Very long print times
Reading other threads concerning UV print times for various processes it seems that my times are much longer than might be expected, particularly salt prints. They are at present taking anything up to an hour or more. The actual time is more as I have to stop every 15minutes for the contact printer and glass to cool down.
I am using FP4+(PyroPMK) in a frame I made myself which has 6mm Pilkington Optiglass (expensive but, so I was told, has better UV transmission). It IS quicker than ordinary glass!
Attached is a picture of my printer which I made and is fitted with 8 x 15w (total 120w) Philips F15 T8 350 BL UVa tubes. They are about 3inches from the frame glass which measures 11x14 inches.
I am wondering if anyone might have some idea as to why this setup is taking so long to make a print. Should I remove the inner frame which supports the print frame; I don't really see that would make any difference. Another might be to have stronger tubes but that would, I'm sure, only heat everything up even quicker. I could use thinner glass but with the times I'm experiencing now, I can't imagine any great reduction in time would be achieved.
For POP for example my UV box was almost useless. Meaning VERY long time and low contrast.
When set into the sun or even just cloudy day my exposure time was much faster and contrasty.
You might try that.
My 2 cents.
I can't see an obvious answer, but it's not clear to me how your lightbox works so may I ask some questions about it?
1) From the picture, it looks to me as if you're projecting the light upwards through glass. Is this correct? This seems like an odd design decision to me because if your negative is in a contact printing frame then you're introducing an extra (and unnecessary) glass layer.
2) This may be a dumb question, but are you sure those are UV tubes? I ask because every UV tube I've seen in recent years has been black not white.
Also, have you tried printing with another lightbox to get a comparison time? I'm half way across the country (Worcester), but you're welcome to use mine if the logistics work. PM me if you're interested.
Along the lines of what Ian says, maybe the bulbs does not output the correct part of the UV spectrum? There is no point in using glass in the box itself. Even if it has good UV transmission, at 6 mm, that's pretty thick. Maybe it also contributes to the heat issues, if it doesn't allow air to circulate. It's enough to have the printing frame glass.
You could try a facial tanning lamp. I use one of these, which covers 8x10" with some room to spare (I believe it would cover 11x14", though I cannot guarantee that) and gives me exposures of about five to ten minutes with vandyke prints.
Thank you for your replies;
Ian and Jerevan; the aperture above the lamps is simply an open hole, the only glass is in the frame which sits within the brass around the edge.
Ian, I think you may be on the right track concerning the type of tubes. When I bought them I thought I had done enough homework and purchased the right tubes. They are marked as UVA but perhaps it should be the blue coloured ones you suggest which may be UVB (or something else). I shall go off now, make further enquiries and see how I go.
Vlad; yes, I started with the facial tanning lamp also but thought I would build my own in the hope it would be better! Obviously not the case. It will be interesting to find the exact type of tubes fitted to the Philips tanner. Again, I'll go and check.
Ian, thank you for your offer and if things get seriously difficult I may well take you up.
I use the 18W tubes listed on this page, but there are 15W versions available to:
My small UV unit has "black light" tubes--but they look white when not turned on. When turned on, they give that classic black light glow to white cloth. In my large unit, I have BLB bulbs--"black light blue." When turned off, they are a deep blue/purple color. Both types of bulb will work. In fact, the white colored "black light" is a bit faster than the BLB. So, long story short, the color of your bulbs doesn't really tell you about the UV content of the light.
Thnaks again for your inputs.
I have been reading Sandy King's article on UV tubes on the Unblinking Eye site again. For the life of me I just can't get a definite answer. He suggests that BL tubes (which mine are) are slightly faster than BLB, but not a great deal, and like you say Allen, they show blue when running. I have also checked supplier's spec charts where available and they only state that UV tubes peak around the 350-360nm area, undoubtedly they start and end elsewhere but it is not shown.
I wonder whether the real bottom line is that perhaps salt prints need a different approach (read 'sunlight') than other processes. In Sandy's - and indeed other - articles salts tend not to be mentioned. I did use sun earlier in the year and found it satisfactory even on a bit of a cloudy day. Perhaps that is where I will have to end my search. I will find another process for winter.
A friend of mine, who makes vandyke and salt prints and uses a facial tanning lamp like mine, says that salt prints do require longer exposures, indeed, but not overly so. His exposure times with salt prints are about twice the times of vandykes, which means at most twenty minutes.
So it appears that your tubes may not be the right ones for this process. You say you've used a tanning lamp before having built your own. How were your exposures then?