Why Do Silver Spoons and Jewellery Turn Black?
― Silver Ignores Oxygen and Teams Up Only with Sulfur
You put a silver spoon or ring away, and some time later it has gone black. Many people assume it's rust, like iron. But silver's partner isn't the oxygen in the air. It's sulfur, which hides in eggs, rubber bands and hot springs. The black you see is a thin film, only about one ten-thousandth of a millimetre thick. And the film can be turned back into silver.
You take a silver necklace out from the back of a drawer for the first time in ages. When you bought it, it shone bright white. Now it has brown patches, and the edges are blackish.
Maybe you wore a ring into a hot spring on a trip. By the time you got out, it had turned pitch black in less than a night. Or you ate a boiled egg with a silver spoon, and only the tip of the spoon went black. People often tell stories like these.
Iron left in the air rusts red. But silver left in the air for years barely changes on its own. So what is turning silver black?
There Are Only Two Reasons It Turns Black
Silver hardly reacts with oxygen. But it bonds very strongly with sulfur. It picks up the tiny amounts of sulfur compounds in air and food, and forms a black substance on its surface called silver sulfide.
At first the silver sulfide film is thinner than the wavelength of light, so it looks yellow, brown, purple or blue. Once it grows to about one ten-thousandth of a millimetre, it absorbs almost all light and looks black.
So tarnish is not like rust, which crumbles the metal right through. Only a very thin outer skin has turned into a different substance. Let's go through it step by step.
Why Does Silver Choose Sulfur Over Oxygen?
Iron rusts by reacting with oxygen and water in the air. Aluminium also bonds with oxygen, and in an instant it forms a clear film on its surface. Silver, though, hardly pairs up with oxygen at ordinary temperatures. The compound of silver and oxygen (silver oxide) is said to break down into silver and oxygen when heated above about 300°C. The bond is that weak.
Its bond with sulfur, on the other hand, is far stronger. Silver sulfide barely dissolves in water, and it counts among the least soluble substances in all of chemistry. For silver, sulfur is a partner it never lets go.
And sulfur is all around us. The proteins in egg white and yolk contain sulfur, and when heated they give off a little of the rotten-egg gas (hydrogen sulfide). Rubber goods get their springiness from sulfur added during manufacture, so a rubber band wrapped around silver can blacken that spot. Sulfur hot springs are full of that gas. Even city air is said to carry tiny traces of sulfur compounds.
The amounts are tiny, but silver keeps collecting them over months and years. That is why it can turn black even if you just put it away.
From Yellow to Black: Why Does the Colour Change with Film Thickness?
Look closely at silver that is partway to black and you'll see yellowish areas, brown areas, and patches that shimmer purple or blue. This is because the silver sulfide film is a different thickness in different places. In Figure 1 below, move the slider and grow the film yourself.
While the film is very thin, light bouncing off its front and back surfaces overlaps. How it overlaps depends on the film's thickness and the colour (wavelength) of the light, so some colours are strengthened and others cancelled out. It's the same mechanism that colours soap bubbles and oil films on puddles.
As the film gets thicker, silver sulfide's own tendency to absorb light takes over. At a thickness of around 100 nanometres (one ten-thousandth of a millimetre), it is said to look almost black. In the cross-section on the left of Figure 1, part of the film sinks below the dotted line. That sunken part used to be silver.
If you boil an egg too long, the surface of the yolk turns greyish green. This is the colour of "iron sulfide", formed when sulfur compounds from the white bond with iron in the yolk. At heart it is the same reaction that blackens a silver spoon. Cooling the egg in water right after boiling makes it less likely.
How Much Silver Do You Lose by Polishing?
Tarnished silver shines again when polished with a cloth containing an abrasive. But the black film you wipe off contains atoms that used to be silver. Each time you polish, a little silver is scraped away.
The calculation in the fold-out section shows that each time you polish a spoon up from black, about 2.5 milligrams of silver is lost. For a heavy solid-silver spoon that is nothing to worry about. But things made of "silver plate", with a thin layer of silver over another metal, are different. Polish too much and the metal underneath starts to show through.
Instead of polishing, there is a way to pull off just the sulfur. See Figure 2. Line a heatproof container with aluminium foil, pour in hot water with baking soda dissolved in it, and sink the silver so it touches the foil. Aluminium gives up electrons more easily than silver does. When electrons move from the aluminium to the silver, silver sulfide turns back into silver and the sulfur moves over to the aluminium. Nothing is scraped away, so this suits silver-plated items too.
