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Old 2nd May 2012, 9:54 am   #1
GrimJosef
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Default Cracked dial glass - should I try a repair ?

First let me say that I wasn't sure whether to start a new thread or to tack my query onto the end of this one from a couple of years back https://www.vintage-radio.net/forum/...ad.php?t=60386. Obviously I don't mind Mods if you want to move this there.

A little while back I bought a Pye PE80 which had been sitting untouched for 30 years. Tragically between buying it and collecting it the dial glass came to grief (see pics - honestly I didn't do this !). The seller was mortified and offered to cancel the sale if I wanted. But since the rest of the radio was in relatively nice untouched condition I decided to go ahead with it. My question is "What to do about the dial glass ?".

I am inclined to try a repair a) because the detailed backing on the glass makes the crack look so conspicuous, b) because I fear that if I don't repair it there is a chance that it will snap completely, or at least chip larger pieces out of the existing crack when I try to remount it and c) because the longer I leave it the more chance there is of dirt getting into the crack and making it harder to fix later. But if anyone can suggest a good-looking alternative to repair then I'd be interested to hear (I will post a request for a replacement in the 'Sets and Parts Wanted' forum of course).

Looking in more detail at the repair process, in the previous thread the only adhesive mentioned in the context of actual practical dial glass experience was cyanoacrylate. Personally my experience in other areas with this has been poor. It just doesn't seem to stick very well. Also it can be very quick-setting which might inhibit its 'wicking' down into what is a very, very thin crack. And while the manufacturers of one of the recommended glues say that it works for a very wide range of materials http://www.zapglue.com/Hints/CA.html they don't actually include glass. Furthermore it's pointed out in the earlier thread that cyanoacrylate has a different refractive index from glass so the repair won't really be invisible. And finally it's suggested that there may be issues with induced stress. But it has at least been known to work in some cases.

Whatever adhesive I choose, I do feel that the point about it needing to have very, very low viscosity is a good one. So I've considered a couple of specialist glass epoxies with this property - Opti-Tec 5001 http://www.intertronics.co.uk/products/opt5001.htm and Hxtal-NYL1 http://www.amazon.co.uk/Hxtal-Epoxy-...5946839&sr=1-1. I'm also aware of UV-curing epoxies, e.g. Sylmasta UV901 http://www.sylmasta.com/acatalog/Onl...onding_10.html, but I don't have easy access to a UV lamp so for now I'm less enthusiastic about them. So can I ask if anyone has tried a glass epoxy for this job ? If you did which one did you use ? Was the wicking successful (I'd like to get the sort of result shown around 6 mins 50 secs in this video http://www.youtube.com/watch?v=UHD6Xb0dFT0) ? Did you try to open the crack up at all ? In my case I might be able to do this by putting a wooden dowel, split lengthways, into the existing screwhole then gently wedging the two halves apart to expand the screwhole a little. Gently, gently of course.

So can anyone help with advice beyond what's in the earlier thread ? I'd particularly value any wisdom from people who have succeeded and/or failed at this.

Cheers.

GJ
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Old 2nd May 2012, 9:59 am   #2
Peter.N.
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Default Re: Cracked dial glass - should I try a repair ?

Ordinary clear silicon sealer is an excellent glass adhesive if fact when it originally appeared on the market in the '50s it was primararaly used for making fish tanks, it will even bond sheets of glass end to end, it doesn't seem to stick to anything else very well but has an affinity for glass.

Peter
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Old 2nd May 2012, 10:12 am   #3
GrimJosef
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Default Re: Cracked dial glass - should I try a repair ?

Thanks Peter. Do you mean something like Dow-Corning RTV http://www.farnell.com/datasheets/22266.pdf ? If so what's the best way to get it into a very tight crack ? And might the acetic acid that's eliminated when it polymerises affect the paint on the back of the dial (the crack goes all the way through from front to back) ? Do you know if the non-acid sealers e.g. http://www.acc-silicones.com/content...ts/as1723.ashx work as well ?

Cheers,

GJ
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Old 2nd May 2012, 10:19 am   #4
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Default Re: Cracked dial glass - should I try a repair ?

First thing I'd do is scan the dial glass onto a computer. Then if it does crack, you can at least print out a transfer to put onto a new piece of glass.

