Mirrors come in many shapes, sizes, and colors. The most common type of mirror found in homes and businesses is made of glass with a thin layer of reflective silver material on the back. This leads many people to assume that all mirrors are silver in color. However, while silvered glass is the most prevalent, mirrors can actually be made using various metals and take on different hues.
How Mirrors Work
Mirrors operate based on the law of reflection. When light hits a reflective surface, it bounces off at the same angle it approached. This allows mirrors to produce images by reflecting light and allowing us to see ourselves. For a surface to be an effective mirror, it needs to be highly reflective. Most mirrors achieve this through a process called silvering.
Silvering involves applying a thin layer of silver or aluminum to the back of a piece of glass. Silver and aluminum have a refractive index close to the refractive index of glass, allowing for optimum reflectivity. The silver acts as a coating that causes the light to reflect back evenly across the entire surface area. Other metals like copper or gold can also be used, but silver and aluminum provide the clearest reflections.
History of Mirrors
The earliest mirrors were made from polished obsidian, a naturally occurring volcanic glass. These types of mirrors have been found at archaeological sites dating back to 6000 BC. By 3000 BC, mirrors made from polished copper were being manufactured in Mesopotamia and Egypt. Copper and bronze mirrors were common in ancient China, India, and Central America as well.
The process of coating glass with molten metal to create reflective mirrors was invented in 1835. German chemist Justus von Liebig developed the silvering technique, which was further refined by German physicist Carl August von Steinheil. This breakthrough enabled the mass production of affordable silvered glass mirrors. Before this, mirrors were luxury items made from precious metals.
Other Metals Used for Mirrors
While silver and aluminum dominate modern mirror making, other metals can produce reflective surfaces as well. Here are some examples:
Copper
Copper was one of the earliest metals used for mirrors. Pure copper has a reddish-orange color and gives mirrors a warmer, softer tone. Oxidation causes copper to deteriorate over time, so modern copper mirrors require protective coatings.
Tin
Tin can be alloyed with other metals like copper to create a reflective surface. Tin by itself has a bluish-white color. Tin mirrors need to be kept in air-tight conditions since exposure causes tin oxidation.
Gold
Gold mirrors have been crafted since ancient times. Gold provides beautiful reflections, but it is expensive and soft. Gold coatings are very thin, often layered on top of glass coated with silver or copper.
Chromium
Chromium displays a steely blue-white hue when polished into a mirror surface. It does not tarnish easily, retaining its shine. Chromium is commonly applied as a plating on top of silver in a process called chroming. This protects the silver from corrosion.
| Metal | Color |
|---|---|
| Silver | Whitish-gray |
| Aluminum | Silvery gray |
| Copper | Reddish orange |
| Tin | Bluish white |
| Gold | Warm yellow |
| Chromium | Steely blue white |
Mirror Coatings and Tints
While plain silvered mirrors have an even gray tone, modern mirrors can be modified to produce different colors and effects. Here are some examples:
Tinted Coatings
Mirrors can be coated with transparent dyes or chemicals to create a tinted effect. This provides a subtle color overlay without distorting the reflection too much. Common tints include blue, bronze, purple, and green.
Dichroic Coatings
Dichroic coatings use thin layers of metallic oxides to selectively reflect certain wavelengths of light. This causes the mirror to appear to change color depending on the viewing angle. Dichroic mirrors often display rainbow-like iridescence.
Two-Way Mirrors
Two-way mirrors have a semi-transparent reflective coating, allowing viewing from one side but reflection from the other. These are made by silvering only part of the glass pane. The unsilvered section lets people behind the mirror see through.
Safety Mirrors
Safety mirrors use a backing that retains the glass if broken. Common options are a mesh wiring, plastic sheeting, or gluing the glass fragments to the backing. The reflective surface usually consists of acrylic or polycarbonate instead of real glass.
Types of Mirror Glass
While ordinary soda-lime glass works for mirrors, specialty glasses can enhance reflectivity, durability, or safety. Some examples include:
Float Glass
Float glass provides a perfectly flat surface ideal for even reflections. It is manufactured by floating molten glass on top of molten tin, yielding surfaces flat to under 10 nanometers. Over 90% of mirrors use float glass.
Borosilicate Glass
Borosilicate glass contains boron trioxide that improves chemical and heat resistance. It can withstand temperature fluctuations that would cause regular glass to expand and distort the reflections.
Strengthened Glass
Strengthened glass is tempered through controlled thermal or chemical treatments. This compresses the surface to resist breakage and improve safety. Strengthened mirrors are up to five times more resistant to impact.
Laminated Glass
Laminated glass uses a plastic interlayer between multiple sheets of glass bonded together. When shattered, the pieces stick to the interlayer rather than falling away. This helps maintain the mirror’s appearance and usefulness after damage.
Mirror Manufacturing Process
Manufacturing quality mirrors requires precision processes. Here are the main steps involved:
Cutting
Sheets of mirror glass are cut to the desired shapes and sizes. This is done using specialized cutting tools and equipment. The cut edges are also polished smooth.
Silvering
A silver coating is applied uniformly across the back surface of the glass. Modern methods include vacuum deposition, electroplating, and chemical deposition. Older techniques involve depositing silver nitrate solution and exposing it to sugars or ores.
Copper Coating
A thin layer of copper is added over the silver. Copper prevents silver oxidation and enhances reflectivity.
Painting and Enhancements
Additional paint layers can be added to the back to improve moisture resistance. Reflective films, safety films, or other coatings may also be applied to achieve certain mirror properties.
Quality Inspection
Finished mirrors are visually examined and tested to ensure reflective quality meets specifications. Optical devices and imaging software can analyze reflections down to the nanoscale.
Why Are Most Mirrors Made with Silver?
Silver makes an ideal material for mirror reflective coatings due to its unique physical and optical properties:
– Has very high reflectivity – reflects up to 95% of visible light
– Reflects light evenly across the visible spectrum
– Smooth surface allows clear image reproduction
– Does not oxidize or deteriorate easily
– Can be deposited very thinly without losing reflectivity
– Bonds strongly to glass for durability
– Is relatively affordable compared to other reflective metals
No other metal combines excellent reflectivity, corrosion resistance, and cost-effectiveness like silver. Aluminum has many similar benefits but is slightly less reflective than silver. That is why silver remains the first choice for mirror manufacturing.
Conclusion
While we often assume mirrors have a silver color, they can actually be created from various metals with different hues. However, silver and aluminum continue to dominate modern mirror making, especially for mass-produced mirrors. Their outstanding reflective qualities, durability, and cost profile make them ideal for silvering the back of glass. So next time you look in a mirror, know that while the reflections appear colorless, there is likely a thin layer of magical silvering bringing your image to life!
