If you are laser engraving a dark wood, like walnut, the solution is rather simple. If you require a higher contrast, you can use liquid shoe polish. We like to use the less-expensive liquid shoe polish, as it is a bit more watered down and is easier to clean up. Using the foam applicator of the liquid shoe polish, apply the liquid shoe polish to the surface of the wood. Because the wood is dark, any migration of liquid shoe polish does not show up. You can even use the thicker shoe polish to accomplish the same task, although the liquid shoe polish is much quicker to clean up using a rag or towel.
—Richard Korbyl, Columbia Awards