"Toughened " versus "Safety" glass.

alisdair4

alisdair4

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midnightdrifter.net
On going down to the boat Yesterday, I found that the starboard wheelhouse window was crazed. There is no indication that anyone (or anything) had hit the window.

The original windows are by Houdini, who tell me that they don't do replacements. Eagle windows were helpful, but due to the design of the window ( Danish, with only one split line) they don't want to touch the job. Thus i am left looking at local (Clyde) suppliers of toughened or safety glass. What is the difference in terms of marine applications - I believe that the latter has a filament moulded into the sheet.

Thanks in advance.
 
I think that toughened is one form of safety glass, the other main one being laminated, as used in cars. I believe that my boat windscreen glass is toughened, which shatters into small fragments when it breaks. In the HR data it is described as tempered. In cars, this introduces a severe risk of eye and other injuries, but, unless you are intending to sail at 40mph, toughened should be satisfactory.
 
alisdair4 said:
On going down to the boat Yesterday, I found that the starboard wheelhouse window was crazed. There is no indication that anyone (or anything) had hit the window.

The original windows are by Houdini, who tell me that they don't do replacements. Eagle windows were helpful, but due to the design of the window ( Danish, with only one split line) they don't want to touch the job. Thus i am left looking at local (Clyde) suppliers of toughened or safety glass. What is the difference in terms of marine applications - I believe that the latter has a filament moulded into the sheet.

Thanks in advance.
Click to expand...

When I had the windows made for my wheel house I specified 2 sheets of toughened glass laminated together. This increased the impact strength but if the toughened sheet did brake the window would remain intact thus preventing any ingress of water.
 
"Toughened" means "increased resistance to cracks" and glass is toughened by putting the middle (thickness wise) in tension and the surfaces in compression. To do that it's heated up till it's soft and then cooled with blasts of cold air. The outside resolidifies first, then as the inner bits later cool they pull the outside into compression while being stretched to tension themselves.

This has two useful effects. First, it's much harder to crack or scratch the outer surfaces, because they are in compression and cracks don't grow that way. Secondly, in the event of an accident all the stored strain energy is released and breaks the glass into tiny fragments. The energy is used up in creating surface energy for the fragments, so the more energy there is to release, the smaller the fragments, because that increases their surface area (and therefore absorbed energy) to volume (and therefore stored energy) ratio.

But I digress.

Laminated glass is generally a sandwich of normal glass with a tough plastic membrane in between. Unlike toughened glass, it can still easily shatter but when it does the fragments are held together by the membrane, avoiding the long razor-like pieces which used to cut jugulars in 1930s car crashes.

Both have advantages and disadvantages, On the whole, toughened is probably better, but each piece has to be specially made. Not expensive, does take time. Any good glazier can supply laminated glass cut to size from stock. I think laminated is more difficult for custom made curved glass.

As rogershaw says, you can laminate toughened glass, which gives the best of both worlds ... and, of course, the worst.

Curious glass fact #17: Kenneth Horne (of Round the Horne) combined his comedy career with being sales director for Triplex Glass.
 
From the way my local glass specialist managed to cut a cooker splashback with curves, apertures for switches and radiused corners I doubt you'll have much trouble getting it made up. They cut a normal sheet and then temper it. Be aware that any attempt afterwards to grind, cut ot drill toughened glass will result in it shattering. If its to fit in a frame the cutting must be precise.
 
When I had new framed windows made up, the makers cut the glass to shape and sent it away for toughening. Seemed a quick turn-round and added ~£10 per window IIRC. Smallish windows though.
 
Normalfornorfolk said:
From the way my local glass specialist managed to cut a cooker splashback with curves, apertures for switches and radiused corners I doubt you'll have much trouble getting it made up. They cut a normal sheet and then temper it. Be aware that any attempt afterwards to grind, cut ot drill toughened glass will result in it shattering. If its to fit in a frame the cutting must be precise.
Click to expand...

The grinding of the edges can be done to a very small extent as one of the windows made for be was a millimeter of 2 too big and the supplied ground down the edge without braking the sheet.

They made it too big and either had to replace of grind down.

Just a note about lamionated glass the edges must be very well sealed against moisture ingress as any moisture will damage the plastic used in the limitation causing bubbles of the plastic going milky.
 
When we went through a breaker on Hayle bar the toughened glass shattered, lots of water and wrecked electrics, minor cuts to face and a crackling ear from which came a fragment days later. Replaced with laminated, which folded and pushed through, in the end we put a 'T' bar of ally across it with rubber behind it.
You pays yer money....

Laminated can be cut with the help of a blowtorch, we actually bought a piece of a very slightly curved lorry windscreen.
 
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