What does copper in sterling silver do to colour, strength and tarnish?

ZKZ Editorial TeamMain answer · Product & gemmology desk

Sterling silver is an alloy of silver and copper, and the copper is the reason it is strong enough for jewelry. It is also one reason sterling tarnishes. For a ring, the practical effect is a durable metal that needs regular wiping and occasional polishing.

What the sources say

Fact The U.S. Geological Survey says sterling silver is the standard of quality for articles containing 92.5 percent silver and 7.5 percent copper and/or other alloys.

Fact A conservation journal article (JAIC, 1990) says that tarnishing is due to the reaction between silver and hydrogen sulfide, carbonyl sulfide or other sulfur-containing compounds in the atmosphere to form silver sulfide, that the rates depend on temperature and relative humidity, and that in sterling silver the tarnish layer contains cuprous sulfide.

Fact A ScienceDirect overview of sterling silver says the copper increases the hardness and strength of the alloy but also makes it prone to tarnishing.

What this means in practice

Our advice Wipe the piece after wear and keep it in a dry, sealed pouch. Humidity and sulfur-containing products speed tarnish.

Our advice Expect a ring made of sterling to scratch and to need an occasional polish. Pure silver would be softer.

It depends Colour differences between makers come from many things, including finish, plating and the exact alloy, and not from copper alone.

Uncertain We did not find a verified, comparable measure of hardness or colour shift for sterling made with other alloying metals. We also did not verify claims for tarnish-resistant sterling alloys, so we do not rate them.

Skin and allergy

Our advice Ask the seller what the rest of the alloy is and how the piece is finished. A well finished piece may still tarnish with time, so treat polishing and dry storage as routine care, not as a defect.

It depends Effects depend on the alloy, the finish and how the piece is worn and stored.

Uncertain We found no source we could cite on how often the copper in sterling causes skin reactions. If you react to a piece, see a doctor and ask the seller for the full alloy.

Related reading: What is anti-tarnish coating on silver, and does it last? · Does 925 Sterling Silver Tarnish? Causes and Fixes

Sources

• U.S. Geological Survey, What is sterling silver?
• JAIC 1990, tarnishing of silver and silver alloys
• ScienceDirect Topics, Sterling silver (overview)

Fact = stated by the cited source. Our advice = editorial recommendation. Uncertain / it depends = limits of what can be said in general.