What are the dark room techniques used to manipulate the *local* contrast of a print?

Question

What dark room techniques are used to achieve effects similar to those of unsharp mask or "clarity" in digital post-processing?

Answer 1

In the darkroom, one can use an unsharp mask to achieve effects similar to the digital tool of the same name. Yes, "unsharp mask" was a darkroom technique before it showed up on your computer.

You start with your negative, and copy it onto another negative (forming a "mask"). The mask is dark where the original negative was clear, and clear where the original was dark. When the mask is stacked with the original negative, the overall contrast is decreased. This allows you to print the negative+mask onto higher-contrast paper than you could without the mask. The twist is that the mask is unsharp; that is, it's a little blurry. That keeps the mask from reducing the local contrast—it just reduces the large-scale contrast. Because you're now printing on higher-contrast paper, the local contrast is increased.

Another technique for local contrast manipulation is to print on variable-contrast B&W paper, whose contrast depends on the color of light from the enlarger. You can do an overall exposure with a low-contrast filter, and then selectively expose parts of the print with a high-contrast filter.