The high-sounding sound of "gold" is most likely not real gold!

Editor’s Note:

Recently, the price of gold has continuously set new records. It is reported that the price of gold jewelry of many brands has exceeded 700 yuan. Gold has always been favored in wealth-related fields such as finance, jewelry and luxury goods, but did you know that gold also plays an important role in the fields of industry and technology?

When you think of "gold", what's the first thing that comes to your mind?

Is it "wealth"? Is it "power"? Or is it "the big gold chain on the jewelry display cabinet"?

What I thought of was a thought-provoking sentence in the textbook:

Gold and silver are not naturally currency; currency is naturally gold and silver.

Gold and silver are not by nature money, but money consists by its nature of gold and silver.

—Karl Marx

This textbook-level statement by Marx puts the naked reality before us: no matter how some people express their disgust or contempt for "money", it cannot be denied that the glittering gold has been deeply embedded in human thought and culture.

In modern society, although gold has withdrawn from the world of currency circulation, the "use of gold" is still the best way to reflect the importance of things. For example, the gold medal in sports competitions corresponds to the first place, the gold medal in various events represents the highest honor, and the stamps with gold wave embossing patterns issued by China Post when Hong Kong and Macao returned to China.

These almost unique existences all use gold or the concept of "gold" to endorse their status.

As society changes, the golden color of gold has become synonymous with wealth and status.

However, real gold is too expensive. Although we see a lot of gold objects in our daily life, they are actually a group of "Xibei goods" that meet people's psychological needs. "Titanium", "Rose Gold", "Champagne Gold"... These simulated gold-colored coatings with gold in their names are the mainstream of the gold objects we see in our daily life.

But remember, these names that sound very high-end and classy are actually not real gold in most cases, but other much cheaper plating. As for why it is nine out of ten but not all, it is because some extremely niche customized models may be made of real gold to create real gold according to the customer's customized requirements.

What many people don’t know is that gold is not only favored in wealth-related fields such as finance, jewelry and luxury goods, but also plays a pivotal role in industry and technology. According to statistics from the China Gold Association, in 2019 alone, China’s industrial/technological gold consumption reached 100.75 tons, accounting for about 10% of the country’s gold consumption.

Part 1

Why is gold also used in industrial production and scientific research?

Gold is used in industry and technology because of its physical and chemical properties. Gold has very good ductility, chemical stability and conductivity, high density and is easy to melt and process.

Therefore, 1 gram of gold can be rolled into about 1 square meter of gold foil without breaking. In addition, gold has a high density of 19.32g/cm3, so the gold foil with a thickness of only a few hundred atoms can no longer be described as thin as a cicada's wing - a cicada's wing is much thicker than it. The slight flow of air can make this extremely thin gold foil produce ripples.

From the outside, the gold foil is already translucent. When viewed against the light, the blue-green light can penetrate the gold foil and make it appear blue-green, while when viewed against the light, the reflected red-yellow light makes it appear golden red.

The 28nm thick gold film grown by deposition, note the transparent green and golden red reflections

Image source: Science, 2017, 355, 1203-1206, DOI: 10.1126/science.aam5830

But please note that you cannot directly pick up gold foil that is this thin, because the sweat and grease on your hands will quickly make the gold foil stick to your hands and break, and it is difficult to remove. So if you don't plan to coat your fingers with gold, don't reach out to grab it.

So, how do we pick up this thin sheet of gold without damaging it?

The right tool for picking up gold leaf is a feather, which can be used to absorb the gold leaf with static electricity and then spread on a suitable surface. Such thin gold leaf is usually the raw material for gold encapsulation. Manually rolling gold leaf and encapsulating gold with gold leaf can even be considered an intangible cultural heritage-level craft.

Picking gold foil with feathers

Image source: Xinhuanet

However, gold foil is sometimes used for illegal activities . Criminals will crush gold foil and use it as an illegal additive, mixing it into some rich food or drinks. Because gold foil is too light and difficult to settle in liquid, the visual effect of alcoholic beverages, cosmetics, etc. mixed with crushed gold foil is quite good under light.

But what would happen if you really ate food mixed with gold foil?

