Even better than my attempt at an explanation below is this fabulous video featuring Santa Cruz woodcut artist Bridget Henry demonstrating the technique.
Reduction printing is a method used in relief printmaking, most commonly with wood or linoleum blocks. In relief prints, cuts are made into the flat surface of the block and ink is then rolled across the surface. Any area that is not cut away will pick up the ink, but the ink will not go down into the cut lines. When run through a press to transfer the ink to paper, assuming black ink and white paper, the resulting print will have white lines against a black background. To create multi-color relief prints, the traditional method is to use multiple blocks of the same size, with each color area cut out on its own block.
In reduction printing, a multi-color print is created using only one block by cutting away more and more of the surface in-between each color printing. After drawing an outline of the image on the wood block, the first step is to cut out any areas that are to remain the color of the paper. The first color is then printed by rolling ink onto the surface of the block and running it through a press to transfer the ink to the paper. Every sheet of paper in the edition must be printed, because after the next step of cutting begins it is impossible to go back and create more prints. Next, the block is cleaned off and any areas of the image that will remain the first color are cut away, and the next color is then printed on top of the first color. And so on, cutting and printing color by color until the print is completed. Generally, colors are built up from light to dark, and depending on the opacity of the ink the underlying colors may influence the tone of the colors printed over them.
When completed, the edition of prints is considered "closed," as most of the surface will be cut away and no more prints can be created from the block. In contrast to the multi-block method, where proofs can be created at any point and corrections made to the blocks, in the reduction method the artist can view and correct the work only as it develops progressively. Once a color is printed and the next round of cutting begins, there is no going back. For this reason, the method is often known as "suicide printing."
From 1958 to 1963, Pablo Picasso created approximately 100 linocut prints using the reduction method. He is often credited with inventing the process, but in fact it seems to have been in use by small commercial printers and taught in art schools for some time before he brought it to wider attention. Picasso was probably introduced to the technique by a commercial printer, and adopted it due to his frustration with the slowness and discontinuity of having to cut each color area on a separate block.
All of the color prints on display here were printed using the reduction method. I use oil-based litho ink and print on a Charles Brand etching press.