Does the blackening of silver jewelry mean that the body is detoxifying? Stop kidding! The real reason is this

Produced by: Science Popularization China

Produced by: Zhiyao Science

Producer: Computer Network Information Center, Chinese Academy of Sciences

Since ancient times, gold, silver and their products have been loved. However, the dazzling silver products will gradually become dim or even black during use, and their beauty will decrease rapidly.

Many people misunderstand this phenomenon as "the body is detoxifying, and the black color represents the toxins excreted by the body"; some unscrupulous merchants even claim that "silver can absorb toxins from the body" in order to sell more silver jewelry.

So, what is the real reason why silver products turn black?

Debunking "detox" rumors

To understand why silver jewelry turns black, let's first look at the two forms of silver jewelry turning black:

1. Long-term storage of silverware: the luster becomes dull, and the surface turns yellow or black.

This is particularly common on ancient silver wares exhibited in various museums. The black substance on the surface is mainly silver oxide Ag2O, which is caused by the oxidation of silver after being exposed to air for a long time.

Since silver is chemically stable and has a difficult time undergoing oxidation, this phenomenon is common in silverware that has been exposed to air for a long time. The corners and sharp parts of silverware are more susceptible to oxidation due to their higher energy.

The antique silver wine jug in the picture below is an example. It can be seen that although the large, continuous blank surface has darkened, there is no large black area; while the black is more prominent in the leaves, vines and other details on the jug.

Left: Antique silver flagon, made by silversmiths John and George Angell for the 1837 London Olympics; Right: Silver oxide powder (Image source: Wikipedia)

2. Silver jewelry is worn close to the skin: it becomes darker and black spots appear where it is close to the skin

This kind of situation is more common in our lives: most silver rings, bracelets and other silver jewelry will lose their luster and even develop black spots after being worn for several months or even weeks.

Left: A brand new silver ring; Middle: A silver ring worn for a period of time, with obvious black spots on the ring; Right: Silver sulfide powder

(Image sources: left: veer photo library; middle: provided by the author; right: Wikipedia)

It is generally believed that the "silver rust" of personal silver jewelry is mainly produced in two ways: one is the same as above, which is silver oxide (Ag2O); the other is silver sulfide (Ag2S).

You may ask: "Didn't I just say that silver oxidizes very slowly in the air? So how is silver oxide produced?"

Yes, it is very difficult for air alone to corrode a smooth silver surface, but close-fitting silver jewelry is more likely to come into contact with human sweat than other silver products.

Human sweat contains various ions, such as chloride ions and sodium ions, which are good conductors of electricity. Therefore, when silver comes into contact with sweat and air at the same time, a primary battery is generated, and the oxidation reaction between silver and air is accelerated to produce black silver rust.

Schematic diagram of the silver-sweat-air primary battery (Image source: provided by the author)

Another source of "silver rust" - silver sulfide is also closely related to human sweat. The protein in the human body often contains sulfur, and some sulfur ions are excreted with human sweat. Under the oxidation of air, these sulfur ions will react with silver to produce black silver sulfide.

So, why do people believe that "silver jewelry turns black because it absorbs toxins excreted by the body" - this may be related to the saying of "silver needle testing for poison"?

Song Ci, a famous forensic doctor in the Southern Song Dynasty, once recorded the famous "Huai'an Strange Case" in his book "Xi Yuan Ji Lu". The method he used to solve the case was "silver needle testing for poison".

In fact, silver needles in ancient times could really detect poison. This is because the poison at that time was generally arsenic (mainly arsenic trioxide), and due to the purification methods at that time, arsenic often contained a large amount of sulfur. Since silver needles would react to produce silver sulfide after contact with it, silver needles became a test for poison.

Until modern times, when many people saw their silver jewelry turning black, their first reaction was "the body is detoxifying". However, this is not the case. It is very normal for the human body to excrete some sulfur with sweat. Whether the silver jewelry turns black or not does not indicate the physical condition.

Why doesn’t some silver jewelry turn black?

Although it is a common phenomenon that silver turns black, some people's silver jewelry always seems to be as shiny as new. This phenomenon can actually be attributed to two parties, one is the manufacturer and the other is the user.

