What causes the hairline cracks that often occur in pocket watch dials? More importantly, what can I, as a collector who likes to tinker with his watches, do or not do to avoid creating new ones?

It helps to understand how it happens if you realize that a dial is a thin sheet of copper or brass with a coating of what is essentially white glass sand fired and melted in place on the dial ,and with the numbers painted on it and usually fired again!!! So be very careful putting pressure on an installed dial , and more importantly take great care re-installing a watch dial and just snug up the dial foot screws do not tighten them too much. Those will help. As for an already hair-lined dial, use a bath of Polident denture cleaner ,it works wonders LOL

To put it in engineering terms, a porcelain enamel dial is a composite structure, made of materials with greatly differing properties, a layer of glass-like material fused to a metal backing.

The backing is a ductile material, but the glass overlay is not, so when the dial is stressed, the metal will deform, but the glass will fracture.

those who knows,knows!!! Well put Ed

I don't know Ed, I've seen Jesus do it.

Perfect dial on table, no lines or cracks, and then all of a sudden it cracks right down the middle.

Curiously it was about the same time the AC came on, but I was taught in college correlation =! causation.

And curiously enough, on the same note, important collections are stored in low humidity environments with a lower (like 68 fahrenhit) temperature that is kept constant. The better the storage facility, the less the variation in humidity/temperature inside through out the day.

As they say, "Dials crack" and scat get old.

Frank, You are a weird dude. What a since of humor.

Simple laymen's terms....

The backing is brass or copper and they are both heat conductors while the porcelain on top is an insulator....and does not expand at the same rate as the backing so when temp changes are large enough the cracks will occur in the face....

Frank is correct in the fact that when kept in temp controlled environments less expansion occurs and therefore fewer cracks....unfortunately most homes are not that well controlled....

Phil was your pun intended....

Regards,

Jerry

There's a whole science of fracture mechanics that governs the initiation of and growth of cracks in brittle materials, which of course is what a hairline is.

Hairlines typically initiate at the interface of the glassy layer with the backing and then gradually propagate to the surface of the dial where they become visible. Sometimes you can see a discontinuity in the reflection of light, before the hairline breaks through to the surface. This is sometimes called a "baby hairline."

Of course things like more stress and thermal cycling will speed along the process of crack growth.

No supernatural intervention is needed Frank, it's all science!

I don't know about Jesus, Guys....
HE only walked on the waters, but Moses is the main guy....he parted them....

Regards,

Jerry