Glass vs Perspex - How big is too big for glass

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JC01
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Glass vs Perspex - How big is too big for glass

Post by JC01 » Fri 14 Jan, 2022 11:41 am

Hi there,

I’m in the process of make a box frame for an a0 piece (841 x 1188) - I’m wondering if there’s a rule of thumb of what size becomes too large to use glass or as long as you use 3mm thickness it doesn’t matter? Especially for a box frame as there won’t be as much support.

Would anyone recommend using perspex, are the results good enough or does it lack that wow factor?

Any advice welcome.

Thanks so much.

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Re: Glass vs Perspex - How big is too big for glass

Post by JonathanB » Fri 14 Jan, 2022 12:56 pm

You'll find that opinions will vary, but I tend to follow the advice given by my suppliers, Wessex Pictures, who suggest that once you go over a square metre, you should consider using 3mm glass. Having said that, there are plenty of experienced framers comfortable with going over that. A good quality cast acrylic will give an excellent result, but if you're going to fit it to a larger piece, you will be looking at 4mm acrylic to avoid the acrylic bowing in a large frame. You should also remember that as a rough rule of thumb, acrylic is about half the weight of glass, so that if you're going to fit it to a large piece you may not be gaining the weight advantage that you hoped for. The main issue with large pieces of glass is safety. If you're framing for something that's going into an area of high traffic or a child's bedroom, for example, there are huge safety advantages in using acrylic. I don't think you should worry about quality issues with modern acrylics, as long as you buy a good quality cast product from a reputable supplier. Be prepared for a big bill with a large piece of acrylic as compared with glass.
Jonathan Birch GCF (APF)

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Re: Glass vs Perspex - How big is too big for glass

Post by JFeig » Fri 14 Jan, 2022 2:17 pm

From my start, 2.5mm glass was the standard and was supplied in boxes of 50 sq ft on sizes from 8" x 10" through 40"x60". Acrylic(Persplex®, Plexiglas®, etc.) was ordered from the same glass vendor in 48" x 96" or larger sheets of 3mm or 6mm. Due to the coefficient of expansion some frames required wider rebates.

My vendor supplied both custom picture framers and hardware stores.

It was always considered safe IF the frame moulding was strong enough for all of the glass sizes IF THE FRAME WAS STRONG ENOUGH.

The primary concern is for the safety of the framer and of the end user.
Jerome Feig CPF®
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Re: Glass vs Perspex - How big is too big for glass

Post by Justintime » Fri 14 Jan, 2022 2:37 pm

I was about to say the same as Jerome, about expansion. I don't have the table handy, I think it's available on the framing.academy group. It's surprising how much allowance has to be made for this with acrylic of this size...

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Re: Glass vs Perspex - How big is too big for glass

Post by prospero » Mon 17 Jan, 2022 12:09 pm

My take on the subject.............. :D

Glass is fine as long as it is cut cleanly, that is no chips on the edge that could be cracks waiting to happen.
It must sit comfortably in the frame. Not too tight and not bumps under the rebate that could create stress points.
Glass will flex quite a lot. 3mm glass will increase the weight considerably and it will crack just as easily as 2mm.
If you use plastic then bear in mind that it will expand/contract (unlike glass) so you might have to widen the rebate,
especially on a large piece. It is also quite 'wappy' and hasn't the rigidity of glass. So that means using thicker stuff, maybe 6mm,
which would be heavier than glass the same size. (and much dearer).


** Plastic glazing is probably preferable if safety issues are relevant. :wink:
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Re: Glass vs Perspex - How big is too big for glass

Post by Not your average framer » Mon 17 Jan, 2022 7:01 pm

I'm not really a great fan of acrylic glazing. Glass is usually a better potion visually, but really large bits of glazing become increasingly problematic. Large pieces of glass can be very difficult, for one person to handle due to the weight. Over the years, I glazed some really large framed items. The bigest frame was glased with 4.4mm laminated glass from Wessex. It was the right choice and 4.4mm laminated glass is really solid and durable. It's the stuff that is used for laminated car windscreens.

I did not make enough profit for all the agro, which this job caused. Really large framed items can take a bit more know how and thinking about. I'm talking about construction techniques and the strengths and rigidity of materials. The finished frame need to be rigid enough not flex when being handled and transported. Even a very heavy solid construction wooden frame can often flex more than was expected when it is support really heavey glazing.

I don't go for acrylic glazing of really large frames. The weight of a large piece of acrylic, with all that weight being supported only at the edges, leaves a lot of weight in the middle, which has to be self supporting. I think that 4.4mm laminated glass out performs acrylic for larger frames whic require glazing. I know that it only my own opinion, but this is the stuff which surprisingly large vehicle windscreens are made of, so it's tough stuff. I also don't like stocking normal anything thicker than 2mm float glass in sizes larger than 48" by 36".