Our gastric juice is not as strong as aqua regia in terms of corroding gold, and even the intestines and stomachs of no known creatures have the ability to digest gold. However, there is no need to worry about "suicide by swallowing gold" after eating it, as the gold foil that can be suspended in liquid will not cause damage to the digestive tract due to gravity pressure.

In addition, since gold is stable enough, gold foil will not cause lesions in the digestive tract due to the release of metal ions after entering the stomach. In short, the current understanding is that after eating it, there is no benefit or harm except that it costs a lot of money. But don't try it lightly!

Now that we have talked about the high density of gold, let us take a look at its stability.

Gold is extremely stable and is difficult to be damaged by oxidation or corrosion. Gold artifacts among unearthed cultural relics may be the only type of artifact that "remains the same as when buried". Stone artifacts, including jade, are difficult to achieve this effect of indestructible gold. Phrases such as "indestructible gold" and "real gold is not afraid of fire" also reflect the ancients' understanding of the chemical stability of gold.

Gold artifacts found by archaeological staff in Liu He's main tomb

Image source: Jiangxi Provincial Institute of Cultural Relics and Archaeology

This stability brings practical effects to gold foil beyond decoration : a layer of gold foil on the surface of an object, no matter how thin it is, will not be corroded by oxygen and disintegrate, which minimizes the infiltration of oxygen and water, and greatly delays the oxidation, rust, dissolution, and decay of the objects inside (of course, the gold foil will break over time, and water and oxygen from the environment will seep into the seams). Although practical gold foil is expensive, there is no better surface treatment method under the old conditions. Royal palaces and Buddhist temples that are not short of money will use gold wrapping, gilding, and even gold plating in large quantities if conditions permit, using gold to protect important building components or statues, the throne room, and the golden body of the statue, not only to make it look rich and glittering, but also to prevent corrosion and rust.

Part 2

Where is gold used in industry and technology?

As mentioned above, in 2019, China's industrial/technical gold consumption was 100.75 tons. The number seems large, but in the same year, the Chinese used 6.7 times as much gold in jewelry. Obviously, gold is a precious metal after all, so it must be used sparingly in industry.

So where was the 100.75 tons of gold used?

Someone said, I know! Industry and technology! I remember the news about Chang'e 4 landing on the back of the moon. The lander was shining with gold! It was probably covered with a layer of gold foil. Our country has made great progress in science and technology in recent years, and has launched many satellites, so the amount of this is indispensable.

The panoramic camera of the Yutu-2 rover takes an image of the Chang'e-4 lander

Image source: China Lunar Exploration CLEP

Unfortunately, the answer is wrong. Although it looks golden, the golden "cloth" on the outer cover of satellites and spacecraft is mostly not gold-plated, but the color of polyimide (PI) used for temperature control. However, sometimes a layer of metal, including gold plating, is plated on polyimide according to actual needs to achieve specific protection needs.

Highlight the key points! The following items are made of real gold.

First of all, because gold has the function of preventing rust and corrosion, the modern electronics industry consumes a large amount of gold every year for the protection of gold fingers, switches, joints and connectors on circuit boards . This is the main way of industrial gold consumption.

These parts often require good electrical conductivity, but there is no way to seal them with protective paint for rust and corrosion protection, so they can only be protected by plating a layer of gold. However, because the gold plating layer can be very thin, the gold plating consumption on countless electronic industrial products each year is still only a fraction of the gold used in jewelry.

These jewellery, used to gild fingers, will keep the small factory busy for a long time.

Image source: Veer Gallery

The name of the gold finger comes from the fact that "the conductive metal sheet shaped like a row of fingers is plated with gold for data transmission and power supply." Of course, in order to save costs, the gold-plated layer on some low-end circuits may be saved or replaced with brass, which will naturally greatly reduce the corrosion resistance, but the cost will also be reduced. As long as it is not in the high salt fog environment of offshore oil platforms, this kind of electronic equipment that is not gold-plated for the sake of cheapness may have a chance to last until the customer decides to change the machine.

Gold finger: power supply and signal interface for plugging in circuit boards or expansion cards

Image source: Veer Gallery

Secondly, some people will notice that some fighter jets’ canopies reflect a rare golden color. This time, I guessed right, it is really the result of gold plating. Of course, only the “rich” version of fighter jets will have gold-plated canopies - the main fighter jets that require stealth performance are not cheap, so they can be gold-plated if they need it!

