I doubt any place other than a frame shop or full-service glass supplier would have UV filtering glass you might 'accidentally' receive.
The architectural type is sold as laminated, and sold more for safety with secondary benefit of UV filtering (I think I had to tell the glass supplier when we put windows in our building what I wanted or guide their research because UV filtering wasn't a main concern in most of their installations). The laminate layer between the 2 lites of glass is what provides the UV filtering (about 99% but I can't tell you exactly what that means in spectral details off the top of my head, just that it compares to minimum 97% for conservation grade picture framing glass).
Non-UV filtering, 'regular' float glass (like hardware store window glass and non-UVF picture framing glass has roughly 43% UV filtering (same problem here, I can't remember the spectral definition that gives the 43% meaning). The green tint as viewed from the side I is from iron in the 'normal' glass recipe. 'Water-white (possibly a trade name) is low iron glass.
I cannot find a UV filtering spec for the non-conservation version of TruVue UltraClear (I think it's no longer sold, but the only product name I could remember). It can be hard to find UV filtering data for products not intended as UV filtering, but they all have varying degrees of it.
I don't think there is an advantage to the low-iron glass for contact printing, especially if you find a variety that only comes in a conservation grade (high UV filtering).
The green cast is annoying on some things, whites & pale colors, but most people only notice it when they're made aware of it, or are matching whites (mats and wall paint, etc. If it's that important we also compare mats under glass.) That isn't an indicator of UV filtering or transmission. UV filtering glass filters wavelengths shorter than 380 nm. UVF Acrylic starts at 400 nm (slightly higher into the blue-violet
I was only able to find UV filtering/transmission information for non-conservation glazing for Tru-Vue glass and Ato-Haas acrylic. Tru-Vue may or may not even publish their non-conservation spectral data anymore...
'regular' acrylic (phone conversation with Ato-Haas polymer chemist) doesn't have a 'spec' for UV filtering because that's not what it's sold for, and may vary from mfr to mfr, but a ballpark figure is 70% UV filtering (not guaranteed). Unfortunately it's wavelength cutoff is different (spectrogram curve is very different not easily compared to glass).
But what I got from this was if I want maximum UV transmission, 'plain' glass was preferable to acrylic by a large margin, despite the apples vs oranges factor. Scratching is another deterrent. Not having found UV filtering (and implied transmission information) for 'waterwhite' glass, unsure where I can get it from my current suppliers, my belief the color cast negligible if any effect on UV transmission and the premium cost are reasons for me to not pursue low-iron/'waterwhite' glass for contact printing, especially alt-processes.
Visually, some things that may help you identify whether glass has intended UV filtering (but not assure you).
TruVue Conservation Clear has a UVF coating that can be seen if your eyesight cooperates, by looking at the reflection of a light bulb, then shift your focus from the lamp reflection to the glass surface, back & forth, while you slightly rock the glass. It's a rare and perceptive person who sees it without having it pointed out. If someone forgets to label the back of piece of framed artwork that required it with a Conservation Clear label, we check for the coating before automatically changing the glass unnecessarily. It's getting a little trickier for me to see as my eyes age, but I don't get 'false positives' I sometimes almost convince myself I'm looking at uncoated glass & try again. Once you eye is 'trained' its' easy to detect. I only know one customer since 1993 (an interior designer) who saw it on her own (& disliked the distracting character it imparted to reflections in her opinion).
Last ditch test is to take a razor blade or Xacto blade (not a glass cutter!) & try to gently scratch both surfaces of the glass (at an edge so you don't render it unusable for framing. The UVF coating on TV Conservation Clear will scratch, and the uncoated side generally won't. Maybe if you try hard enough you could.
DenGlas, probably no longer made, had a 'dip' coating that ran on the edges years ago. One would cut the edges off so they were not visible. I haven't seen enough of it to comment on it's reflectance or scratch properties.
Laminated UVF glass is for all practical purposes (maybe one could measure a difference in reflectance due to 6 surfaces vs 2) as clear as ordinary glass, but the center laminate layer can be seen from the side.
'Ordinary' vs UVF acrylic is indistinguishable to the eye. Can only be identified by trusting the factory paper coating's ID label (if not cut off when cut to size) or spectral testing.
While we have had suppliers send us wrong acrylic (both upgrade and downgrade), someone would have to be careless to accidentally give you a premium UV filtering product when you want 'regular'. The cost difference helps keep them from making that error.
