As far as I know, once it's gone, it's gone. Some people have used gunsmith's cold blue to darken the cladding, if I recall correctly.
the unnatural chemical stuff is scary from a food safety standpoint.
What's the chemical reaction that it causes?Not if you understand what the process is doing. Cold bluing reacts with the metal and is removed afterward. There is no residue left to contaminate food.
Rick
What's the chemical reaction that it causes?
KU from quench should be burnt oil
Sorry, what is there before the quench?
You could do a dark patina with vinegar etch. Paint the edge and blade road with a resist like nail polish. Saturate a paper towel with vinegar and wrap the blade for a while. You could also dip it in hot vinegar instead.
I wanted to do this on some spots where the kurouchi went off.
Could vinegar hurt/weaken the kurouchi that is still on?
I've been wondering about this. Saw a blade Straub posted on I.G. a while ago with a "parkerized" finish. Is there a significant difference/advantage between this and an acid etch? My understanding is that it is similar to a heated acid etch with additives that create the corrosion resistance.Find someone who does black parkerizing in your area. I've done it to one of my Moritakas. It is more durable than KU and takes a very trained eye to tell the difference. Parkerizing is a common firearm finish and will work on iron cladding but not stainless cladding. It is safe for the steel as it only reaches 100°c maximum. Mask off the blade road with a waterproof tape like used in powdercoating. You can then media blast the remaining finish down to bare metal. It takes about 1-2 minutes in the solution and you will have an even, durable KU finish. I'll do it for you if you want to send me the knife. It would cost a bit of postage, but I would do it for free to help out a member here.
Given all the discussion above, plus 40+ years in the shipbuilding business (including with Bethlehem Steel), the simple answer is, you can't. The series of steps that an experienced smith goes through to create his version of KU cannot be undone and redone without creating a completely different knife than the one you started with. [I've never understood the value or purpose of KU finishes anyway - if they were so damn good, why do we not see them deployed in other "using" metal products, whether swords, machined parts, industrial products, whatever ? They are decorative and not particularly and not particularly attractive to my eye. I have gradually been removing the KU from my two Takedas and like them better and better.]
Tom
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