Even the brightest silver will change with time, losing its brilliant gleam as it turns to an almost solid black. Some people take this transformation as the rusting of silver, but that isn’t the case.
Silver doesn’t rust, it tarnishes. Although rust and tarnish are both products of oxidation and result in some degradation, rust is a term reserved for the reaction of ferrous metals (iron and its alloys) with atmospheric oxygen. Silver is neither ferrous nor does it react with oxygen in the air.
This article explains how rust differs from tarnishing; it explores why silver tarnishes, the safest ways to restore silver, and how to prevent silver from tarnishing.
Corrosion: Rust vs. Tarnish
Corrosion refers to the wearing of metals caused by chemical reactions. Rust and tarnish are forms of corrosion but with considerable differences.
Rust is the resulting iron oxide from the chemical reaction between oxygen and iron (and its alloys), in the presence of moisture. It appears as a reddish-brown flaky substance on the surface of the metal. Rust is undesirable, as it compromises the strength of the metal, and with time, the entire metal will turn to rust.
Tarnish results from metals reacting with non-metallic compounds, particularly oxygen and sulfur. The topmost layers of the metal react to form a thin gray or black coating that essentially shields the underlying metal from further chemical reactions. This shielding allows the tarnished metal to be restored to its original luster when polished.
Rust vs. Tarnish
Summarily, these are the differences between rusting and tarnishing:
- Rust is reddish-brown while tarnish ranges from grey to black.
- Only oxygen causes rust, but tarnish may result from compounds of oxygen or sulfur.
- Rust affects only ferrous metals. Copper, silver, aluminum, and brass get tarnished.
- Rust is indefinite, but tarnish is self-limiting. Rust will continue until the metal gets wholly degraded, but tarnish affects only the top layers, the underlying metal is preserved.
- Tarnish results in no loss of the metal, only its physical appearance changes. Rust can severely compromise the iron’s (or it’s alloy) structural integrity.
- Tarnish can be electrochemically reversed, but rusted metals can not be restored, as the rust would have flaked off.
For more information about the differences between rust, tarnish and patina check this article.
How Does Silver Tarnish?
Silver tarnishes from its reaction with sulfur, resulting in silver sulfide (Ag2S). Sulfur is readily available in the environment, being present in water, wool, air, and even in the oils of the skin. Consequently, it is near impossible for silver not to tarnish.
Like gold, silver is a noble metal in its purest form, meaning it is resistant to corrosion and oxidation. But pure silver is soft, so soft that you can bite into it and leave teeth marks. As a result, it’s impractical to use silver in its purest form if not as pendants and earrings.
Other metals (usually copper), are added to silver to make it sturdy. It is these impurities in silver that causes it to tarnish. Accordingly, the rate of tarnishing is directly proportional to the impurities present in the silver, and the amount of sulfur in the air.
Tarnish causes silver to lose its luster. It starts as a yellow tint, gradually progresses to gray, then finally becomes black and disfigured. Still, it is possible to restore the silver object to its former brilliance by polishing and electrochemistry.
You can remove tarnish by rubbing steel wool, sandpaper, or baking soda over the silver.
Polishing is a destructive method of restoring silver, as it involves stripping the tarnished layers away. Continuous polishing and scrubbing will eventually wear off any markings or images on the silver, or completely strip off silver from silver-plated objects.
Electrochemical Restoration of Silver
Silver reacts with sulfur to form silver sulfide (Ag2S), a thin black coating on the silver’s surface. You can reverse the reaction and restore silver to its previous luster by reducing silver sulfide to silver.
The reversal is a reasonably straightforward procedure. You will need the following materials:
- Aluminum foil
- Baking Soda (½ cup)
- Salt (1 tablespoon)
- A large container, preferably metal or Pyrex dish
- Boiling water
The procedure is as follows:
- Cut a sizable amount of aluminum foil, enough to cover inside the container completely.
- Line the container with the aluminum foil, keeping the shiny side turned up. If possible, use a rectangular container, as it will better accommodate the aluminum foil without creases.
- Lay the silverware in the container, making sure that they are well spaced. Ensure that jewelry like chains and bracelets do not curl back on themselves, as the tarnish will remain on the points of intersection.
- Pour baking powder into the water first, followed by the salt.
- Stir the solution with a spoon or spatula. It is usual for bubbles and foam to appear as the baking soda and salt react with water.
- Pour the solution over the silver in the container. You will observe bubbles forming on the silver’s surface and might catch a whiff of sulfur too. Both of these indicate that silver sulfide has begun to reduce to silver.
- Leave the objects in water for at least 10 minutes. It is best to wait until the water cools, so as not to stop the de-tarnishing process prematurely.
- Rinse and dry with a flannel cloth.
Depending on the amount of tarnish or the size of the object, you may need to repeat the process a few times until there isn’t any tarnish left. Ensure that you use a new aluminum foil each time.
The electrochemical method works by taking advantage of sulfur’s affinity for aluminum; thus, in the hot baking soda solution, sulfur will leave silver for aluminum. This method is the best way to remove tarnish from silver, as it restores luster, without degrading the silver.
The only downside is its impracticality for large to life-size silver sculptures. This impracticality is perhaps why museums use kinder abrasives like calcium carbonate to polish silver artifacts when there is a need to do so.
If you would like to know how to remove rust from jewelry that contains iron or steel, read this article.
Tarnish Proof Ways to Store Silver
Although you can restore silver nondestructively, it might not always be convenient to do so. And while scrubbing and polishing silver may not be as complicated as electrochemical restoration, they are still labor-intensive. Thus, it is better to store silver in a way that prevents tarnishing.
Before putting silver in storage, use the electrochemical method to remove any tarnish from the silver. You can store silver in either of these ways:
Acid-Free Tissue Paper and Polythene
Wrap the silver object in an acid-free tissue paper, and place it in a Ziploc bag. Add an anti-tarnish strip before sealing the bag. The strip will absorb any sulfur present in the air.
Sulfur Absorbing Flannel Pouches
A sulfur absorbing flannel pouch will stop sulfurous gases from reaching the silver object inside. For even better protection, put the flannel pouch into a Ziploc bag with an anti-tarnish strip.
If you are unable to perform the electrochemical method for some reason, you can use a traditional polish on the silver before storing it.
How to Keep Silver Jewelry From Tarnishing
All jewelry except gold will eventually tarnish from the constant exposure to air and moisture. While tarnishing is inevitable, you can delay it if you take these steps:
- Keep jewelry dry. As much as possible, keep jewelry away from moisture, dry it off immediately if it gets wet. A coat of nail polish applied to the parts in contact with skin will prevent sweat and skin oils from touching the jewelry.
- Limit how often you use the jewelry, as extended periods of exposure will hasten tarnishing.
- Store it properly.
Thankfully, silver doesn’t rust, it only tarnishes, and tarnish can be reversed. So if you’re unable to prevent tarnishing, you can always restore silver’s brilliant gleam with some baking powder, aluminum, hot water, and science.