The material we arrived at to protect precision equipment
The sapphire crystal that protects the Retina display of the Apple Watch has been talked about as "it won't scratch with a drill! It won't break with a hammer!" If anyone around you isn't surprised at how robust it is, that person may like watches. The material called sapphire crystal, which feels new in the digital gadget world, is a familiar material in the wristwatch industry as a representative material of glass that protects the windshield = dial.
Glass has a long history and existed long before mechanical watches. A clock is a precision machine that hates dust, but if you cover the dial, you will not be able to know the time. It was only natural that glass was used so that the clock could be displayed while being sealed. The fact that the glass is fragile is not a big problem with table clocks.
However, the windshield is a big weak point in a pocket watch. To make up for the shortcomings of old pocket watches, a "hunter case" that completely covers the windshield with a metal cover was common. A hunter is a hunter. At that time, only the upper class owned a watch and enjoyed hunting. Even during hunting, which is a luxury activity, the hunter case was a watch that responded to their desire to use the watch with peace of mind. The act of taking the watch out of his pocket and opening the lid made me feel the status of the person holding it.
Pocket watch with hunter case
During the Second Boer War of 1899, pocket watches were worn around the wrist. I couldn't put the watch on my arm because I couldn't take it out of my pocket one by one, so I couldn't open and close the watch lid while fighting. The watch chosen on the battlefield was an open case with a bare glass windshield.
The glass windshield, which is exposed to impact and dust on the battlefield, was protected by a grid of metal guards. Measures for glass will continue thereafter. Cartier "Santos", which is said to be the first wristwatch designed as a non-military watch, was born in 1904. The screwed Santos bezel (the frame surrounding the windshield) was actually a detail to facilitate glass replacement. Rukuruto's masterpiece "Reverso", which was born in 1931, is a mechanism that protects the glass by rotating the case 180 degrees. Each was an idea born for a gentleman who wants to wear an elegant wristwatch in activities such as balloon flight and polo competition.
Organic glass, which was industrialized in 1934, changed the common sense that glass breaks and is chipped since the dawn of history. It's so-called acrylic. It is 10 to 16 times stronger than glass, and even if it breaks, it will not have fine debris, so it will not affect the movement. It was also an era when aircraft and automobiles became widespread, and wristwatches were required to be tough. Acrylic becomes the mainstream of the windshield of wristwatches at once. Acrylic windshields have greatly contributed to the evolution of wristwatches. However, acrylic has the weakness that it is hard to break but easily scratched. You might think, "Is it scratched even though it is strong?", But there are two factors in toughness: hardness and strength. Acrylic has high strength but is inferior in hardness to glass. If the number of scratches increases, the visibility will decrease, and above all, the watch will look old.
Hardness is more than 3 times that of gorilla glass
Sapphire crystal is the last material the watch industry has noticed as a scratch-free and unbreakable glass. As a reference value, acrylic has a Vickers hardness of 30HV and tempered glass has a hardness of 640HV, while sapphire crystal has a hardness of 2300HV (by the way, stainless steel has a maximum of 140HV, and gorilla glass has a maximum of 701HV). The tensile strength is around 30MPa for glass, around 120MPa for acrylic, and 2250MPa for sapphire crystal. It is strong against chemical substances and has high transparency. It is an ideal material for the windshield of a wristwatch. Rolex Day-Date has been using sapphire crystals since the late 1970s. Most of today's luxury watch windshields are made of sapphire crystal.
However, sapphire crystals are not very popular except for luxury watches. This is because mass production like glass and acrylic is not possible and the cost is high. Sapphire crystal is a single crystal of high-purity alumina. High temperature and high pressure of 2000 degrees are required for formation, and there is no choice but to crystallize over time. Furthermore, if it is so hard, it is not easy to process. The only way to mold sapphire crystals is to polish them with diamonds. That's why the windshield of most watches is polished flat.
Also, sapphire crystal is heavier because it has a higher specific density than ordinary glass. It can be inferred that one of the reasons why the Apple Watch Sport, which uses aluminum for the case and is lightly finished, uses Ion-X glass instead of sapphire crystal is the weight problem.
Some wristwatches dare to express the windshield of free design with sapphire crystal. For example, Franck Muller. Have you ever seen the soft curved windshield of "Tonow Carbex"? That is a thick sapphire crystal that has been polished on both the front and back sides and molded over time.
The windshield of the Apple Watch also draws a beautiful curve to match the case body. You can see how hard it is and how long it took for the watch to reach the sapphire crystal. Fortunately, those who have an Apple Watch now and those who will be users will definitely be active, like the gentlemen who wore watches on horseback with hunting and watches instead of drills and hammers. We want you to enjoy its robustness on the spot.