Oh, TruVue's Anti-Reflective (AR) and Museum Glass have <1% reflectance compared to about 8% typical of most of their non-surface-treated (Perfect Vue Low Reflectance, if still made was about 4.6%), and AR used to be described at having 78% UVF (better than regular glass, but not a conservation product) and Museum has >97% UVF. If you move either around long enough while having a bright light source above or behind it, at certain angles you can see a pink or green reflection. (I have seen blue reflections in a local art museum but don't know whose glazing produt that is).
Too much info? (Maybe it's good for something else some day).
Last edited by Murray@uptowngallery; 07-19-2008 at 11:03 AM. Click to view previous post history.
When you saw the show at Freestyle you can just pick out the 3 or 4 framed prints that are not behind UV glass. I had some older frames that are the same except for the glass. UV glass will have a slight pink/yellow cast to it if I had all the same frames you would not see it. After we hung the show I looked around the room and saw it I asked about it and other people could not see it.
Thanks for you comments on Montana de Oro I have 10 negs of that image the fogg just kept changing.
Bill, I'm going to have to mark my calendar for next year. I've never been up your way. It would be great.
Murray.... You're making me crazy!!!! I thank you for the excellent response. Very complete and educational, thank you. It may take me a while to digest all of the info. I hope others read this great information.
Jan, I will have to take a look the next time I'm at Freestyle. All of the work is great. I just instantly new the Montana de Oro image. Great place to work isn't it? Beautiful prints.
So, I can figure that the regular glass I have for my frame will not crack and shatter into pieces under the UV light for 15 minutes? Is this correct? Thanks.
I don't know what how much/how fast temperature has to change to make glass break under heat, but suspect that other than Pyrex, Kimax, and that type that are made to tolerate large temperature changes (usually made as tubing & vessels/glassware for chemistry), sheet glass you can easily buy probably doesn't have any special thermal expansion properties...
I could be wrong, but thought the advantage of tempered glass is that it breaks into very small pieces instead of large jagged pieces that could be more dangerous (car windows, for example).
You raise an interesting concept - I have read posts from people who had an enlarger that broke internal sheet glass while illuminated for a very long exposure. This puzzled me; the initial thermal shock of turning on the (incandescent) lamp would seem to be the worst change. Even if the temperature got a lot higher over several minutes, I would think it would be more gradual than that initial temperature change.
Maybe it broke because it was constrained in such a manner that a long exposure created a large amount of thermal expansion, stressing the glass as it began to expand against the mounting hardware or frame.
It would seem to be a good idea to make sure the glass has enough room to expand freely and not exert pressure against its constraints. I guess this is relevant to thickness as well as length and width (I really do mean guess).
Getting in on this a little late and don't know if you'll check this or not. I have used the absolute cheapest glass I could find (very thin stuff from Home Depot) and have no problems at all other than the fact that it does flex a little due to it's thickness (or lack of). However, even so, it still seems to work fine on the 8 x 20 printing frame I made. I have also tried some thicker glass from the local glass shop (called double strength) and it performs just like the Home Depot stuff. My commercial 11 x 14 split back frame has glass thicker still (close to about 3/16" thick) and I see no real difference between the three glass types/thicknesses.
So - is carbon becoming a reality yet?
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Dan, thanks for the input. I built a frame for my 8x10 negatives. It is a 10x12 frame and I put in some home made pin registration. I used some glass I got from Lowes and it worked ok during my 1/2 hour test with my UV light. The fan I used kept things cool.
I hope to try multi-layered carbon prints eventually. I have my UV light ready to go. I've been really sick the last week and did not have the energy to do any thing. I may try to pour some carbon tissue this weekend and then get to printing soon.
Last edited by Jim Fitzgerald; 07-27-2008 at 10:43 PM. Click to view previous post history.
Reason: forgot some stuff
Hi Jan, long time no see.
Originally Posted by Jan Pietrzak
Congratulations on the show at Freestyle. I wish I could see it. I looked it up on the site but sadly no images.
Tim, the images are really beautiful. It goes to show you that they do not have to be big to be beautiful. Jan's work is really inspiring. Next time I'm in I'll take a look for you!
Tim, and Jim
Thank you both for your kind words, the show was fun. Sorry Jim that I am stealing your thread. But these are contact prints made with glass on top.