Take the F-22A as an example. The integral bubble canopy is indeed plated with gold. The reflection in the picture below shows the existence of the gold plating layer. Due to the support of the canopy, the gold plating can be so thin that it is almost transparent, with limited impact on the pilot's vision.

However, this thin layer of gold can effectively prevent radar waves from penetrating the canopy, making the canopy surface and the aircraft skin form a reflective whole, thereby effectively controlling the reflection angle of radar waves. This is to avoid a situation where radar waves shine into the cockpit to form a strong scattering source, destroying the overall low RSC design of the fighter, and thus reducing the radar stealth effect.

As for "low-end" fighters or non-stealth fighters that are not so demanding on stealth, they do not care about the poor RSC, so it is rare to see their canopies reflecting gold.

By the same token, in some systems with limited size but high requirements for electromagnetic compatibility, a shielding layer plated with gold is also a means of reducing electromagnetic interference.

Again, the same principle of gold plating conductivity is also used in scanning electron microscope (SEM) microscopic imaging. When working with non-conductive non-metallic samples, a thin layer of gold film needs to be sprayed on the surface of the sample to act as a conductor so that the scanning electron microscope can effectively form an image.

For example, if you want to use a scanning electron microscope to observe the microscopic details of a mosquito, you have to spray-coat the mosquito with a layer of gold. It is really an extreme honor to coat a dead mosquito with a layer of high-purity gold whose purity (price) is far higher than jewelry-grade 999 gold and 10,000-pure gold.

The head of a mosquito under a scanning microscope. Although this photo is in black and white, the mosquito has already become a golden body.

In aerospace, gold is also a very important protective material for protecting against certain types of radiation in space. In addition to the gold plating on the polyimide cladding mentioned above, gold-plated parts are required for extravehicular space suits.

In the picture above, the shiny golden hemispherical object on the upper part of the mask of the Russian "Sea Eagle" space suit is actually a half-opened gold-plated mask, which is the outermost openable and closable anti-glare protective cover of the space suit mask, and is plated with a layer of real gold. This layer of translucent gold is plated on a transparent supporting material, and astronauts can open and close it manually to protect against strong sunlight, especially ultraviolet and infrared rays in sunlight, during extravehicular activities.

In addition, gold also has many uses in the laboratory. For example, the "gold electrode" is a commonly used tool for electrochemistry and electroanalysis. The sulfur atoms at the end of organic compounds are easy to combine with gold, making the gold electrode particularly suitable for surface modification of sulfur-containing compounds (thiol compounds) and then conducting related research or detection.

As for the "nanogold" that countless researchers love and hate, it is truly an amazing tool that spans chemistry, materials, nanotechnology, biology, and medicine!

Nanogold materials of different shapes and sizes have different light absorption properties and display different colors.

Nanogold can have various geometric shapes, its surface is easy to modify, and it has the property of easily binding to proteins and nucleic acids. It can also be detected by fluorescence, surface enhanced Raman scattering (SERS), dark field imaging, electrical analysis and countless other detection methods, and has a myriad of rich usage methods. These characteristics make nanogold a very good basic material for making biochemical test kits.

What? You said you have seen nano-gold health products or cosmetics? Don't believe the scams of unscrupulous merchants...

In addition, gold is also one of the common catalyst components in chemical laboratories and industrial production . In the periodic table, gold's neighbors are all very important catalyst elements, so gold will also appear in the composition of some catalysts, and is used and consumed in industrial production such as petrochemicals and corresponding theoretical research.

Nano-gold with a very high surface area ratio is also a promising new catalyst. However, like platinum group precious metal catalysts, if the problem of catalyst recovery and reuse is not solved, the amount of precious metal materials cannot be saved, and the production cost cannot be reduced, which is a very distressing thing.

Finally, I want to emphasize that although the cost of using "gold" is not low, the purpose of using "gold" in industrial production and scientific and technological research is by no means to "show off wealth", but it is a precious metal raw material that must be used. I would like to add that in actual industrial production, the material cost of gold is usually only a fraction of the total cost...

Produced by: Science Popularization China

Author: Lao Xu Novich

Producer: China Science Expo

This article only represents the author's views and does not represent the position of China Science Expo

This article was first published on China Science Expo (kepubolan). Please indicate the source of the public account for reprinting