Author Topic: Annealing Cracks  (Read 3204 times)

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Offline nigel benson

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Re: Annealing Cracks
« Reply #10 on: April 15, 2007, 05:50:46 PM »
Hello,

To take this idea of drilling holes to stop a crack continuing to its conclusion, it would seem that if I own a piece of Monart (or other glass for that matter), which has a number of cracks in it, the answer is to drill 5mm holes at either end of all of them (or at least the ones chosen as 'at risk') - thus producing an item that would be more suited as an impliment in the kitchen to strain the veg's? I seriously think not!

Just because a reputable restorer, such as Wilkinson's, suggested this invasive practice as a means of repair, it does not imply that it is indeed Museum practice. I would go further and suggest that this method of "repair" is a hang-over from the days when staples were used to put together either ceramic or glass items. The item in question would be drilled on both sides of the crack (or break), at intervals, and a staple inserted to hold the vessel together. It is frowned upon by museums - and was long ago left behind as a means of repair. Nowadays, any repairs untaken in a museum have to be reversable for a whole range of reasons.

The idea that drilling holes in an item as a repair is also a misnomer in my opinion. I can see that putting something back together is a repair, but scarring it with holes............?

It maybe that in certain circumstances that this method might be considered as a means of curtailing a crack, however I think that I would rather let time take its course as I would find the holes in an item frustratingly irritating.

Nigel


Offline Frank

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Re: Annealing Cracks
« Reply #11 on: April 15, 2007, 06:54:21 PM »
More like an 0.05 mm hole Nigel, but that is only of use if there is a crack AND that crack is unstable... i.e. still growing.

I don't have a problem with such a soution, if it was suggested to me by a competent restorer as the only way to retain a single item rather than two partial items, I know which I would choose ;D

 >:D Two pieces and put one on eBay >:D
Frank A.
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Offline taylog1

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Re: Annealing Cracks
« Reply #12 on: April 15, 2007, 07:58:52 PM »
The principle is no different to ships and planes having rounded corners on doors and windows (the original Comet had square windows, which caused all the problems). By drilling the hole, when the crack reaches it the aggregated stresses are spread out over a wider area, and so the forces are less at any given point. One should then be able to fill the hole in (with light sensitive superglue for example) to make it less apparent if you wanted to.   

[J.E.Gordon - The new science of strong materials - is an enlightening read, and has a chapter on cracks in glass. It helps explain why cracks become unstable as they grow, the stresses are proportional to the length of the crack, but concentrate on the ends. Greater stress equiates to greater propensity to grow.]   


One of the references I came across referred to using a flat computer screen and a polarised filter to look for signs of stress in a piece of glass (in this instance a telescope reflector blank).  http://www.usm.uni-muenchen.de/people/cries/christoph/schleifseite_eng.html
I've just tried it with a camera polarising filter (common on the non digital SLRs) and the screen is definitely polarised.   
If anyone's got a piece of internally stressed glass and a polarising filter on their camera I for one would be very interested in the result - the principle's sound, but is the effect big enough to show up on camera ?

taylog1


(and on an entirely different matter, in looking for a reference above I came across this site, which I found interesting. Apols if referenced elsewhere. http://www.glassblower.info/glossary.html)


Offline nigel benson

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Re: Annealing Cracks
« Reply #13 on: April 20, 2007, 09:53:23 AM »
Hiya,

I've just been mopping any awfully large 'egg-on-face' off my brow :-[

My apologies Frank - goodness knows where I got the idea that the holes would be that large. All I can say is that Sunday was a rough old day for me, so I went onto the GMB for a bit of light relief - only to get that wrong!! Shouldn't have got up that day I don't think. Ho hum.....

As for the comments I made about stapling and whether or not it was a museum approved method of repair, they stand.

I fully understand the principle behind the idea, but comparing an industrial solution to use on a piece of art glass does not seem to be comparing like with like.

If I relate this solution to whether or not to have damage, such as a chip, taken out of a rim and I find it difficult to to accept one, but not the other, since both are interferance in some way of the items original condition.

Ultimately "you pays yer money and ya takes ya chioce". 

Nigel


Offline Frank

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Re: Annealing Cracks
« Reply #14 on: April 22, 2007, 03:50:22 PM »
No need for the egg Nigel, a hole is a hole :)

I will be eBay testing the impact of a biggish stress crack on an unusual piece of Monart, later tonight.
Frank A.
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Offline Frank

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Re: Annealing Cracks
« Reply #15 on: April 23, 2007, 05:19:08 PM »
I have added the powder box to the catalogue on Scotland's Glass so you can get get a good look at the crack.

Link

Due to the extra thickness of glass, the stress cracks under Monart handles are usually very different to those found on the surfaces of vases and the only difference to a crack caused by a knock is that it only affects one-layer.
Frank A.
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Offline Frank

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Re: Annealing Cracks
« Reply #16 on: May 08, 2008, 02:29:45 PM »
An interesting aside with staple repairs, god museums can be stuffy, is that they were not intended to repair an object of desire but to continue the useful life of an object. The technique is often described in early books of household repairs. For museums to discount social history contained in objects is of course ridiculous. It is a pity that today we (in the West) consider it better to throw away damaged pieces of household stuff and buy new ones - a trend that needs reversing to help combat the damage to our environment caused by excess consumption. There is a nice trade in repairs of household items, including disposable lighters, carried on on the sides of streets throughout India and presumably most countries where the cost of living means that impressing the neighbours is of lesser importance.
Frank A.
Please help preserve glass web-sites for posterity by donating to The Glass Study Association a non-profit organisation.
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