Summary
Silver turns black because it bonds with sulfur, not oxygen, forming a film of silver sulfide on its surface. While the film is thin, overlapping light makes it look yellow, purple or blue, and at about one ten-thousandth of a millimetre it turns black. To keep silver white longer, keep it away from egg dishes, rubber bands and hot springs, and seal it in a bag to cut off the air.
Tarnish isn't rust. It's a "sulfur diary".
Where the silver has been and what it has touched is written on its surface.
If you'd like to compare this with how iron rusts, try "Why does iron rust?". For how iron combines with oxygen inside a disposable heat pad, see "Why does a disposable heat pad get warm when you open the bag?". The danger of sulfur gas collecting in hot-spring areas is covered in "Why shouldn't you step into a hollow in volcanic or hot-spring areas?".
- Get a tarnished silver spoon, or a piece of silver jewellery you no longer use. If nothing is tarnished, leave a piece of boiled egg yolk on the surface for about an hour, and that spot will turn brownish.
- Line a heatproof container with aluminium foil and add about one tablespoon of baking soda. An adult pours in hot water, and the silver is sunk so that it touches the foil.
- Wait a few minutes, take it out, rinse it in water and wipe it dry. Check whether the black areas have gone back to silver, and compare the sides that touched the foil with those that didn't.
A faint rotten-egg smell will come off, so open a window. Take care with hot water, and let an adult handle any step involving boiling water. Don't use this on jewellery with pearls or stones, or on pieces with a surface finish, as they can be damaged.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled by level, from junior-high science to university-level courses
- Junior highCovered in junior-high science
- High schoolCovered in high-school chemistry or physics
- High school+Advanced high-school material, or textbook sidebar content
- UniversityUniversity specialist courses (corrosion science, inorganic chemistry), not taught in high school
- ResearchNot taught even at university as settled fact; researchers are still investigating it
Junior highTerms: this phenomenon has names
- Silver sulfide: the black substance formed when silver and sulfur bond. It is what tarnish is made of.
- Tarnish: a thin film of compound forming on a metal surface and changing its colour and gloss. It is distinguished from rust (corrosion), which eats into the metal.
- Hydrogen sulfide: the rotten-egg gas. It comes from heated eggs and sulfur springs, and is one of the main things that blackens silver.
Junior highHigh schoolChecking with a formula: how much silver does one black film use up?
The silver that makes up the black film was once silver on the surface. Silver atoms are neither created nor destroyed, so from the film's thickness we can work backwards to the amount of silver lost.
| In symbols | m ＝ A × ( d ÷ R ) × ρ |
| In words | Mass of silver lost ＝ surface area × (film thickness ÷ volume ratio) × density of silver |
| Where it comes from | In the reaction 4Ag ＋ 2H₂S ＋ O₂ → 2Ag₂S ＋ 2H₂O, the number of silver atoms is conserved. Turning silver into silver sulfide swells its volume by a factor of R, so the thickness of silver that made a film of thickness d is d ÷ R |
| m | Mass of silver lost (grams) |
| A | Area of the blackened surface (square centimetres) |
| d | Thickness of the silver sulfide film (centimetres) |
| R | How many times larger the volume of silver becomes as silver sulfide (no unit) |
| ρ | Density of silver (grams per cubic centimetre) |
| Volume of 1 mole of silver | about 10.3 cubic centimetres |
| Volume of silver sulfide holding 1 mole of silver | about 17.1 cubic centimetres |
| Film thickness at which it looks almost black (a rule-of-thumb value) | about 100 nanometres |
| Blackened area of one spoon (assumed) | about 40 square centimetres |
| Density of silver | about 10.5 grams per cubic centimetre |
| Volume ratio R | 17.1 ÷ 10.3 ≒ 1.66 |
| Thickness of silver that made the film (nanometres) | 100 ÷ 1.66 ≒ 60 |
| Converted to centimetres | 60 × 0.0000001 ＝ 0.000006 |
| Volume of silver lost (cubic centimetres) | 40 × 0.000006 ＝ 0.00024 |
| Mass of silver lost (grams) | 0.00024 × 10.5 ≒ 0.0025 |
Each time you polish a blackened spoon clean, about 2.5 milligrams of silver is lost, about 60 nanometres in thickness. If the silver plating is, say, 3 micrometres (3000 nanometres) thick, then 3000 ÷ 60 ＝ 50, so the plating would be worn through after roughly 50 polishings from fully black.
