There's never any shortage of people saying ludicrous things about gum printing, and not long ago there was a newly-self-appointed gum expert proclaiming that the "correct" exposure time for a "gum layer" is six minutes. This of course is egregious nonsense, or as my gentle mother would have said, proof that "a little knowledge is a dangerous thing." The correct exposure time, of course, varies widely and depends on many different things, as we'll discuss below.
To determine correct exposure, use a step tablet (a Stouffer 21-step is not expensive; I keep six of them handy at all times). The test strip, on the same paper with the same emulsion and same light source, under the same environmental conditions, that you intend to use for the print, will give you the optimal exposure for that emulsion on that paper under those conditions. If conditions change, or if you mix a different emulsion or decide to print on a different paper, then you need to make a fresh test strip to determine the optimal exposure for the new circumstances.
You can do okay just settling on a generic exposure time and always exposing the same, especially if your goal is a more traditional gumprint look that's not intended to have a photographic look or tonal scale but be more pictorial in nature. Your different mixes will be over or under-exposed but you can adjust for that by developing shorter or longer to compensate. But you won't get the full advantage of the characteristics of each coating mix this way.
Expose for maximum number of steps retained after development, not for DMax. For most photographic processes, you're taught to expose long enough to achieve DMax; when the bottom step, or bottom two steps to be very sure, are the darkest possible value the emulsion will print, that's your optimal exposure; the other steps fall in line automatically. It doesn't work that way for gum, and in my observation, one of the greatest barriers to success in gum is coming to gum with an expectation that it works like other photographic processes. With gum, DMax arrives before what I consider an optimal exposure is reached; in other words, DMax is a necessary, but not a sufficient condition for establishing correct exposure for gum. If you expose for DMax alone, you'll probably be underexposing and not getting the full value of the coating mix, because you won't be printing long enough to retain all the steps that coating mix can print.
An exception to this rule: when you're making a multiple gum print in order to print a longer tonal scale, you'll use a heavier pigment mix to fill in the deepest shadows in the scale, and you'll want to expose that mix just long enough to ensure DMax but not long enough to print the highlights and lighter midtones, for reasons explained on the page on gum and tonality. Just the fact that this way of printing the darker end of the scale has been recommended by gum experts for decades is another indication that it's long been understood that for gum, a shorter exposure is necessary to ensure DMax than is necessary to print the number of steps possible from that coating mix.
Illustration showing how to determine proper exposure: consider the step prints below. This is the way I determine exposure for a new mix or new set of conditions: Coat and dry a small piece of paper, tape four step wedges to it. Bracket exposures around where you expect the correct exposure to be, controlling the exposure by moving a piece of black paper to cover an additional wedge after each exposure increment (here I moved it in 1 minute increments, except for the last exposure, where I accidentally let it go longer). Then develop for a set period of time; traditionally 30 minutes has been a sort of "standard" for gum development, but it's not set in stone. You can expose longer and develop longer, but there's no particular advantage to doing so, as long as you've established that the shorter exposure/development gives optimal results. Or in other words, exposing beyond the minimum optimal exposure doesn't get you more steps or more DMax, it just necessitates a longer development time to develop the same number of steps and open up the steps down to the basic DMax. The number of steps, and the DMax, are determined by the emulsion itself.
The first thing to notice here is that the gum border is the same DMax and color saturation at the lower edge as at the top of this paper, even though the bottom border was exposed for only one minute and the top border was exposed for 4 minutes and 24 seconds. You see no value/saturation change at the edges where the paper was moved at one-minute intervals, as you'd normally see in such a test strip done on silver paper. In other words, the exposure of 1 minute was sufficient to produce DMax under glass (without film) for this particular coating mix. The added base density of the film of course means that under the film, 1 minute wasn't quite enough to produce DMax in step 1, but 2 minutes was more than sufficient, and the top two strips, quite overexposed, have the same DMax as the slightly underexposed 2-minute strip. That the two oveexposed strips have the same number of steps just confirms that more overexposure doesn't get you more steps. That the steps aren't well-differentiated shows that they need longer development to compensate for the overexposure and open up the steps.
Obviously the one-minute strip is underexposed, as it lost several steps in the standard development time, and the two-minute strip, while closer and while passing the traditional "DMax" test, is underexposed as well, as it also lost at least one step in development. One quick further test at 2:30 (not shown here) determined that 2:30 was a good exposure for this mix and this development time, giving the maximum DMax and color saturation, the maximum 7 number of steps for the mix, and the best tonal separation (the amount of desired tonal separation is a subjective judgment that will vary depending on what your printing goals are for a particular printing).
As noted, the maximum number of steps available from this coating mix is 7; the two overexposed strips (3:00 and 4:00 minutes) just push those steps up farther along the scale since the lower steps are blocked, but don't add more steps per se; if these strips were developed longer to compensate for the overexposure, they will move back down.
A more in-depth discussion of gum and tonality.
Thoughts about Influences on Exposure Time:
Light source: it should go without saying that a major determinant of exposure time is the light source being used.
Dichromate concentration: Dichromate concentration is another very important factor in exposure time, as there is a strong and linear relationship between dichromate concentration and speed of the emulsion. This is demonstrated not only in my own tests but is a relationship well established in printing industry laboratories for decades. Read the linked page for more information about this important relationship.
Humidity: There is a very strong, indeed almost vertical relationship between humidity and speed: the higher the ambient humidity, the faster the emulsion will print, and vice versa.
Pigment concentration: a more heavily pigmented emulsion requires more exposure than a less pigmented emulsion, using the same pigment. Different pigments may require different exposures depending on characteristics of the pigment, although there's no clear identifiable relationship between particular characteristics of pigments and speed.
Pigment Hue Range: I find no evidence to support the newly popular theory that pigments in particular hue ranges require different exposures because they act as UV filters depending on their hue range; i.e., blue prints faster and red prints slower, because red blocks UV more than blue. In all my experience printing gum, I've never seen that to be the case, and tests summarized at the linked page confirm my casual observations: there's no such relationship. This is one of those many bits of gum misinformation that someone says unthinkingly and then everyone repeats without bothering to test it for themselves, creating yet another new gum myth.
Paper: Different papers print at different speeds. If you always print on the same paper, this won't be an issue, but if you change papers, you may find that you need to change your exposures to adjust for the change in paper.
Copyright Katharine Thayer, all rights reserved