SAVOIR FAIRE What is this Sapphire Crystal?

Watchmakers first began to use glass protection in the early 17th century, made from rock crystal quartz, a big step forward from the earlier uncovered dials enclosed in metal cases with a hinged cover that needed to be opened to reveal the time. However, the glass was not always a perfect fit, and those first protective layers of flattened quartz were very easy to break and crack. 

The invention of the first semi-synthetic plastic in 1862 by England’s Alexander Parkes opened the door to the production of the first fully synthetic plastic in 1907. Introduced by Belgian chemist Leo Baekeland in 1907, who named it Bakelite, it was followed in turn by Perspex plastic made by British Imperial Chemical Industries in 1932, a material quickly implemented to create hundreds of products that we use in our daily lives until today.

Synthetic plastic / acrylic / plexiglass / lucite is flexible, lightweight, durable, shatter resistant, and relatively inexpensive to produce, becoming, for these reasons, the norm of protective dial shields for many years, especially for tool watches. However, if its smaller scratches can be polished away, it is nevertheless more likely to scratch than other surfaces, scoring only 3 on the Mohs Hardness scale introduced in 1812 by German geologist and mineralogist Friedrich Mohs to measure the comparative scratch resistance of a mineral from 1 to 10 (talc is rated 1 on the Mohs scale, and diamond is rated 10). A new material had to be found that would be tougher and more scratch resistant.

Enter Mineral glass / silica, still common today on mid-range watches. More expensive than plastic / acrylic crystals and much more scratch resistant, it is a form of glass created from the melt of sodium silicate, produced by melting sodium silica sand with sodium carbonate.

The high sodium content results in a glass whose specific chemical and physical properties include transparency, a high gloss, chemical resistance and temperature stability perfect for optical, industrial and electronics applications. For watchmaking purposes, it is usually ‘Tempered Mineral Glass” made by heating the surface of the glass to a high temperature, resulting in a more scratch resistant and less likely to crack or shatter material. However, if it does get scratched or chipped, unlike acrylic, it cannot be polished and ‘repaired’ and must be replaced.

Sapphire is one of the two precious gemstones derived from the mineral corundum, the other being ruby, and corundum is the third hardest mineral on earth. Made of crystallised aluminium oxide, naturally clear, the blue and red colours of the precious stones come from trace elements of other minerals. Most importantly, with a ratio of 9 on the Mohs scale, it can be scratched only by diamonds with the highest ratio of 10, and moissanite with 9.5.

In 1902, French chemist Auguste Verneuil developed a process to manufacture synthetic sapphires and rubies, using a flame to melt the aluminium oxide powder within the natural stones that was slowly deposited on a seed crystal. Corning Glass Works improved this in the 1930s by producing larger and higher-quality crystals, still used today. While synthetic rubies were soon adopted by watchmakers in their watch movements to replace the more costly natural stones, the sapphire crystal windows to protect our watch dials were used rarely, and mostly for tool and diving watches that needed extra strength and protection. It was not until the 1970s that they were generally recognised as an improvement to plastic and glass protection and started to appear on high-end watches.

More expensive to produce than its predecessors, the synthetic sapphire that forms in round masses when heated needs to be sliced into pieces with diamond–coated saws; the discs are then ground and polished into watch crystals.

In addition to their high resistance, crystals can be manufactured into different shapes, flat or domed, with an average thickness of 1.0 to 1.5mm, up to 5mm and more for rugged tool and deep diving watches. More reflective than mineral glass, sapphire crystals are often coated with one or more layers of anti-reflective material, preferably on the inside, as external coating may be subject to scratches, even while the crystal itself remains unscratched. And if the front crystal allows you a beautiful transparent panorama of your dial, on the back it provides fascinating views of the movement inside. 

Sapphire crystal has become the preferred choice for TAG Heuer and many other luxury watchmakers due to its exceptional durability and scratch resistance. The advancements in synthetic sapphire production since the early 20th century have made it accessible for a wide range of timepieces, allowing for both aesthetic appeal and functional resilience. As watch technology continues to evolve, the enduring qualities of sapphire crystal ensure that it remains a hallmark of quality in the world of horology, blending elegance with practicality for discerning watch enthusiasts.