Topic: Look what happened to my glass!

[Chris Harrison]

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:

[Leni]

Leni said,

... But that lives in a cabinet, in a room which is kept at a very steady temperature, so - fingers crossed - it should be OK

This message sounds like a good opportuntity to mention those cabinet items of yours with the interesting reactions, Leni.

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!

But I will, I promise! 

[krsilber]

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:

I worry about this sometimes when I think of glass being shipped by plane, though until now I've only much thought about the effect of change on air pressure (and temperature) on bubble wrap.  It's hard for me to imagine outside air pressure having that much effect on a tiny bubble encased in glass.

A lot of old glass simply deteriorates with time.  A tiny, invisible crack can slowly spread on its own, and some glasses are inherently unstable because of their composition.  I have a Pairpoint candlestick with a large bubble in it that is crizzling on the inside.  You'd think there wouldn't be enough moisture in there to cause it.  I've heard what sounds like the same phenomenon called "gaffers breath." 

I've also heard that sometimes gaffers would take a mouthful of water before blowing, and that the steam created this way helped inflate the bubble.  Anybody else heard of that?

[tropdevin]

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. 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%.

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?  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.

Alan

[Leni]

Good grief!  We mustn't dust our glass; now we mustn't wash it either! 

I admit I have always worried about glass in the hold of planes, and have bought very little from abroad for that reason.  However, I regularly send glass to Australia for Marinka, and I do worry about it!  Particularly as it is possibly up to 100 year old glass!   

In fact, at Gaydon last weekend I saw a Derbyshire hand vase in uranium and I thought about getting for her, but it had a large crack in it!  It was very cheap as a result, but I thought the odds of it arriving whole after a long flight would be just about nil!  (Sorry, Marinka!  )     


Just a thought:  does packing in polystyrene packing peanuts protect glass more in airplane holds?  Only it does have some insulating qualities, doesn't it?  :huh:

[Frank]

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.

Where did you hear that one about lead, if true there would be a pile of lead under glasses that have been left for a long time. Ditto water, bottles have been found from ancient times with contents. Water might get into cracks and clouded bubbles can be the result. Immersing poorly annealed glass into lukewarm water is a quick way of ensuring a large temperature gradient between glass in water and that in air, hence the cracks. Wash with wet cloths not by immersion.

Any lead secretion into contents is likely to be the result of chemical reaction with the surface of the glass and its contents that causes part of the glass to dissolve, once dissolved the lead can be separated. There are some problems with some compositions that lead to an oily secretion and this is discussed in detail, see archive forum.

[tropdevin]

I heard the one about lead from Richard Golding. He said that when the concerns arose about lead content of glass for food and drink vessels, and that items had to be tested, Whitefriars lead decanters were unsuitable for wine or spirits, because lead leached into the liquid inside over time. So Whitefriars experimented with a lead free glass inner, and a lead crystal outer, judging the coefficients of expansion correctly so that the glasses matched.  When first tested, they were fine - no lead.  Six months later the same decanters failed the tests, because the lead had diffused into the lead free glass. I guess Whitefriars did not choose the inner glass carefully enough, because there are US patents for this very process, using an alumino-silicate inner glass layer.

It is not a case of lead trying to escape from the glass: so you will not find free lead under your lead glass items!  It is diffusion of lead within the glass from a region of high content to one of lower or no lead content. The leaching is a chemical reaction involving the surface of the glass.

Regarding the effect of water, glass cutters know empircally that a drop of water can help start a crack. But more scientifically, research from the 1920s onwards has recognised the role of water in crack growth in glass. Here is a link to an early paper (pdf document) that discusses the role water plays at certain stages of crack growth.  There has been much work since - the effect is to do with modification of the stress field at the crack tip.

Alan

[Frank]

The traditional cure to stop cracks spreading is to drill a hole just beyond the ends of the crack, the crack stops growing after reaching the hole. Not a task for the amateur though.

[krsilber]

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

"There are some problems with some compositions that lead to an oily secretion and this is discussed in detail, see archive forum."  The "oily" secretion is, as you probably know, actually highly alkaline water from the glass dissolving in reaction to moisture from the environment.  This is a problem especially with non-lead glass low in lime.

I agree with Frank that the idea of lead moving through glass on its own, without the aid of moisture to transport it, sounds pretty odd.  Being a solid, lead isn't subject to diffusion.  Any idea how thick the inner layer was on the Whitefriars pieces tested?  And was there liquid in the vessels over the course of the test?

[tropdevin]

Hi Kristi

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.

Alan