Light Source

Graphic arts plateburners, mercury vapor lamps, quartz lamps, photoflood bulbs, sun lamps, tanning beds, sunlight, and blacklight fluorescent bulbs are some of the UV sources that people have used to expose gum prints.

I use a #2 photoflood bulb, type EBV, 500 watts. I suspect I am unusual in that respect; many gum texts don't recommend this type of light as they are said to require very long exposures (like 30 minutes or more), to produce so much heat that a fan is required to cool the contact glass and avoid heat-fogging the coating, and to vary significantly in photointensity over the life of the bulb. My experience is that none of these issues are of practically significance. My exposures run from 1 to 5 minutes, depending on the type of material the negative is made of and the ambient humidity, and the exposures don't change significantly as the bulb ages, in my experience.

The situation with regard to heat is slightly more complicated: While there is no problem with heat in the contact glass or the print (my print platform hangs 13" below the bottom of the bulb, and the glass and print are still at room temperature at the end of the longest exposure time) the bulb does produce significant heat at its base. For that reason, I use a heavy-duty porcelain light fixture. As I mention in the method section, the one problem that affects the prints is fall-off on larger prints, which I work around by moving the print platform during exposure to ensure full exposure to the edges.

The EBV is very important; in an earlier version of this page I just specified a #2 photoflood bulb, which is the only specification I had from the text that gave me the idea in the first place. But someone read that and went out and bought BCA photofloods, the blue kind. When he told me it wasn't working very well, I discovered that I had some BCA bulbs on hand and tested one of them against the EBV. The BCA took 5 times as long to expose a print as the EBV, everything else held constant.

It's difficult to locate information about the UV output of the bulb, but from what little I've been able to find out, the UV output of the bulb is very low. I've seen citations to some printing industry research that seemed to indicate that the peak sensitivity for the gum emulsion is at a higher wavelength than for dichromated gelatin or albumen. I haven't been able to find the actual technical paper and read it for myself; the citation didn't give specific wavelengths but just said vaguely that for gum, the peak sensitivity was shifted significantly "toward green" whatever that means. So one might guess that the reason the photoflood bulb works well for gum in spite of having little UV output is that the actual sensitivity of gum is more in the visible range, so the violet and blue output from the photoflood suit the sensitivity range well. But the BCA bulb has significantly more violet and blue than the EBV, but prints 5x slower. So it's a mystery.

The one problem I find with the photoflood bulb as far as printing is fall-off in light intensity at the edges of larger prints; I compensate for this by moving the printing platform, which is suspended by chains from above, around during exposure to expose the entire print area evenly when I print larger than 11x14.)

I like the photoflood because it doesn't take up any space, it prints nicely, it's not so bright or hot that it "cooks" the emulsion or produces dichromate stains; I personally have found it a great light source for gum.

Copyright Katharine Thayer; all rights reserved.

Back to Method page