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View Full Version : UV-A Blocking/Transmission Properties of Contact Print Glass



Cletus
22-Nov-2013, 15:10
Ok, this is probably a question for Sandy King, or on the other hand, it might be a completely "over-nuked" and altogether irrelevant issue in the first place, so here goes -

I'm looking to replace the glass in my B&S contact printing frame with something a little thicker and more durable than what's already in there, which I mildly suspect is ordinary, garden variety soda lime float glass, i.e., cheap window glass.

My first thought was to get a piece of tempered glass from an architectural supply house, but that turns out to be unrealistic from a cost standpoint and for other reasons. High quality picture frame glass was my next guess, but therein lies the problem. Good picture frame glass tends (I assume) to have coatings or other properties that are designed to block UV light, rather than efficiently transmit it, which is counter to my intent.

So, my question is: Has anyone ever experienced problems finding suitable contact printing glass for Pt/Pd and other UV dependent alt process applications? Or, has anyone ever actually used a piece of glass in a contact printer that happened to have some anti-UV coating or characteristic and caused problems with printing?

My "research" turns up that borosilicate glass would be a good choice in that it is durable, resistant to chemicals and scratches and it also transmits light well, including UV-A in wavelengths from about 350nm and up at around 90%, which is good. I guess. Since 400-500nm, or UV-A to near visible seems to be within the range required to "expose" ferric oxalate sensitizer.

The bottom line is, I'm looking for a piece of glass that's a little thicker and more durable than what I'm using now and which has nice smooth finished edges for easier, constant handling. I have a quote for a sheet of 3mm AR coated "Water White" glass and 3mm uncoated borosilicate glass, either of which should work fine. I'm just wondering whether anyone has ever looked at, or even considered UV transmission properties of glass for contact printing with UV processes. Maybe all this concern with UV light transmission is totally negligible and I should just use whatever glass I can get my hands on?

Vaughn
22-Nov-2013, 15:19
Over-nuked...I would just go to a glass shop (dealing with window glass), and just buy the right size double-strength window glass -- and have them sand the edges. Get two and store one, since you are there.

Just sanding the edges will make the glass easier to handle and less likely to crack. Have something to slip your frame into if you wish to be more protective of the glass.

Vaughn

Jim Noel
22-Nov-2013, 16:04
My preference is not heavier, but lighter. The less glass I can have between my UV source and the print the better. I have been using "Non-glare" glass from a framing shop in all contact frames up to 7x17" for several years and am very happy with it.

Cletus
22-Nov-2013, 17:09
Thanks for your input guys, I think this will save me a good bit of time and money. I was concerned about using just any old glass, as I know a lot of it is specifically designed to filter or reject UV light - or at least that was my presumption. Apparently that's only the case with specific high end picture frame glass and then it's mostly designed to reject light in the UV-B range, which isn't what I'm concerned about anyway.

There went about 20 hours of my life (learning useless information about glass) that I'll never get back!

Vaughn
22-Nov-2013, 18:57
Jim, am I correct in assuming that you have the "non-glare" side away from the negative?

Vaughn

sanking
22-Nov-2013, 19:34
Bottom line is that there are some glasses that will transmit a higher percentage of UV light below about 350 nanometers but in practice the extra cost is not warranted IMO with most types of alternative printing processes. I have not had to replace the glass in any of my printing units for a long time, but if that were necessary I would do just as Vaughn suggests, go to a regular window glass store and have them cut a sheet to the size you need, and sand down the corners.

How thick? Plate glass 1/4" thick is OK for my work, and is reasonably tough.

Sandy

Jim Noel
22-Nov-2013, 21:26
Jim, am I correct in assuming that you have the "non-glare" side away from the negative?

Vaughn
No, I have it against the negative. I am amazed, buy it cannot be seen in the image.

Jan Pedersen
22-Nov-2013, 21:41
I did exactly as what Jim did. The glass that came in the frame from B&S had an annoying imperfection that would show in even gray parts of the image.
The non glare glass from a local glass store against the negative also solved another problem for me, newton rings.