Pattern Welded or “Damascus Steel” (conversation for another day, trust me.) is brought to life via etching. There are multiple schools of thought on the process, so I’ll share mine.
For any piece that is to be etched, I will polish up to at least 1,000 grit then degrease thoroughly. Hardening steel will generally give a much better contrast than if the steel was left unhardened, this can be used to play with different types of patterning elements.
Generally if you’re doing standard mix of 15n20 and a simple carbon steel such as 1084 or W2, go with 4:1 water to ferric chloride and etch in 15 minute cycles. Make sure to use 3,000 grit to rub off oxides while scrubbing the piece down with running water, then etch again/repeat the process until desired look is achieved. You can then place the etched piece into a highly concentrated instant coffee solution to darken up the carbon steel portion of the mix even more. Please be careful when working with ferric chloride, as it is corrosive.
Stainless Damascus uses muriatic acid to etch and can vary greatly. I personally use 304 and 410 stainless steel for my patterns.
Hot (150 degrees Fahrenheit) distilled white vinegar works for carbon steels, it just takes longer than ferric.
If you’re etching a hamon, I recommend 10:1 water to ferric ratio and soak for 10 minutes per cycle and polish at a grit 1000 higher than your finish polish. Though ferric tends to be a bit too aggressive for a nice hamon. White vinegar works great for hamons. I do a 4:1 water to vinegar mix for them.
If you can get your hands on a commercial product called pH Down, that works better than ferric chloride.
If you can safely get nital, that’s a wonderful etchant for hamons. Though nital and nitric acid are a bit difficult to pick up and a bit dangerous to handle.
Oxalic acid (wood bleach) works great in 30% concentration when etching a hamon. Very good for revealing delicate patterning and metallurgical structures like precipitation of pearlite or cementite banding.