Regular 2mm clear glass is the best option. Tru-vue do make a super clear glass, but that is more about removing that slight green tinge than increasing UV transmission. Don't use the Tru-Vue conservation glass because that hardly transmits any UV at all, great for framing, but would make a VERY slow sunlight printing frame!
Oh, forgot to mention that Perspex/Acrylic blocks a lot of UV, together with the static and scathing problems, would make that a bad choice as well.
That, spelled correctly as calcium fluoride, is the Fluorite already mentioned.
Originally Posted by Poisson Du Jour
Thanks for info!
So I will use regular glass:)
Regarding the thickness, 2 mm sounds good, although I am probably going to use something thicker and thus heavier
You say heavier. Is that because you think weight will add more pressure? The pressure comes from the springs (usually three) on the back of a contact frame or the suction of a vacuum frame.
Originally Posted by revaz_kv
If you contact print often or all the time there is wear and tear on the glass. Once or twice a year I replace the window grade glass in my contact print frame. Scratches, pin holes and other such damage are just a part of the work for me. It happens. These of course show on the print. I keep two spare glass pieces wrapped in paper in case I need a replacement suddenly, such as breaking it or scratching it. If glass is so easily damaged that is another good reason not to use plastic.