The last word–

CUT-GLASS ACCIDENTI recently removed some crystal

glasses from a cupboard. Two of

them left 1 centimetre of their rims

behind on the shelf (see Photo). The

glasses are more than 30 years old

and were upside down. The rings of

glass are not an even width, and

there appears to be a starting point

for the “cut”. What caused this?

No definitive answer here, but there

are three effects that could have

played a role. The third suggestion

was presented by some readers as

apocryphal, but we now know that it

really happens – Ed

● Glass looks and behaves like a solid, but it is actually a gel – a very slow-flowing one. The structure of lead glass or crystal causes it to flow more readily than other types of glass. You might have seen this in windows in old buildings, where the glass is visibly thicker at the bottom than at the top. Lead glass, from which these crystal glasses are made, also becomes more brittle over time.

The base and the stem of the glasses are quite thick, to provide a base that is heavy enough to stop them falling over when you use them. The sides of the glasses are elegantly thin.

While the glasses were stored rim down, the glass will have been slowly flowing towards the rims, making them fractionally thicker. The rest of the glass will have become slightly thinner, and all the glass more brittle. While the glasses were upside down, the force on the rim was compressive –

with the weight of the heavy base compressing all the glass below it. When you lifted them, you changed this to a tensile force. After long storage the sides had become too thin and too brittle to support the weight of the rim, and the glass broke.

Slight variations in the thickness of the glass to start with mean the break will not be even. Lead glass has impurities, and one of these probably provided the point of weakness that started the break, which propagated from there.

Cold comfort I’m sure, but if the glasses hadn’t broken when you picked them up they would have broken when you filled them, and showered you into the bargain.Helen Jenkins

Ohope, Bay of Plenty, New Zealand

● The body of the glass is blown as an egg-shaped globe or bubble and the stem and foot are added while it is still hot and malleable. After cooling, the rim line is scratched round and the

cup part snapped off, leaving a sharp cut edge.

This edge is reheated, and it flows in a treacle-like state to form a smooth rim. Finally the whole glass is heated in an annealing oven to soften it slightly and allow the stresses that

have built up within the glass due to heating and cooling to relax. Then the whole object is cooled evenly.

In this case the annealing process was probably not carried out correctly. If the glass has a scratch, it can create a stress line around which a crack can easily run. Such failures can occur spontaneously: in the late 1940s there was a batch of toughened glass tumblers on the market that were prone to exploding spontaneously.David Stevenson

By email, no address supplied

● This reminds me of a curious incident at a pub in which I used to work. Occasionally, when we were pouring beer into pint glasses from the hand pumps, the glass would break into two parts cleanly around the middle. It left a perfect cut and a clean edge – and, of course, beer all over the floor.

This happened several times, and we examined the glasses on the shelves. Many of them seemed to be scored as if by a glass cutter. We assumed for a while that a customer was playing some sort of prank. Eventually the landlord had a flash of inspiration and asked whether any of the barmaids were left-handed.

One confirmed that she was, and she wore an engagement ring which carried a large diamond. When she dried pint glasses she held the cloth in her left hand, inserting it into the glass and rotating it to dry the inside. The diamond acted exactly like a glass-cutting tool and left a score mark all the way round that caused the glass to neatly divide under the shock of beer hitting it.

I wonder if we may be seeing a similar situation with your reader’s crystal glasses.Richard Hooker

No address supplied

THIS WEEK’S QUESTIONBounce back

When a car crashes and its protective airbags are inflated, where do the airbag covers go to stop them breaking your nose?Damien Hadley

Chandler’s Ford, Hampshire, UK

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“In some windows in old

buildings, the glass is thicker at

the bottom than at the top”

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