David Wolman, reporting in Wired:
Kuhl's intricate production process combined offset printing with silk-screening. The hardest features to forge with any level of sophistication are on the front of the note: the US Treasury seal, the large "100'' denomination in the bottom-right corner, and THE UNITED STATES OF AMERICA at the top. Real US currency is printed on massive intaglio presses (intaglio is Italian for engrave). The force with which the presses strike the paper lying over the engraved steel plates creates indentations that fill with ink, giving the bills a delicate 3-D relief and a textured feel. Its absence is a telltale sign of a counterfeit. For Kuhl this was the most critical puzzle piece: how to create that texture convincingly without the benefit of actual engraving. "I had an idea," he says, "and I was itching to try it."
Kuhl, the counterfeiter in question, led a truly amazing life, from faking Warhol prints and actually meeting him in person, to having his fake bills extolled by Germany's central bank as the best in the business. He was a true artisan who strove for perfection.