Jewelry manufacturers have tried every possible way to combat "silver tarnish" so that their jewelry can maintain its beautiful appearance for a longer period of time. The strategy they use is simple: since silver turns black when it comes into contact with sulfur in the air or sweat, they just need to use "silver protectors" to prevent them from coming into contact.

Silver protectors, or silver sealers, are transparent but airtight materials that can cover the surface of silver with a single-molecule-thick film by immersing it. This process is like putting a layer of clothing on silver jewelry to prevent it from being corroded by air and sweat. Generally speaking, the effect of silver protectors in preventing the formation of surface sulfides can last for several years.

In addition, the user's wearing habits, personal characteristics, and living environment will also affect whether the silver jewelry will turn black. For example, if your friend is very rich and wears a new one every day, it will indeed be as bright as new all the time.

Of course, there are only a few rich people like this. The real reason may be that their sweat glands are less developed, the amount of sweat is less, and the sulfur content in sweat is lower, which can cause less reaction that makes silver turn black; or, they like to wear rough clothes, so the friction between silver jewelry and clothes is like a polishing process.

How to make blackened silver jewelry shine again?

Is there any way to make silver jewelry shine again? Yes, there is. Here we briefly introduce several convenient treatment methods and their principles.

1. Aluminum foil/tin foil restoration

Prepare several pieces of shredded aluminum foil or tin foil, soak them in a baking soda solution (if baking soda is not available, salt water will do); then place the silver jewelry to be treated in the aluminum foil/tin foil and immerse it in the solution, heat the solution (you can also use boiling water directly), and take it out after a few minutes. If there are white impurities on the surface, you can use hydrochloric acid or white vinegar to clean it.

This method can reduce silver sulfide and silver oxide into single silver, thus preventing the silver from decreasing in amount and the jewelry from becoming smaller with washing.

The principle is that aluminum and tin have very strong reducing properties. When heated under alkaline conditions, they become good reducing agents and transfer electrons to silver, thereby restoring silver to its metallic form. However, after the reaction is completed, aluminum hydroxide may adhere to the surface and condense into white impurities. Therefore, if white is observed, it can be removed by acid washing.

Illustration of silver washing by reduction method (Image source: www.instructables.com)

2. Physical polishing

The physical polishing method is relatively convenient and easy to operate, and it can be done with a tube of toothpaste or an eraser.

Toothpaste usually contains polishing agents, which are hard tiny particles that can remove impurities on the surface of silver jewelry by rubbing against them. Therefore, you can also remove surface rust by gently rubbing the surface of silver jewelry with toothpaste on a toothbrush. However, the toothpaste you choose should not be the type with larger and harder polishing particles, otherwise it may leave scratches on the silver surface.

Generally speaking, toothpaste with calcium carbonate as abrasive is more appropriate. If it contains silica particles, it may not be suitable.

If you don't know which toothpaste to use, you can usually achieve the goal by directly wiping the surface of the silver jewelry with an eraser. The various silver polishing papers sold on the market also work on the same principle.

It should be noted that if the surface of the silver jewelry is coated with silver protector, the polishing method will destroy it completely. Therefore, some protective agent can be added after polishing to make it last longer.

3. Soak in carbonated beverages

Since there is often very little silver sulfide on the surface of silver jewelry, it can be dissolved in an acidic solution (such as cola, soda, etc.).

Most commercially available silver cleaning solutions are based on a similar principle, which is to dissolve silver sulfide and silver oxide into water to achieve the purpose of cleaning the surface.

Well, without further ado, everyone in front of the screen should take action to save their dirty silver jewelry!

References:

(1) Li Danhong, Wang Fujie. Research on anti-discoloration performance of silver coating protective agent[J]. Technology and Market, 2019, 26(07): 44-45.

(2) Liu Jinhong, Wang Yihong, Guo Zhirui, Gu Ning. Study on the anti-corrosion of self-assembled monolayer for silver protection [J]. Materials Protection, 2004(01):6-8+65.

(3)Wikipedia-Silver, Silver Oxide, Silver Sulfide.