The short answer
Ordinary silver tarnish forms when sulfur-containing substances react with surface silver, commonly producing silver sulfide. This is surface corrosion, even though it is different from iron rust. Collectible coins should not be polished simply to remove the color.
The surface has reacted
If a silver piece turns gray or black, some of the metal at its surface may have reacted with sulfur-containing gases. The Canadian Conservation Institute identifies silver sulfide as the characteristic tarnish on pure silver. A silver-copper alloy can also develop copper sulfide, so the chemistry of sterling need not match fine silver exactly.
The word rust usually describes iron corrosion. Calling silver tarnish something different does not make it chemically harmless: surface silver has changed into a compound. Nor is the layer simply loose dirt. Understanding that distinction is more useful than treating every dark patch as something to rub away.
Why an egg appears in the story
CCI demonstrates tarnishing with a freshly hard-boiled egg, which gives off hydrogen sulfide. The example shows how a familiar material can create a reactive atmosphere around silver. It is not a suggested experiment for a collectible coin.
Thin tarnish films can produce yellow, red, or blue interference colors before the surface looks black. A colorful coin therefore deserves careful observation, not an automatic assumption that the color proves age or authenticity. Appearance alone cannot establish how the surface acquired its present condition.
The collector’s useful response
Preserve the piece before trying to improve its shine. CCI advises leaving corrosion removal from coins to a conservator and warns against coin dips and metal cleaners. Abrasive products can scratch a surface, while repeated tarnishing and removal can take away silver. Silver plate deserves particular caution because its precious-metal layer is thin.
Choose individual, suitable protective holders and handle coins by their edges. Ask what a holder is made from rather than relying on the word protective. CCI lists archival-quality polyethylene and polypropylene among suitable plastics and warns against PVC flips.
For a valuable or unfamiliar piece, take clear photographs and seek a conservation assessment before treatment. The sensible first question is what needs preserving. A brighter surface is not necessarily a better-preserved collectible.
About this edition
Original source and editorial notes
Adapted from original material by Rich Silver. Original source card: Silver fact: supplied original card (publication date not established); Silver fact: supplied revised card (publication date not established); Silver fact: supplied alternate revised card (publication date not established).
Reviewed 2026-11-04. Retained the original and both revised Silver fact records as provenance without selecting either diagram. Replaced the simplified equations and harmless-tarnish claims with a conservation-based explanation of surface corrosion. Removed routine polishing advice and kept the egg example as an explanation rather than a treatment. The cards have no verified publication dates or public URLs; none have been inferred from file metadata. No source-card graphics are reproduced.