UV curing resins - you can get UV LEDs for pennies these days. I've got a laser pointer that's also got a UV LED in it for checking banknotes etc (though I find it useful to point at my watch to make the phosphor glow a bit more). Cost me less than £2. OK, so a UV LED doesn't kick out a huge amount of light, but it should be enough to cure the resin.
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Old 2nd May 2012, 10:24 am   #5
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Default Re: Cracked dial glass - should I try a repair ?

Scanning is a good thought. But the glass is huge - 528mm long - so I'd need at least an A2 scanner and printer and transfer film.

Cheers,

GJ
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Old 2nd May 2012, 11:41 am   #6
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Default Re: Cracked dial glass - should I try a repair ?

or you do multiple scans and stitch them together. Not sure than A2 flatbed scanners exist realistically. I guess you can't feed glass into a drum scanner.

Or use a digital camera and rescale in software.
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Old 2nd May 2012, 11:53 am   #7
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Default Re: Cracked dial glass - should I try a repair ?

Although it looks a bit of an eyesore, the crack should not propagate any further than the hole. I would be inclined to do nothing since it should not get any worse and is on the outer area of the scale. Any type of bonding agent or sealant could affect the reverse scale printing so best left alone.
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Old 2nd May 2012, 12:40 pm   #8
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Default Re: Cracked dial glass - should I try a repair ?

I have succesfully used Loctite Glass Bond glass adhesive to mend glassware. [ RS code 693-810 when I last bought some a few years ago.] I haven't had occasion to mend radio glass, but I fixed a broken 1930's glass lamp shade that my wife managed to break when trying out the tennis game on her Nintendo [lucky it was her, not me!]. A good source of UV is one of the black light lamps that you can get from various sources, such as Maplin, for a few pounds. Essentially a low energy bulb with no phosphor.

What about car windscreen replacers? I believe that they can fix cracked windscreens by using a penetrating adhesive that does match the refractive index of glass and so doesn't interfere with vision.
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Old 2nd May 2012, 12:45 pm   #9
Peter.N.
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Default Re: Cracked dial glass - should I try a repair ?

Dow corning rings a bell, I think they were the originators of it.

Peter
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Old 2nd May 2012, 3:46 pm   #10
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Default Re: Cracked dial glass - should I try a repair ?

Quote:
Originally Posted by GrimJosef View Post
Scanning is a good thought. But the glass is huge - 528mm long - so I'd need at least an A2 scanner and printer and transfer
Scan it in two halves and bring it together with software. An A4 printer is restricted by it's width (21 cm) so a sheet a sheet of A2, cut down the middle, should feed through.

David
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Old 2nd May 2012, 4:42 pm   #11
GrimJosef
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Default Re: Cracked dial glass - should I try a repair ?

Quote:
Originally Posted by emeritus View Post
I have succesfully used Loctite Glass Bond glass adhesive to mend glassware ... I fixed a broken 1930's glass lamp shade that my wife managed to break ...
Was that a clean break into two pieces or were you actually able to get the Loctite into a tight crack ?

Quote:
What about car windscreen replacers? I believe that they can fix cracked windscreens by using a penetrating adhesive that does match the refractive index of glass and so doesn't interfere with vision.
Yes, I believe they use the UV-curing low-viscosity resin of the type I mentioned in the first post. They are usually faced with a different problem which is that the damage they are dealing with does not fully penetrate the glass. Under normal circumstances this means they struggle to get all of the air out of it. So while the treatment can strengthen the glass somewhat and can also stop the crack spreading, the damage can still remain visible where the air is trapped. They use various vacuum (not pressure) devices to try to suck most of the air away. These can help a lot, when they work but, as the windscreen on my own car testifies, they don't always work. In the case of the PE80 dial the crack does go all the way through and is also open at the ends where it meets the screwhole and the edge of the glass. This would make the vacuum devices useless. But provided I can get good capillary wicking they should also be unnecessary since adhesive applied to the front of the crack should expel the air out through the rear and the ends.

Cheers,

GJ
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Old 2nd May 2012, 6:19 pm   #12
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Default Re: Cracked dial glass - should I try a repair ?

Sorry to hear about the dial glass. How unlucky!

