One of the greatest technical challenges in photography is to capture as much of the dynamic range of the original scene as possible. This means preserving as much of the finest hghlight detail and deepest shadows as we can. Photographers working in black and white have an easier time with this because their film and paper both have a wider range of sensitivity than does color material. With black and white sheet film, we can follow Ansel Adams' Zone System and tailor the film exposure and processing to better squeeze out the subtleties we desire. Even with that, the original scene will have held far more tonal detail than any film/print combination can hold.
Working with color film and paper is even more limiting. Color negatives have a wide dynamic range than positives, or slides. Color slides have other advantages, however, and if you've read this far, you probably already have color slides you want prints from. How do we get a print from a slide with as wide a dynamic range as we can realistically expect?
When to mask
If we make a print with a single exposure under the enlarger, for the middle tones, let's say, then most likely we will have shadows that are too dark and higlights that are washed out. Choosing a lower contrast paper will help here some, but the color saturation will suffer. If we want the higher saturation of a higher contrast paper, the printer will need to manipulate the exposure with dodging and burning to bring extra luminance to the darkest areas and hold back the light areas. But some images are inherently hard to dodge and burn sufficiently or accurately.
Some examples of these kind of images would be portraits in speckeled light coming through a tree, highly reflective subject matter such as snow and granite accompanied by heavy brown trees and dark green foilage. A light evening sky with pale and subtle colors above a sharp horizon, below which is important but darker shadow detail presents a common problem. Even if the contrast is not so great that it couldn't be handled with dodging and burning, the sharp dividing line between sky and earth makes this difficult. As we dodge back the sky with our hand or a piece of cardboard, the shadow we cast will have diffuse edges. It is impossible to accurately manipulate the dodging tool without having either a "halo" effect in the sky where we slipped and didn't hold it back enough, or a "shadow" along the top of the horizon because we overlapped it too much.
How the mask is made
One very effective solution to these kinds of problems is contrast masking. Here is how it's done.
The original film is removed from the slide mount and taped into a window cut into a piece of unused printing paper (I say paper, but Ilfochrome Classic is actually all plastic-much more dimensionally stable than paper.) Two small holes are punched in the end of the paper mount using a pin registration punch. Then a piece of Kodak Pan Masking Film is similarly punched and registered with the film for exposure. After the exposure, the masking film is removed and processed.
Since the mask is black and white negative film, what we will end up with is an exact sized black and white negative of our slide which we will then sandwich back on top of the film, using the pin registration easel to make sure everything is perfectly aligned.
The negative will have density, or be darker, in the areas of the image that are light on the slide. The shadow areas of the slide will be reproduced very thinly on the negative. Then the two are re-registered, we will have added density (neutral in color because it is black and white film) to the highlight areas of the slide but relatively no density to the darker parts of the picture.
Kodak Pan Masking film is specially formulated for the job of masking. Because it has no anti-halation layer on the back, light from the exposure is not prevented from scattering in the emulsion layer. In regular film, this would cause an unacceptable degree of softening of the image similar to lense flare around a bright object in the picture. For masking, however, we want the mask to be soft to make registration less critical. Also, by removing the anti-halation layer, it's often purplish cast will not affect our color balance.
Now, with the highlights blocked up, or masked, we can give the whole image a much longer exposure when printing to bring up the darker areas, but the highlights won't be washed out because the mask is holding them back. It's just like dodging and burning, but with much more accuracy!
Masking also can increase the apparent sharpness of a print. Image sharpness and contrast are directly related, whether measuring lens resolving power, film sharpness or print sharpness. Higher contrast images look sharper than softly lit ones. When we mask a slide, we protect the highlights from being blown out, allowing us to bring up darker shadow detail through increased exposure. But increasing exposure also increases contrast (areas of relative brightness get brighter at a quicker rate than those that are darker.) The darker, shadowed areas of the print, where the lighting is flatter, will see a slight increase in contrast by the longer exposure, making subtle detail more noticeable.
Remember, when looking at a slide with the mask attached, our goal is not to make a print that matches it, because the addition of the mask will make it look dark and flat. But the masked image will better match the dynamic range of our paper, resulting in a print that is much closer to the look of the unmasked slide.