Copper changes colour, and sometimes spots, if it is stored badly. Most of it is preventable with four habits: keep it dry, keep it clean, keep it away from a short list of materials, and leave it alone. This guide covers the conditions that matter, what to put the rounds in, what the marks on a spotted one usually mean, and what not to do about them.

The short version

  • Dry. Humidity is the main driver. Aim as low as your home allows.
  • Sealed. An airtight holder keeps out moisture and the gases that react with copper.
  • Inert. Use holders made of materials that give off nothing: not soft PVC, rubber, wool or unsealed wood.
  • Handled by the edges. Skin oils and salts leave marks that can start corrosion.
  • Never cleaned. Polishing does more harm than the tarnish it removes.

Why copper changes at all

Copper reacts with oxygen, moisture and sulfur-containing gases in the air. A slow, even darkening is ordinary toning and is not harmful to the metal. Spots, uneven dark patches and green deposits are different: they mark places where something has reacted with the surface, and they are worth preventing.

The Canadian Conservation Institute's guidance on caring for metal objects puts numbers on the humidity part. It calls dry environments, 30% relative humidity or lower, ideal for metals, and says that above 65% all metal corrosion speeds up significantly (see its page on caring for metal objects). It also singles out hydrogen sulfide, a gas that reacts with silver and copper even at very low concentrations.

Where to keep them

Pick a stable, dry spot indoors. Avoid kitchens, bathrooms, basements and attics, where humidity and temperature swing, and avoid a window that gets sun. The U.S. Mint's coin care guidance makes the same point: sharp changes in temperature and moisture cause discoloration (see its advice on caring for your coin collection).

A small hygrometer shows what your room is really doing, and it is cheap. If the humidity runs high, put the rounds in a sealed box with silica gel, which absorbs moisture from the air inside. Follow the gel maker's instructions for drying it out and reusing it, and replace it when it stops working. Use a box made of polypropylene or polyethylene, and check that any gasket is not rubber, because the Institute lists rubber among the materials that give off sulfur gases.

What to put them in

  • Hard plastic capsules. They seal each round individually, stop rounds rubbing together, and are the simplest long-term answer.
  • Mylar flips. Look for flips made of Mylar or similar inert film. They are stiff and slightly crinkly, not soft.
  • Acid-free cardboard and paper. The Mint recommends acid-free holders free of polyvinyl chloride (PVC).

Avoid soft, flexible PVC flips and album pages. Their plasticizers can break down over time and leave a sticky film that eventually turns green and etches the surface. If a holder feels soft or oily, or smells like a new shower curtain, do not use it.

Also keep rounds away from wool and felt, rubber bands, unsealed wood and fresh paint. The Institute lists wood, wet paint and rubber among the materials that give off acids or sulfur gases that attack metal. And do not drop rounds loose into a jar: copper is soft, and rounds rubbing together pick up marks.

How to handle them

Hold a round by its edge, between thumb and forefinger, over a soft surface in case you drop it. The Institute warns that fingerprints and perspiration carry salts, oils and moisture that transfer easily to metal, and recommends not touching it bare-handed. If you wear gloves, use clean nitrile gloves, or cotton gloves that have been washed first, since new ones can carry residue. The Mint adds one more: do not talk or breathe over a coin, because tiny droplets of saliva leave spots.

What the marks usually mean

  • An even brown or deeper red. Normal oxidation. Slowing it is a matter of dry, sealed storage.
  • Dark, uneven patches. Often sulfur exposure, from rubber, wool, some papers or polluted air. Find and remove the source.
  • Marks shaped like fingertips. Skin oils and salts. Handle by the edges from now on.
  • Small round spots. Moisture or saliva droplets. Check your humidity.
  • A sticky or pale green film. Usually a holder or a coating. The Institute notes green corrosion on copper forms from contact with polish residues, beeswax, lanolin, linseed oil and leather, and soft PVC does the same.

What not to do

Do not polish or scrub a spotted round. The Mint warns that polishing and cleaning reduce a coin's value, and the Institute notes that localised green corrosion on copper is often the result of harsh cleaning compounds, so the cure leaves a worse mark than the problem. Do not coat rounds with oil, wax or lanolin products for the same reason. If a round is dirty, leave it and fix the storage instead.

Does any of this affect what it is worth?

Not the metal, which is worth well under a dollar regardless (see what a copper round is worth). It affects how the round looks, which is the reason to own one. Bars show handling more than rounds do, as we cover in copper bars vs copper rounds.

If one arrives with a problem

Everything we sell is .999 fine copper and ships insured. If something looks wrong when it arrives, tell us within 48 hours of delivery; the details are on the shipping and returns page. You can see what we have in stock on the copper rounds and copper bars pages.