It would surely pay to experiment on some plain glass (e.g. from an old picture frame) first, or maybe a radio dial that is otherwise damaged and therefore useless.

I suspect all the suggested techniques have pros and cons, and will no doubt be highly "technique sensitive".

Nick.
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Old 2nd May 2012, 9:08 pm   #13
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Default Re: Cracked dial glass - should I try a repair ?

Hi GJ
I have attempted a few glass repairs using Glass Bond and in my oppinion you should be able to get a reasonable repair on that one.
Super glues don’t bond well to glass and they do not have the right refractive index to make a near invisible join.
They say glues such as Glass bond require UV to cause the setting process. My recommendation would be to wait for a nice hot sunny day.
Put a piece of sellotape or PVC tape over the back of the glass on the printed side to help stop the glass bond running through and dissolving the print.
Run a line of Glass bond over the crack and verrrry gently flex the glass slightly to help the glue run into the crack. You should be able to see if the glue has run into the crack when the crack will tend to disappear.
Put the glass on a flat surface and leave in the sunlight for a good time depending on the intensity of the sunlight.
The glue on the surface should be nice and hard after a few hours on a really bright summers day.
Scrape of the excess glue on the front of the glass with a razor blade.
Good luck.
Mike
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Old 2nd May 2012, 10:11 pm   #14
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Default Re: Cracked dial glass - should I try a repair ?

[QUOTE=GrimJosef;526431]
Quote:
Originally Posted by emeritus View Post
I have succesfully used Loctite Glass Bond glass adhesive to mend glassware ... I fixed a broken 1930's glass lamp shade that my wife managed to break ...
Was that a clean break into two pieces or were you actually able to get the Loctite into a tight crack ?



It was in several dozen pieces, and the glass was opaque and multi-coloured, so no issues with refractive index. It did run into cracked parts that were still attached, but not right to the internal end. I didn't like to try flexing the glass too much as I didn't want to risk propagating the crack further. I would agree with newlite 4 that your crack will not propagate further as it has definite ends. A bit of gentle differential flexing might open it up a bit, but possibly not as the crack is unlikely to be perpendicular to the surface.
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Old 3rd May 2012, 8:27 am   #15
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Default Re: Cracked dial glass - should I try a repair ?

Beware of the uv cured glass glues and as they can attack the gold paint, however i still find it the best method when sed in conjuction with a piece of clear acetate film laid over the top of the glue. When glued use razor blade to remove excess.

steve
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Old 3rd May 2012, 10:13 am   #16
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Default Re: Cracked dial glass - should I try a repair ?

If you decide to use cyanoacylate adhesive (superglue) be aware that unlike any other common adhesive, it is cured or set by the presence of moisture.

If you wish to delay the setting as much as possible, ensure that the surroundings are very dry. Work in a room with low R/H, avoid exhaling near the work, and wear latex gloves to prevent moisture from the skin promoting rapid curing.

When the adhesive and work are to your satisfaction, rapid curing may be achieved by exhaling onto it.

It is well known that such adhesives bond skin in seconds, the moisture in/on the skin ensures a more rapid bond than when mending glass etc.
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Old 3rd May 2012, 3:07 pm   #17
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Default Re: Cracked dial glass - should I try a repair ?

Thanks to everyone for your advice. Honestly I hadn't expected so much help or such detail. So let me try to summarise:

It's been pointed out that the glass is probably pretty stable at the moment so could be left unrepaired. But quite a few of you have had some success with repairs yeourselves and also seem convinced that I could do a worthwhile job on this crack. So I think I will take the risk. If it's successful it would make a big difference, at least as far as I'm concerned.

Silicone: This has good adhesion to glass and is flexible when set, so won't add stress. I'm not convinced that I could force it into this very tight crack though. And a bit of research suggests that the refractive index of the ordinary stuff is not quite the same as glass. The fibre-optic people have recently developed a better-matched variant but I don't think it's widely available. Furthermore the flexible result won't add much stiffness to the joint so won't resist any further stress. For these reasons silicone won't be my first choice.

