Glass: 2mm or 3mm

[Uncle Sumo]

Talking to a glass supplier, they offered me the rule of thumb that if the length or width of a frame is one metre, use 3mm glass not 2mm glass. I didn`t ask about acrylic. Has anyone got opinions on this? Other rules of thumb regarding glass thickness?

[The Crofter]

I frame a lot of panoramic pictures up to 1200mm glass size. However the height is rarely more than say 400mm so I feel happy with 2mm glass. I do twitch when people bring in big posters and have refused to frame the biggest as it would have taken a full sheet 915x1200. So my rule of thumb is more like a gut reaction !.

[Roboframer]

I'd say it depends on how long the other side is too!

Read once, long time ago, in a trade mag (ABT) that 2mm is OK up to one square metre, so I wrote to Pete Bingham's agony column in The Picture Business saying I thought that was unsafe; what did he think?

He said it depends on what you are backing it with and that, say, 5mm MDF would make it safe.

I wrote back saying how's about the glass is spaced half an inch away from the artwork and any backing, so the glass is backed with fresh air? Got no reply. I'd say a square metre, which is not far short of a full 48x36" sheet, is unsafe.

I'd also say that something, say, one metre by half a metre would be safer portrait than landscape.

I just play it by ear, and if I don't feel it's safe I upgade, but never to thicker glass, always to acrylic. This is after several nightmares with heavy glass on large pieces - 2 man job to get it on the bench, flip it over to clean the opposite side and then drop it in the frame.................. and then remove and replace it about 500 times to get rid of the sneaky flumbs!

Beats me how we have no regulations for safe sizes per sq metre, I was talking to my glass rep once and he said that if the wooden panels in the bottom of our shop doors were glass, they would, by law, have to meet different standards than the glass ones above them.

Yet we trust great lumps of 2mm glass in things not permanently fixed to the wall.

[framejunkie]

2mm picture glass is probably more flexible than you think.....

bending 2mm float.JPG

I took this photo a couple of years ago to freak out a friend of mine who has a phobia of 2mm glass. I had to trim the image to post it but the whole thing shows a 48" strip of 2mm float with a roll of wire as a weight in the centre deflecting the strip down by about 4". Its resting on a roll of gaffer tape at either end and after i took the photo i added more weight and got the glass to touch the bench without breaking, but the lightest tap with the brass handle of a glass cutter made it shatter. (if you're gonna try this at home i recommend you don suitable protective gear!)

(Not sure this adds to the debate, but i thought i'd share it with the group anyway.)

For my part, i'm happy to use 2mm up to 3'x4', but as mentioned by others its ok for long thin frames.

I've done the odd box frame with 2mm museum glass bigger than this, but it gives me the heeby-geebies - especially fitting up the frame on my own. The trick is never to move the glass from the vertical until its in the frame - i fit large pieces(bigger than 5') with the frame leant against the bench.

One time a certain large supplier of Glass and Mirror products sent me a 66" cut-to-size piece of 2mm AR glass. It was a rush-job so i'd made the frame before the glass came, and the glass arrived 3mm longer than ordered - just a tad too big for the (slightly tight?) frame i'd made. It was late at night when i found out, and the frame was being collected for an exhibition first thing the following day. Had to get the thing flat on the bench to trim a sliver off. Took me 15 mins to figure out how to do it without chopping my own legs off, and another 15 to pluck up the nerve.

Never again!

It's gonna keep me awake, now that i've brought it back to mind

[Uncle Sumo]

Illuminating as always.

Roboframer, why do you think one metre by half a metre is safer hung portrait rather than landscape?
I would have thought there would be less flexing / movement the other way round.

[prospero]

A big sheet of glass is no more dangerous than a small sheet. And no more likely to break. In fact it is harder to break than people think. That's if it is fitted into the frame properly......

Check the edges for chips. A chip is a start of a crack.
Don't fit the glass tight in the frame. Any shocks to the frame wil transmit though to the glass.
Make sure the underside of the rebate is smooth where it contacts the glass. Any small 'nibs' will create a stress point.
Never expect the glass to flatten out a twisted frame. It will up to a point, but will always be in a state of stress.

I'm not sure there is much to be gained in using thicker glass. The extra weight will tend to negate the extra strength.
If you really want safety, use 6mm laminated.