Exhibition caseback

An exhibition caseback is a case back containing a transparent window, so the movement remains visible with the watch fully assembled and sealed. The window is almost always synthetic sapphire, held in a metal surround that is either screwed into the case or secured by screws around its edge. It is the reason movement decoration matters to buyers at all: a finish nobody can see is a finish nobody pays for.

How it is built

The window is a disc of synthetic sapphire, hardness 9 on the Mohs scale, pressed into a metal ring against a gasket. That ring is the part that engages the case: either a threaded back that screws in and is opened with a die-type wrench matching the notches cut into its edge, or a flat back held by four to eight screws over an O-ring. Better examples carry anti-reflective coating on the inner face, which is why some movements look as though there is no glass in front of them at all.

Not every transparent back is sapphire. Cheaper watches use mineral glass, including Seiko's Hardlex, which is a great deal softer and picks up scratches from a desk edge or a belt buckle in a way sapphire does not. The comparison of crystal materials covers the difference, and it applies as much to the back as to the front.

What it costs

There are trade-offs, though they are smaller than folklore suggests. A sapphire window adds thickness and cost, and it is one more sealed joint in the case. Solid backs remain standard on watches where the back is doing structural work, on antimagnetic watches that need a soft-iron inner cage, and on many diver's watches, partly for robustness and partly because there is nothing to gain from a window nobody will look at underwater. A well-engineered display back can still be pressure-rated for serious depths, so the presence of a window is not by itself a sign of a fragile watch.

The numbers settle the water-resistance question quickly. The Tissot PRX Powermatic 80 pairs a display back with a 100 meter rating, the Seiko 5 Sports does the same, and the Omega Aqua Terra carries a transparent back at 150 meters with a screw-in crown. What a display back does rule out is the traditional route to magnetic protection, since a soft-iron shield has to be a closed box. Omega reaches 15,000 gauss on the Aqua Terra by removing ferromagnetic parts from the movement instead of shielding it, which is exactly why that watch can have a window where an older antimagnetic watch could not. The guide to magnetism explains the trade.

What to look for

A display back rewards a loupe. Look at the bevelled edges of the bridges and whether the inward corners are crisp. Look at whether decoration continues onto surfaces that are hard to see, or stops the moment it leaves the visible area. Look at the screw heads: polished, with clean, aligned slots and no burring, is a sign of care.

Then look past the decoration. Striping and circular graining can be applied by machine in seconds and tell you very little on their own. The finish that costs real hours is anglage, the beveling and polishing of edges, and above all inward angles, which no machine can cut. Our guide to movement finishing separates the techniques by what they actually take to produce.

Remember also what side you are seeing. Through the back you are looking at the bridge side, where the rotor covers a large share of the view on most automatics. Some makers skeletonize or offset the rotor for that reason; others simply decorate the rotor and leave the plate beneath plain.

Living with one

The gasket behind the window is a consumable, like the crown and crystal seals, and it is replaced at every full service. Any time the back comes off, for a battery, a regulation or an inspection, the case should be pressure tested before the watch goes near water: indicative US figures are $25 to $60 for a test on its own. Moisture that reaches the movement usually announces itself as fog under the crystal, and by then it is a bench job rather than a drying-out job.

Common misconceptions

An exhibition back does not mean the movement is in-house, high grade, or even finished at all: the same window can sit over a bare, unadorned caliber. Plenty of watches show a plainly finished workhorse through sapphire, and the decision is a marketing one as often as a horological one. It also does not make a watch more delicate in normal wear, since sapphire is far harder than the metal around it. The one genuine risk is edge impact, the same weakness as any sapphire crystal.

Two smaller confusions are worth settling. A display back is not the same thing as a skeleton dial, which opens the front of the watch instead. And a solid back is not a sign of secrecy or of a cheap movement: Rolex fits solid backs across its current catalog, for reasons of case rigidity and sealing rather than shyness about what is inside.