Synthetic sapphire crystals like the ones used in watch crystals have the same crystalline structure as ones found in nature, but without the impurities that make the coloured gem varieties desirable. The sapphire crystals used in our watch crystals are grown, literally, from a seed.
Bauxite, an aluminium ore of which most comes from Australia is processed into aluminium oxide (Al2O3). This is then put into a special furnace with a small seed of pre-existing sapphire crystal, under a very specific heating and cooling regime, for around two weeks. By now a pure corundum ‘boule’ has formed; a giant mono-axial crystal with superior optical properties, thermal stability and hardness (inferior only to diamond and silicone carbide). The boule is then sliced or cored along an axis particular to the application of the final product. These cores are then wire cut in a diamond slurry to form wafers, the wafers are ground, lapped and polished. Most Schofield crystals feature a polished top edge bevel and a matt ground bottom edge bevel, the former for interesting optics and the latter for a better fitment to the gasket.
Sapphire unlike glass cannot be moulded, all sapphire shapes must be ground. Domed sapphire crystals are considerably more expensive than flat ones because of the assortment of grinding tools needed to gain the shape.
Sapphire has a higher refractive index than mineral glass or plastic and therefore needs to be coated to remove unwanted glare.
Anti-reflective coatings (ARC’s) are applied to the surface of lenses to reduce reflections and improve contrast. These are built up in layers each with a different refractive index. These layers are often different materials and/or different thickness. Schofield uses 8 layers for the top crystals and 5 layers for the case-back crystals. The bluish hue commonly seen on ARC’s is due to the coating reflecting slightly more blue light than other visible wavelengths. However it is possible to tint ARC’s, as done by some watch companies.
There are various materials used for ARC’s, typically fluorides and oxides and these are deposited using a Physical Vapour Deposition (PVD) process. Layers of magnesium fluoride (MgF2) for example, of differing thickness will give less than 1% reflection on crystals if coated on both sides, reflections occur at the surface (known as the Interface) on both sides of the crystal.
Schofield crystals are coated on the inside only, a deliberate choice made for two reasons:
1. Coating the outside of a watch crystal results in it wearing off, becoming scratched and looking unsightly because the coating is softer than the sapphire.
2. Coating both sides of the crystal renders it almost invisible, this is not the aesthetic result we want as some of the optical qualities of the sapphire are desirable; minor glare, diffuse and specular reflections and refracted light, occasionally prismatic.