I agree, coffee is great for etching, and (for me) the best is, it is holding up far better i could have imagined. I am not good enough yet making my own knives without help, but I hope I will be sometimes. So I used the etching to build up a grey "patina" on the very sensible outer part of my blades of my bought kitchen knives. They have a core of so called "blue paper steel", outside there is an unknown steel, corroding within minutes if you let the knife lie wet after cutting and don't care wiping it dry.
The patina I built up that way is there now more than a year although the knife gets a lot of use and washing (no dishwasher of course). I used no instant, just usual filter-coffee, little bit stronger than I like it for drinking.
I'm at work now and have no access to my pictures while reading this part of ifi, so I will add a picture later.
To answer frosty's question: I believe, one could mix acidic (food-)components with interesting results, because there are people using mustard (there's vinegar), onions, ans so on, to force a patina on fast corroding steels.
Blood is used also, especially warm, to get a blue patina. The colour, one get's, differs with the steel as well as with the patinate.
