I just tried this potato treatment on an inexpensive Mora carbon steel blade, - I have this thing where I like to fool around with the carbon steel blades - and try and 'force' an aged patina. The blade was left in the potato overnight, - about 16 hours total - it came out with a very nice fairly even toned patina. Much quicker than the prescribed Wayne Goddard mustard finish. That one may take several different treatment session to achieve the results you want. The mustard treatment specifies to dab on little dots of mustard, leave for several hours, wash, rinse & repeat. If your not happy with your resluts, a nylon kitchen scrubby and some dawn dish soap can remove the darkened patina, and you can start over. Vinegar is an acid, and can also be used with some good results. Try a regular paper towel, sprinkled lightly with vinegar - a lot of them are embossed with a textural design, which will transfer to the knife blade steel. Set paper towel on blade surface, then gently dampen with vinegar, let set 6 or 7 hours. The one I tried left a very fine octagonal 'fish net' effect on the blades, very neat looking and attractive. I did this once on an old SC505 Schrade stockman, and had several friends convinced it was a 'custom order damascus' blade from their custom shop, - ( I am given to lying once in a while). I imagine ketchup & hot sauce would also work, haven't experimented with them yet, but the tomato & vinegar acids should work just fine.