I can get 4mm thick acrylic cut to size my my local hardware shop if I need to, but it's not all that often. Since having my stroke I am not super anxious to take on to many physically demanding tasks, it does not worry me about turning down less straight forward framing jobs. Low cost 4mm thick acrylic can be a good alternative to glass for small ready made frames which are required to be didpatched by post and in smaller sizes the cost can be very reasonable as well.
Mark Lacey

“Life is short. Art long. Opportunity is fleeting. Experience treacherous. Judgement difficult.”
― Geoffrey Chaucer

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Re: Glass vs Perspex - How big is too big for glass

Post by Richard Photofusion » Mon 17 Jan, 2022 10:36 pm

I regularly switch out printed and mounted (my side of the work) images for a client who has many sets of frames. Largest are 60x40", with 3mm AR glass. Most of the frames have a 6(ish)mm piece of ply, with a ply sub frame (12ish mm x 4ish") that screws (pocket holes) into the main frame (main frame around 10mm thick). Interior of the sub frame is bevelled 45 degrees and the work is hung on french cleats. These frames are rather heavy, particularly with the prints bonded to 3mm DiBond, but they are sturdy thanks to the sub frame/backer. They used to scare the living daylights out of me, but I do the dissassembly / assembly in the morning, take it slow, and they work just fine.

One set of frames has a much lighter assembly, last lot, the backing was stapled in place. These (even the small 24x36 ones) offer to let the glass pop out as soon as you look at them, they have so little dimensional stability.

The backing subframe is the key to the larger frames. They hang in a Mayfair gallery (and in the City, and a fancy Knightsbridge department store etc), they travel, and they inspire confidence in the person using them (me, the gallery, the art handlers, the customers), and they're using 3mm glass. But, if they were in a family, or travelled (corridor/fire route) setting, I'd go for AR acrylic...

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Re: Glass vs Perspex - How big is too big for glass

Post by Not your average framer » Tue 18 Jan, 2022 10:19 am

Hi Richard,

The ply sub-frame sounds great. Do you join the sub frame, as a complete sub frame assembly, if so how? This sounds like a really neat idea. I use bare wood pine mouldings quite often, as they are readily available in different cross section sizes and they are very economical as well. I also use the same mouldings to produce various frames as well, so nothing gets wasted. I'm really big on stacked moulding frames and having a good stock of bare wood pine mouldings in my workshop is very helpful as the middle sections of stacked frames are easily made from flat profile mouldings.

I like using the flat pine mouldings, because there's no machining and they can be cut to length in seconds on the Morso and joined on the underpinner. As most forum members will probably already know I am very much keen on quick, simple and easy, especially since my stroke! I'm not too much bothered about paying a little extra to use a basic pine moulding, when the result is to save be both time and effort when making things. I now work very limited hours and need to make every minute count.

I mainly use ply for smaller simpler items, so that I can cut them to size on my band saw and clean up the cut edges on my bench top belt and disc sander. This works very well for bits and pieces, which are smaller and easy to hande, above a certain size in is much easier on a table saw, but you need more space. I set up my table saw outside the rear of my shop, often on a Black and Decker workmate, but this only works when it's not raining. It can get quite windy at times at the back of my shop and a good windy day clears away most of any residual saw dust. I don't use dust extraction, for my table saw! Dust extraction? What's that?
Mark Lacey

“Life is short. Art long. Opportunity is fleeting. Experience treacherous. Judgement difficult.”
― Geoffrey Chaucer

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Re: Glass vs Perspex - How big is too big for glass

Post by Not your average framer » Tue 18 Jan, 2022 11:22 am

I'm guessing that plywood sub-frames must be really solid and strong for perhaps quite thin sections. Interesting idea, I would think! Plywood does not cut very cleanly and needs a bit of tidying up, this is o.k. for me when making small fairly simple things of from plywood which can be cleaned up afterward on the belt sander. I'm currently working my way through a large number of pine wood slats salvaged from an old Footon bed. These are quite helpful as they are nicely machined all round and when cut into smaller strips, leaving two edges still nicely finish as originally machined.

Adding a rebate to these strips is quite easy using my hand held electric planner, which already came as new with factory made stops for rebate planing. I originally was not very impressed with these, but to my amazement they actual work much better than I was expecting. Yet again stuff like this needs to be quick, simple and easy, or why bother. As a result I was thinking that this would take make to much time to be viable, but this is simply not the case.

I add extra width to a fairly basic narrow basic pine mouldings, by making a stacked moulding frame the inner moulding, does not just add great looks and extra width at negledgible cost, but it also acts as a sub-frrame adding extra strength. Added to that, I have a huge abundance of these Footon bed slats, which when sliced into more usable profiles further increase the availabe number of sub-frame lengths as will mean that I will have plenty of these for quite a reasonable time.
Mark Lacey

“Life is short. Art long. Opportunity is fleeting. Experience treacherous. Judgement difficult.”
― Geoffrey Chaucer

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