Special glass for passing more UV light

[thefizz]

I understand normal glass blocks most UVB light and that special glass can be got which allows more to pass. As a lot of alternative processes are sensitive to UVB light, I would like to get the best glass suited. Can anyone tell me what type to look for?

Peter

 

[Photo Engineer]

I believe it is quartz glass, but then some old uncoated lenses also pass a lot of UV.

Quartz lenses cost an arm and a leg. Have your wallet ready.

 

[thefizz]

Sorry PE its glass for contact printing which I need rather than lenses.

 

[David A. Goldfarb]

I haven't seen any print frames with quartz glass, so I suspect it's not a realistic option.

 

[Photo Engineer]

Sorry PE its glass for contact printing which I need rather than lenses.

It would still be quartz glass. Regular glass blocks some UV. But then, the big UV contact printers that I have seen use regular glass and just blast away!

 

[David A. Goldfarb]

Here's a price list:

Dead Link Removed

So 3/16" is $20/sq. in., which if I've had enough coffee amounts to $1600 for an 8x10" sheet, presuming they make it that large. Don't drop it! Maybe better get 1/4" to be sure it can take the pressure of the springs.

 

[Ian Leake]

I suspect that you're over-engineering a solution to a problem that is purely theoretical. Ordinary glass is fine, and the simplest way to decrease your exposure time is to add more lights.

 

[AgX]

It would still be quartz glass.

Not necessarily Quartz. CalciumFluoride has high UV-transmission too and is thus used for UV-lenses.

 

[Bill Burk]

Here's where you can really do a reality check...

For the process you are about to test...

Put one sheet on top of "any" glass you want to test...

And one sheet underneath

Expose a step-by-step exposure series in f/stop series.

Develop them up and find the "effective" density added by the glass.

If it is negligible (or if the effective density is something you can work with), then you don't need anything better than the glass you just tested.

 

[Mustafa Umut Sarac]

Use ultra thin glass ! There are even paper thin flexible glasses available , good optical flat plastic would be answer also , fluro plates are better than cheap glass when it comes to index. I dont know they transmit uv or not but if they do , they are optically excellent.

 

[RalphLambrecht]

I suspect that you're over-engineering a solution to a problem that is purely theoretical. Ordinary glass is fine, and the simplest way to decrease your exposure time is to add more lights.

sounds reasonable to me.

 

[Hexavalent]

Most alt-processes do just fine with UV-A. Save your eyes/skin and don't worry about UV-B transmission - glass that passes 360 nm and longer is totally usable.

 

[David A. Goldfarb]

Realistically, what you do is pick your light source and expose and develop your negs to give you a reasonable exposure time for your process.

I make albumen prints and use the sun. A good neg for me prints in 10-15 minutes in direct sun, 30-60 minutes in indirect sun. Some can be as short as 5 minutes or as long as 90 minutes, but anything longer than that, and there start to be heat artifacts from the rivets that keep the springs on the back (maybe I need to design a back without rivets that go all the way through, hmmm....). I haven't started albumen printing yet since relocating to Hawai'i, and our balconies face north, so no direct sun, but it's very consistent. It would be ironic if I had to build a UV exposure unit out here, but I might have to.

 

[gmikol]

DAS-based processes are sensitive at 335 nm (compared to 360-365 for dichromate). I've looked at super-white and borosilicate glasses as an alternative for my NuArc. Some examples are Schott B270, Borofloat and BK-7. They have transmittance of 70-80% at 335nm, compared to 40-50% for plain-old soda lime glass.

Given that there are 2 sheets of glass between the light source and the print in my unit, that can make a 1-stop difference in printing time. (0.7^2 = 0.49, 0.5^2 = 0.25). I haven't actually tested this, it's just theory at this point, but I hope to once the weather cools and the nights get longer.

--G

 

[Richardk550]

I went to the glass shop and asked for a glass that would pass UV to replace the regular glass I had been using in my print frame. They called it "low E" glass and charged me about $17/sq ft. It made less than 1/2 stop difference in exposure, so if it breaks, I will just replace it with plain glass.

 

[gmikol]

You have to select carefully. One ultra-clear glass I looked at, PPG Starphire, actually blocks *more* UV than soda lime.

Also for dichromate or iron-based UV processes there won't be as much of a difference. 1/2 stop sounds about right, based on the transmission curves I've looked at.

--G

 

[chrisaisenbrey]

I can just confirm that normal glass works perfectly well. I’m also using DAS.
With a face tanner as light source at a distance of ~1m I get exposure times in the range of minutes. (I’m using 200mg DAS and 500mg of lamp black for 5g of gelatin)

 

[Jim Noel]

I have non-reflective framing glass in all of my contact frames and it works well. The slightly rough surface is against the negative and helps prevent Newton Rings which I used to have occasionally.

 

[gmikol]

I can just confirm that normal glass works perfectly well. I’m also using DAS.
With a face tanner as light source at a distance of ~1m I get exposure times in the range of minutes. (I’m using 200mg DAS and 500mg of lamp black for 5g of gelatin)

Your face tanner probably puts out more UV-B than my NuArc. The NuArc lamps just weren't designed to have much output in that region. It's been a while since I've done anything with DAS, but my exposures were more like 20 minutes, and the tissue/negative sandwich was getting *hot*. If I could cut that by even 5 minutes (10 would be better) it would be a huge improvement.

--G

 

[chrisaisenbrey]

Those face tanners are quite cheap second hand. My one consists of 6 fluorescence tubes (15W each).
I did need quite long exposure times at the beginning until I noticed that I have to use more lamp black to get into a good gamma range. This reduced then the exposure time automatically.

Chris

 

[Bill Burk]

... It's been a while since I've done anything with DAS, but my exposures were more like 20 minutes, and the tissue/negative sandwich was getting *hot*. If I could cut that by even 5 minutes (10 would be better) it would be a huge improvement.

--G

gmikol,

Thanks for giving a good reason to want to reduce the time. I was thinking only pre-press production workers would need special glass to boost productivity, but for an artist... time is not a big issue.

I'm used to seeing special UV passing glass in small pieces... up in the top of the unit protecting the point-source lamp. Not necessarily as the vacuum frame top glass.

 

[gmikol]

That's why individualized testing is important, IMO. There are subtle differences in everyone's setup. Even "plain window glass" can have some variability in how much UV it passes. Negative substrates can differ slightly (acetate vs. polyester, especially, but minimizing base + fog as well). So can the mylar you might use between the neg and print or tissue. Each of those things eats light, and it adds up, some more than others.

To the OP: If your exposure times are short enough, who cares...whatever plain glass you use is fine. But, like me, if they start to run the risk of damaging the negative, or preventing a good print, then something needs to be fixed, and the glass is as good a place to start as any, IMO.

I wish the top (bulb-protecting) glass in my NuArc was small. It's 11"x14" !! No quartz glass for that.

--G