Superglue (cyanoacrylate): This has been shown to work provided a suitably low-viscosity version is used. There are some worries about poor adhesion and, conversely, also about stress though. And again the index doesn't match glass really well. The setting time is often short (sometimes very short) and while it may be possible to extend this by working carefully and in a dry environment unfortunately my most convenient workspace is a garage which has just been exposed to the wettest April on record . This is an issue because the crack is in fact rather long - about 85mm - so the job will inevitably take quite some time. Once again I feel cyanoacrylate doesn't come out top.

Loctite Glass Bond: This is another acrylic compound but this time it's methacrylate not cyanoacrylate and is cured essentially by exposure to UV light, not moisture. Actually finding any definitive data about this adhesive was challenging. It turns out that Loctite uses the name 'Glass Bond' for different adhesives in different countries. It's pretty clear that the UK stuff is this http://www.amazon.co.uk/Loctite-Glas.../dp/B0001P0DK6 which is not the same as this Norwegian product http://www.loctiteproducts.com/p/sg_...Glass-Glue.htm (a cyanoacrylate). Nor is it the same as their US 'Glass Bonder' http://henkelna.com/faceted-search-1...UID=0000000IIQ which is an epoxy. The best I could do for technical information on the UK product was this http://www.wynns.uk.com/pTIS/GRP_LOC.../853358_EN.pdf which I think relates to the stuff we're talking about. The manufacturers say there that the setting time in outdoor daylight varies between 10 seconds (strong sunlight) up to several minutes (dull and cloudy). At least one of the reviewers on Amazon asserts angrily that it stays liquid for a whole evening if that's when you choose to use it . So it sounds as though the setting time could be workably long provided I keep away from the windows. The manufacturers say that the viscosity is 'medium' - their average figure is ~2500 mPas - which isn't ideal. And I note emeritus' comment that it wouldn't run into the tightest parts of the cracks in his case. I might be able to prise my crack open a little. But I might not and for now this is my biggest worry with this adhesive. I also note that it might damage the paint on the back of the glass but I fear that that could be the case for almost any of the glues I'm considering.

Opti-Tec 5001: This is an epoxy, not an acrylate. Its attractive features are that it has a long pot life under any lighting (2 hours for a 4g sachet), an exact index match to glass and, in particular, it is also 'very low viscosity'. The quoted figure is 100 mPas so it should be 25 times runnier than the Loctite. I hope that would improve its chances of filling my crack. As an epoxy it does not contain acrylic acid which most of the acrylates, including Glass Bond http://docs-europe.electrocomponents...6b8001f44c.pdf do. Some of the windscreen repair resin suppliers are now starting to offer acid-free products http://www.gtglass.com/files/Diamond...esin%20PDF.pdf. I don't know if the acid in acrylate adhesives would pose a long-term threat to the paint on my dial but if I don't have to take the risk then I'd sooner not.

So for now I have to say my choice is between the Loctite Glass Bond and the Opti-Tec adhesive with the Opti-Tec looking more promising on paper. I think the next step is to get hold of some Opti-Tec and some Glass Bond and, following Nickthedentist's advice, see what I can learn about them in practice. I'll let you know how it goes !

Cheers,

GJ
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Old 3rd May 2012, 5:16 pm   #18
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Default Re: Cracked dial glass - should I try a repair ?

The UK stuff from Amazon looks identical with the product I got from RS. It is still in their range : http://uk.rs-online.com/web/p/solven...633D4E4F4E4526

It comes in a yellow tube and mine was manufactured in Eire.

As an acquaintance who was a chemist for a paint manufacturer told me many years ago, a trade mark is no indication of the nature of the product.
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Old 6th May 2012, 1:02 pm   #19
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Default Re: Cracked dial glass - should I try a repair ?

There's a write-up of a repair by Paul Stenning here, in which he mentions using "B&G glass adhesive" to repair a cracked scale. You might be able to pick up some hints here.

Ed.
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Old 6th May 2012, 3:49 pm   #20
G8HQP Dave
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Default Re: Cracked dial glass - should I try a repair ?

I repaired a broken Bush VHF80 scale with PVA. Maybe not ideal, but I used what I had to hand. It wicks into cracks quite well, and tends to shrink and go translucent as it sets. As the glass had broken in two, I strengthened the edges by adding tiny metal shims (miniature transfomer laminations!). In your case I might be tempted to try watered-down PVA which should wick even better.
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