Coffee Treating Patternwelded Steel
Note: this information is from Mareko Maumasi of Maumasi Fire Arts. Mareko is bladesmith who specializes in high-end kitchen cutlery. He makes a habit of weekly sharing details into how he does things: usually it's a damascus pattern, but a little while back he did a series of videos on Instagram detailing how he finishes his damascus blades, using a coffee-based treatment. With his permission I transcribed it to text, which can be read here. This is the shorter article that I rewrote using the information from Mareko's original videos. Enjoy, and let me know if you try this!
Coffee treating has two great advantages: one, it's a very beautiful, high contrast finish on damascus, a smooth dark gray over the bright silver. Second, is being coffee, it's completely food safe, and so very suitable as a final finish on kitchen cutlery.
The first thing to get set is the temperature and mix ratio. Coffee is an acid, albeit a mild one. So, the hotter the mix, the faster the etch. If the coffee mix is boiling hot, it can get where you want in about ten minutes, whereas a cold mix can take about 24 hours. Mareko uses "tap hot" water, water that's about as hot as you can get from the tap, or just about a simmer on the stove.
For the mix ratio, use three cups of instant coffee mix to a half gallon of water. Mareko specifically uses mix from a seven ounce container of Nescafe Clasico Dark Roast.
This mix is plenty sufficient for your average to large knife: when I tried this finish, I mixed my solution and poured it into a 3" diameter PVC pipe with the bottom end capped off. Plenty of solution to fully submerge my 9" blade.
So that's the solution. Now for prepping. The damascus is first etched in standard ferric chloride (Mareko uses a 4:1 ratio of water to acid). Etch it to the depth you want. This gives height difference between the contrasting steels, and so a shimmery 3D look, but the coffee gives the final coloring. After etching to the preferred depth in ferric, neutralize the acid on the blade with baking soda water, and wet sand with a worn out, used piece of 2500 grit sandpaper. Scrub the whole blade down nice and evenly.
Now, if it's just the bare blade at this point, you're good to go right into the coffee. If the handle is already fixed on, paint a resist (such as nail polish) over the handle where it meets the blade; this will prevent the coffee solution from staining the wood handle.
Once you've got your knife prepped, the coffee mixed, and the temperature set, you're ready to dip the knife. Hang the blade in the coffee solution, and check every ten minutes or so to make sure it's darkening evenly. It'll take forty minutes or so, give or take depending how dark you want it.
If it looks like the blade isn't darkening evenly, there's likely something wrong with the prep. Take it out and scrub it down with fine steel wool. It's a mild abrasive, but it'll even everything out and break everything down. Then, put it back in and try again.
Uneven darkening can happen at the second or third check too, so be ready in case that happens. If it does, take it out, scrub it down, and try again.
Another issue you may run into is if you have too much "bright" steel in the pattern as opposed to dark. Damascus patterns are typically two steels, one "bright" and another "dark" (silver and gray/black), usually something like 15n20 for the bright and 1084 for the dark. If it's too much bright steel, the bright steel tends to darken instead of staying untouched. Mareko doesn't know why, it just does, so if the particular pattern you're doing has a larger amount of bright steel then the coffee treatment may not be the best way for it.
Once you've reached the desired darkness, take it out and neutralize it with baking soda water and wipe it down with a paper towel. A bit of the oxide should come off and it'll lighten a small amount but the rest should stick. If you're not happy with it then it's steel wool and repeat, but if you are, it's a light oil and you're done!
P.S. Mareko noted that if you have any questions, he is more than willing to answer them and help other bladesmiths out. You can email me, or contact him through his website.