Ever observed the backside of a mirror? You might be wondering if it seemed to be painted in silver. But in reality, it’s not a paint, its coating or plating.
In order to turn glass into a mirror, it is passed through a chemical procedure. In which a thin layer of silver is generated on the glass surface. This procedure is known as the coating.
So far lots of metallic coatings are used for mirrors depending on applications such as gold, copper, silver, aluminium, and zinc. For household mirrors, an aluminium coating is a top choice.
Why silver is used in coating of mirrors? Silver is used for coating mirrors where high reflection is needed in optical mirrors. It reflects 95% of light across the visible spectrum. Protected silver has low emission and high adhesion to the glass surface. However, in typical household mirrors, the aluminium coating is more preferred.
Silver coated mirrors are mostly used in telescopes, When Gemini Observatory switched from aluminium mirror to silver mirrors, experts found a lot of improvements They noticed low emissivity and more telescope sensitivity. Though it needs tarnish layer to get extra protection from environmental factors.
What Is Silvering/Silver Backing or Coating on Mirror
Before we move on to the topic, read a few lines to understand, silvering, silver coating or silver backing. After this, it will become easy to understand the reasons for using silver metal for this purpose.
Silver backing or silvering is a chemical process of creating a thin layer of silver on the backside of the glass to turn it to a mirror. This thin silver layer is not a silver paint, it is a metallic layer that is responsible for causing a chemical reaction to the surface of the glass. This layer is then further protected by paint that is named silvering.
Silvering has a long history. Thanks to German chemist named Justus von Liebig who invented “silvering” in 1835. But now it’s usage for typical mirrors has stopped as aluminium is a better alternative to it.
7 Reasons Why Mirrors Are Coated With Silver
There are so many metals that are used for coating silver. They are silver, gold, aluminium, zinc, copper and platinum. The two most commonly used metals for mirror plating are gold and silver. Below are the top 7 reasons why silver is used in making mirrors:
1. Protected Silver Offers Versatile Plating material
Silver metal is versatile in itself. Its plating offers versatile features according to its application.
It is classified as noble metal in the periodic table. For years, it has been widely used for coating glass to make mirrors throughout the world. It is versatile in all its properties like thermal conductivity and metallurgy.
2. Protected Silver Plating Is Highly Reflective
Protected silver coating provides the highest reflectance within 500-800nm in the visible and infrared spectral region. This high reflective property makes it a top choice in various infrared applications. The possibility of damaging in ultraviolet light is high in silver plating. To counter this tarnish effect various specialized coating materials are added to create an extra protective layer to avoid damage.
3. Protected Silver Coating Has Low Emissivity
The choice of plating material is made by its emissivity, that is the energy it emits from its surface. Though bare silver has high emission that is unaccepted for plating purpose. But protected silver has low emissivity that makes it the perfect material for practical optical mirrors used in telescopes and other instruments. If emissivity of any coating material is high, it tends to make it less reflective.
4. Protected Silver Coating Has Strong Adhesion
Though bare silver is performing badly when it comes to adhering to the glass surface. But protected silver offers the best adhesion to the glass surface in the mirror. Moreover, this coating creates an extra protective layer that resists tarnish and damage in the case of ultraviolet rays reflection. When such rays touch the mirror surface, a proactive silver layer acts as a reflective dielectric layer. This way, it protects the mirror from any damage and improves the adhesion strength.
5. Silvering Is Affordable Alternative of Gold
Centuries age gold was used for plating applications. With the increase in the cost of gold plating, silver offers its best alternate. It is inexpensive metal and widely mined throughout the world. Because of low cost and high efficiency, it is used in automotive, telecommunications, electronics and solar power industries as an affordable alternate to gold and other metals.
6. Silver Plating Is Ideal for Observing All Wavelengths
When it comes to coating mirror, silver plating may have surpassed aluminium. The reason is that silvering is ideal for observing all wavelength whether they are infrared or other visible spectra. This fact is evident while studying the Gemini telescope in which silver plating was used for optic mirrors. However, this fact makes silvering risky as it tends to absorb more energy from ultraviolet rays
7. Protected Silver Coating Is Effective Against Humidity
Experts say that the protective silver coating is effective against humidity. As it absorbs more heat energy when ultraviolet lights fall on it. This light energy turns into heat and is found effective against corrosion. To make silver highly resistant to corrosion, plasma-enhanced atomic layers are deposited along with silvering. This layer acts as a barrier for highly absorbent mirrors.
Disadvantage of Silver Coating on Mirror
The choice of plating depends on two aspects: reflectivity and emissivity. The high reflectivity of silver makes it risky for various applications. Nowadays, this plating is not used for typical mirrors. Still, it is used for specialized optical instruments where high reflective properties of metal plating are required. Below are three disadvantages of silver coating for mirrors
- Silver coating is expensive than aluminium coating. That’s why it is used in different applications where high reflection is required in applications like a telescopic optical instrument.
- The silver coating has a small life span. When silver absorb more light energy out of ultraviolet rays, its adhesion to glass surface may decrease
- The risk of catching fire and corrosion is higher when bare silver is used for plating. It can heat up early. For this reason, protected silver is used for mirrors plating.
- The usage of silver plating is highly dependent on its applications. It is unaccepted for many optical instruments.
- The silver coating may require maintenance and extra layers to perform well in different atmospheres. It can add up to its cost.
- According to experts silver coating can distort reflection in household mirrors. If you want to get image precision, prefer aluminium coating rather than silver.
When Did Silver Stop Being Used In Mirror
Most people stop plating silver on mirrors from 1860 to 1880 in favor of aluminium coating. Probably they did so because silver plating was done chemically, while aluminium coating requires the easy procedure of evaporation.
One big reason for stopping this was the low cost of aluminium plating. However, it has not stopped fully. Many people still make silver-plated mirrors for different applications. But for typical mirrors, its usage has almost stopped.
Frequently Asked Questions
Why Is Silver So Reflective?
The main reason for using Ag or silver in metallurgy is that it is highly reflective up to 95% in the visible light wavelength region. This high reflective nature increases its luminous efficiency. Because of this luminous nature, it reflects more.
Why Are Gold and Silver Used for Coating Mirrors?
Because of high reflective nature gold and silver are used for coating mirrors. Though gold coating is very expensive now. It is done depending on its applications. Silver coating is still used in many medical and optical instruments. However, for typical mirrors, aluminium is more preferred metal for plating.
Conclusion
Now you better understand the reasons why silver is used in coating mirror. Though there are many substitutes for silver. But it is still used in various optical mirror coatings where no metal can outperform.
Thanks to the latest coating methods, now you can get a long-lasting plating effect. One more thing: this silver layer is extremely thin. Never try to recycle it as you may not get any cash out of it.
Have you ever witnessed the procedure of silvering a mirror in your life? If yes, share your experience with us.