... But that lives in a cabinet, in a room which is kept at a very steady temperature, so - fingers crossed - it should be OKThis message sounds like a good opportuntity to mention those cabinet items of yours with the interesting reactions, Leni. :o
Leni said,You're quite right, and I will Kevin. I've just been very busy since your visit, and now I'm knackered after a day on my feet at Gaydon!Quote... But that lives in a cabinet, in a room which is kept at a very steady temperature, so - fingers crossed - it should be OKThis message sounds like a good opportuntity to mention those cabinet items of yours with the interesting reactions, Leni. :o
Beware changes in altitude, too.
I bought a nice Czech candleholder in a charity shop, put it in my backpack in bubble wrap and went on a cable car ride with a 600m change of elevation. 2/3 of the way up, there was a very distinct cracking sound. There must have been a bubble I hadn't seen that expanded as the outside air pressure decreased.
It's now a 2-piece candleholder :cry:
Moisture. There would always be some moisture in the exhaled air of the gaffer. However, glass can allow the slow migration of atoms through its structure, (eg lead moves through it over a period of months) so maybe water can move around too? And the presence of liquid water can help a crack to grow in a stressed environment - hence the accounts of items suddenly cracking when washed in lukewarm water.
Several points in recent posts here bear comment.
Transport by plane. Items flying at altitude experience a decrease in external pressure of around 8 psi, which will put increased stress on the items whether they have bubbles in or not. The effect of a change of 8 psi on a solid like glass would be negligible. And items in the cargo hold can get cold too. I have heard of paperweights sent whole arriving in kit form.
Effect of heat or sun on bubbles. If the item gets so hot you can hardly touch it (say 65 deg C), you get an increase in pressure in an internal bubble of around 25%. It's not just bubbles that are the problem, but also an uneven heating of the glass, causing different rates of expansion in different parts of it.
I suspect that things that go ping have not had enough appropriate annealing, or have been heated (or cooled) unevenly.
Moisture. There would always be some moisture in the exhaled air of the gaffer. However, glass can allow the slow migration of atoms through its structure, (eg lead moves through it over a period of months) so maybe water can move around too? Although water can move into glass, I doubt enough of it would move through it to have a significant impact, otherwise crizzling in air traps would be much more common. I suspect that the glass was too alkaline to begin with and there was a bit of moisture in there. And the presence of liquid water can help a crack to grow in a stressed environment - hence the accounts of items suddenly cracking when washed in lukewarm water. My understanding of the effect of water on cracks is a slower chemical process. I think once again temperature is usually to blame when things break while being washed.
...Regarding the effect of water, glass cutters know empircally that a drop of water can help start a crack. It can help start a crack? But water was a necessary part of cutting, used to cool the glass so it didn't crack. I don't understand this point at all.
Alan
The effect of a change of 8 psi on a solid like glass would be negligible.- I disagree: put 8 psi into a thin walled vessel and it will explode. The extra pressure in a bubble in a paperweight will causes additional stress, which could lead to crack growth and failure, particularly with any differential heating.
My understanding of the effect of water on cracks is a slower chemical process. I think once again temperature is usually to blame when things break while being washed.- I agree that the mosy likely cause of cracks when washing would be thermal shock. But water molecules can have a physical effect on crack tip stresses, and affect the growth of the crack, and lead to failure. See the research paper I cited in an earlier posting.
Regarding the effect of water, glass cutters know empircally that a drop of water can help start a crack. It can help start a crack? But water was a necessary part of cutting, used to cool the glass so it didn't crack. I don't understand this point at all.- Apologies - I was not clear in what I was saying. By glass cutters, I did not mean engravers and fancy cutters, but those people who cut window panes, mirrors and so on from sheet glass. They score the glass to generate many micro cracks, and sometimes wet the start to get the main crack propagating.
Hi KristiQuoteThe effect of a change of 8 psi on a solid like glass would be negligible.- I disagree: put 8 psi into a thin walled vessel and it will explode. The extra pressure in a bubble in a paperweight will causes additional stress, which could lead to crack growth and failure, particularly with any differential heating. Sorry, I should have been more clear - I meant without bubbles, responding to "whether they have bubbles or not."QuoteMy understanding of the effect of water on cracks is a slower chemical process. I think once again temperature is usually to blame when things break while being washed.- I agree that the mosy likely cause of cracks when washing would be thermal shock. But water molecules can have a physical effect on crack tip stresses, and affect the growth of the crack, and lead to failure. See the research paper I cited in an earlier posting. I did, and I meant to thank you for posting it! Interesting article. The only mention of a possible strictly physical effect I saw was unexplained ("Gurney [6] then showed that moisture enhanced fracture could be explained in terms of thermodynamic concepts"). Otherwise it seemed to be about chemical effects of moisture, including humidity.
Alan