High schoolHigh school+Seen as electron transfer: the reaction that blackens and the one that restores
High schoolThe reaction that blackens silver is "oxidation", in which silver loses electrons. Even when the partner is not oxygen, losing electrons is still called oxidation. The reaction that restores silver with aluminium foil is the opposite, "reduction". Aluminium turns into ions more readily than silver (it has a higher ionization tendency), so aluminium gives up electrons and the silver ions in the silver sulfide take them and return to silver. The whole reaction can be written 3Ag₂S ＋ 2Al → 6Ag ＋ Al₂S₃. Aluminium sulfide reacts with water to give off hydrogen sulfide, which is why you smell it during the process.
High school+The colours of a thin film are explained by "thin-film interference". The light reflected from the film's surface and the light reflected from its base (the boundary with the silver) travel different distances. When that difference equals half a wavelength for a particular wavelength, that colour is cancelled. As the film thickens, the cancelled colour shifts from blue towards red, so the colour you see changes in turn.
UniversityCorrosion of silver in the atmosphere
In real air, the main agents that sulfidize silver are said to be hydrogen sulfide and carbonyl sulfide, at very low concentrations on the order of parts per billion (ppb). The reaction still proceeds, and is thought to occur within a thin layer of water adsorbed on the surface, so the higher the humidity, the faster the tarnish. The growth rate of the film also depends on how fast atoms diffuse through it. The volume ratio when a metal becomes its compound is called the "Pilling–Bedworth ratio", and is used as a guide to whether the film protects the surface. The poor solubility of silver sulfide is expressed by the "solubility product", whose value is known to be extremely small.
📖 Derivations and further reading: Redox reaction (Wikipedia, in Japanese) ／ Thin-film interference (Wikipedia, in Japanese)
ResearchWhat is still not clear
- Which step sets the film's growth rate? Whether the limiting step is gas reaching the surface, the reaction in the water layer, or diffusion through the film changes with humidity, dirt and salt, and research continues for each set of conditions.
- How to protect silver artworks. Museums are testing materials that absorb the faint sulfur gases inside display cases and thin protective coatings that prevent tarnish, but a way to protect silver for a long time without changing its look is still being sought.
- Changes in nano-sized silver particles. Researchers are studying how silver microparticles used in clothing and medical products are sulfidized in the environment and how their properties change.
In other words, this article too is "an explanation of what is understood so far". Treat the link between colour and thickness, and the thickness at which the film looks black, as rough guides that vary with conditions.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science, Year 2: "Chemical changes" (metals bonding with sulfur) | Silver and sulfur bond to form silver sulfide |
| High school | Chemistry: "Redox", "Ionization tendency" | The blackening reaction, and restoring with aluminium |
| High school+ | Physics: "Interference of light (thin films)" | Why colour changes with film thickness (Figure 1) |
| University | Corrosion science, inorganic chemistry | Sulfidation in air, Pilling–Bedworth ratio, solubility product |
| Research | Conservation science, environmental chemistry | Preventing tarnish on artworks, the fate of silver particles |
| ― | Link to daily life | Storing silverware, caring for silver-plated items |
- Wikipedia, "Silver sulfide" (硫化銀, in Japanese)
- Wikipedia, "Hydrogen sulfide" (硫化水素, in Japanese)
- T. E. Graedel, "Corrosion Mechanisms for Silver Exposed to the Atmosphere", Journal of the Electrochemical Society, 139 (1992)
- J. P. Franey, G. W. Kammlott, T. E. Graedel, "The corrosion of silver by atmospheric sulfurous gases", Corrosion Science, 25 (1985)
- The Chemical Society of Japan (日本化学会), ed., Handbook of Chemistry, Fundamentals (化学便覧 基礎編), Maruzen Publishing (丸善出版)
※This article is a general-audience science explainer. The figures given are rough guides to help you understand the mechanisms. For care of expensive items, cultural property, or jewellery set with gemstones, follow the advice of the shop where you bought it or a specialist.