid in a bowl of lukewarm water and a soft brush do the same job. Rinse in a bowl or glass of clean water rather than under the tap, so a loose stone can't disappear down the plughole, then dry with a soft, lint-free cloth.

For light tarnish, a silver polishing cloth is the first thing to reach for. These cloths are impregnated with a fine abrasive, but far less of it than a liquid, paste or foam polish, which is why the Canadian Conservation Institute describes them as gentle and most useful for lightly tarnished silver.

If a piece needs more, use a polish made for silver, not a general metal polish. Multi-purpose metal polishes are more abrasive and remove more silver. Keep the lid on: a polish that has partly dried out has clumped into larger, scratchier particles.

The foil and soda method

This is the trick that looks like magic: line a bowl with aluminium foil, add hot water and a spoonful of soda, lay the silver on the foil, and watch the tarnish fade.

It isn't magic, it is electrochemistry, the same family of reactions as a battery. Aluminium is the more reactive metal. With the two metals touching in a salty, alkaline solution, the aluminium gives up electrons and corrodes, and the silver sulphide takes them and is turned back into silver metal. That is why the silver must be in contact with the foil: a small electric current flows between them.

The conservators' version uses a warm solution of washing soda (sodium carbonate). The common kitchen version uses bicarbonate of soda, often with a pinch of salt, and a university chemistry demonstration of it notes that the reaction still works without the bicarbonate, just more slowly. Either way the solution is there to carry the current.

Unlike a polish, this method doesn't rub silver away. But it has drawbacks:

• A dull finish. The silver from the tarnish is left on the surface as fine, rough particles, which look matt until buffed with a polishing cloth.
• Faster tarnish afterwards. The Canadian Conservation Institute notes that silver cleaned this way may tarnish more quickly than polished silver.
• Pitting. If the aluminium becomes coated and stops working properly, the silver can pit.
• Hot water. The kitchen version uses very hot or boiling water, so pour carefully.

When not to use it

• Plated pieces, including gold plated silver and vermeil. On plating, any tarnish removal can be very damaging.
• Hollow pieces, such as some lockets, beads and bangles. Conservators say objects with sealed hollow parts shouldn't go into any watery solution, because it can get inside and can't be rinsed out.
• Deliberately blackened silver. The method is designed to remove dark films from silver. It can't tell tarnish you don't want from a finish you do.
• Pearls, opals and other delicate stones. Gemmologists warn that heat and sudden temperature changes can damage some gems, and pearls in particular shouldn't be soaked.

Silver dips

A silver dip is a liquid you lower the silver into, and the tarnish vanishes in seconds. Dips work by dissolving the tarnish faster than they dissolve the silver underneath. They are made of an acid and a complexing agent, a chemical that grabs hold of the dissolved metal.

Here is what one silver dip contains, according to its 2018 safety data sheet:

The sheet gives the dip a pH of 1.8 to 2.2, so it is a fairly strong acid, and classifies the mixture as causing serious eye irritation (H319), suspected of causing cancer (H351), suspected of damaging fertility or the unborn child (H361) and harmful to aquatic life (H412). Those classifications come mainly from the thiourea. They are reasons to wear gloves, work somewhere airy and keep the bottle away from children, which is what the label asks, not reasons to panic.

For jewellery, the bigger concern is what a dip does to anything that isn't plain silver:

• Plated and costume jewellery. The maker of one popular dip says it isn't suitable for gold, plated items or costume jewellery.
• Oxidised silver. Dip labels routinely warn against artificially oxidised silver. The dip will strip the dark finish along with the tarnish.
• Stones. The same label warns against contact with opals, coral, emeralds, pearls and paste stones, and against fully immersing pieces with water-soluble glues.
• Stainless steel. Conservators warn that dip acids damage stainless steel, and the British Stainless Steel Association names hydrochloric acid based silver cleaners among the things that must not touch it.
• Over-cleaning. Leave silver in too long and it can pit. Dipped silver can also come out with a yellowish stain, which a polishing cloth removes, and can look oddly flat, because the darker tarnish in the recesses that gives engraved or textured silver its depth has gone too.

If you do use a dip, the conservators' method is gentler than the label's: dab it on with a cotton bud where it is needed, then rinse thoroughly in clean water, rather than plunging the whole piece in.

Why not toothpaste?

Toothpaste cleans teeth partly by scrubbing. The American Dental Association lists its abrasive ingredients: calcium carbonate, dehydrated silica gels, hydrated aluminium oxides, magnesium carbonate, phosphate salts and silicates.

Those abrasives are chosen for tooth enamel, not for silver, and nobody grades them for jewellery. The Gemological Institute of America says abrasives such as household cleansers or toothpaste should never be used on diamond jewellery, because abrasives can scratch gold and other metals. The Sheffield Assay Office puts toothpaste among the home remedies that may cause long-term damage to silver and gemstones if used often.

Conservators are fussy about abrasives for a reason. The Canadian Conservation Institute recommends testing every new pot of silver polish on clear acrylic first, to see the scratches it leaves, and even specifies the kind of chalk to use in a homemade polish, because coarser chalk leaves coarser scratches. Toothpaste comes with no such guarantee.

Which method for which piece

If a piece is valuable, old, stone-set or you aren't sure what it is made of, a jeweller can tell you, and clean it for you.

Back to the egg

The egg in the box makes one last point. Silver left alone in clean, dry air tarnishes slowly; silver shut in with a source of sulphur tarnishes in minutes. Most of the work of keeping silver bright happens before any cleaning at all, in where you keep it and what it touches. Do that well, and a soft cloth now and then is usually all a silver chain will ever need.