Kolrosing is a scandinavian technique whereby the carver makes shallow cuts into the surface of the wood and drives a pigment into the cuts. It is similar to scrimshaw designs in ivory and even tattooing in skin. While the wood does not "heal" as a tattoo would, the fibers can be burnished to lay over the cut, and once a drying oil like flaxseed is applied and dries, the pigment is pretty well held in place.
There are lots of great resources on the web to get you started. Del Stubbs at Pinewood Forge has a great intro to kolrosing page and sells really nice kolrosing knives, though you can use most any knife. His are nice because the blade has a sweeping radius, which allows for tight turns without the blade "jumping" out of the piece, and the angle of the blade lets the edge enter gradually, compressing/cutting the fibers neatly even if you are cutting against the grain. I also like the width of the blade, which forces the wood fibers apart and creates a good pocket for the pigment. Finally, the round handle reminds me of a pencil, and as such can be spun to achieve tight turns (if you are careful!)
No need for deep cuts! They will be marginally darker and wider than shallow cuts, but only barely, and if you really auger down into the wood you will have a hard time making smooth lines. Watch the grain direction as your lines come together. You can inadvertently dislodge a chip under certain circumstances (see the stem of the leaf on the right in the image below.) Use really fine powder. Well ground coffee works great, but I like taking the powder that remains at the bottom of the grinder. Powdered coffee like Nescafe works well too. I have also experimented with milk paint powder for a colored effect, though the tone is hard to distinguish. You don't necessarily have to sand the wood surface before hand, but treating the wood with a rub of beeswax or another sealant can keep the coffee from staining the surrounding wood and hiding your design. Lots of people like basketweave designs, but I prefer more freehand and flowing lines. It is challenging to get fluid curves. Sometimes it works out, like here:
Other times it does not...
post on some of my work, the spoon third from the left has some cross hatching to indicate shadow. The one on the right was done with green milk paint. It also lost some of its coloration, as I did not burnish the surface enough. You can also sand the surface lightly to close the fibers.
I find that the hardest part of kolrosing is letting go. After finishing a spoon that you feel proud of, it can be hard to take a risk and scratch it all up. Like getting your first tattoo, or your first "visible" tattoo, it can be hard to get your head around permanently altering a pristine surface. But once you cross that threshold, it can be hard to stop!
After my pal Oliver came to stay for week, we traded spoons. He chose one of my more heavily kolrosed spoons to take with him, saying something like, "Well, I have to take that one, because it is rad!" His comment got me thinking. What is "radical" about that spoon?
Is it the spoon's design? Probably not. There is nothing extraordinary about this spoon. It is a nice spoon, but the design is not out of the ordinary. Even some of the more particular elements of this spoon, like the asymmetrical bowl or the tiny finial, are in fact inspired by Oliver's spoons. He would not find them radical. Was the floral design itself radical? Not so much. Flowers, especially made up ones like this, are not "radical." So what was "rad" about this spoon? I think Oliver's response to the spoon is akin to the response many of us have to tattoos. When we meet a tattooed person, we seldom explore the subject of the tattoos, or admire linework, shading or coloring. Instead, one of the first and most prominent feelings many have is that the commitment of the tattoo, the commitment to permanently alter a pristine surface, is in itself radical, regardless of the subject or execution. Kolrosing is rad, in itself.