A Guide to Crystal Glass

By J. Daly, Dublin, Ireland

March 31, 2021

When looking for the right watch, it is important to arm yourself with as many details as you can to make an informed purchase. When it comes to watches, the dizzying array of variation and jargon can make the task daunting at first, but fortunately we have compiled a list of some of the basics elements to help you have all the right information. Here we will discuss one of the most important and obvious features of any watch, the glass. Unsurprisingly, there are numerous materials, compounds and manufacturing processes used to make glass for watches, but they largely fit into three broad categories which we will cover from most fragile to most durable.

Acrylic

Prior to the advent of sapphire crystal in watchmaking during the 1990s, manufacturers used acrylic for their crystals as standard. Sometimes referred to as hesalite, plexiglass or perspex, this material types are actually forms of hardened plastic. Historically, strength requirements meant watches with this material were often thick, with domed tops. Its lightweight and flexible properties do allow for excellent shatter resistance, but this comes at the cost of much higher susceptibility to scratches. Minor scratches however can be buffed out with careful polishing. The material is inexpensive to manufacture and hence is regularly seen on lower priced watches but sometimes luxury brands will use it on certain models.

Mineral

Moving on from acrylic, we have mineral crystal. This is a compound made from tempered glass derived from silica and offers much better scratch resistance than acrylic, but not to the same standard as sapphire. Additionally, mineral glass is often treated with anti-reflective coating ensuring good visibility in poor lighting conditions, resulting in less eye strain for the wearer. As with sapphire, scratches cannot be polished on mineral glass in the same way as acrylic, but the inherent scratch and impact resistance coupled with excellent durability makes this a great material for mid-range watches.

Sapphire

Holding the top spot on the watch glass food chain, we have sapphire crystal. Used today by most luxury watch brands, this material is made from a synthetic, colourless, lab-grown sapphire known as corundum. This synthetic corundum is actually a crystalline aluminium oxide that is the second-hardest material after diamond. The material was invented by a French chemist and usually is made in solid disc form, that is milled, ground and machined by specialized laboratories. Sapphire crystal can be less shatter resistant than mineral or acrylic, but easily offers the best scratch resistance available. Watchmakers have recently begun to experiment more with this material, which has resulted in models like the Bell & Ross BR-X1 Tourbillon Skeleton Sapphire.

Magnifying window

Occasionally a magnifying window can be featured on the glass. This is normally positioned above the date window on specific models, which can be difficult to read at times due to the limitations of the dial dimensions. The Rolex Submariner is an iconic example where the magnifying window is employed.

Exhibition caseback

While many watches will use metal casebacks, most commonly with quartz movements, others with more complex mechanical or automatic movements will use an exhibition case. This is a glass caseback that allows the wearer to view the internal workings of the movement while maintaining perfect protection.