Disclaimer: I am not an expert. This project involves dangerous chemicals, and this tutorial is meant as a guide only. Do your research, be safe, and be a competent adult--in this and in all things.
Everyone who fiddles has started a project that didn't quite end up the way they imagined (though they may love it later). I'd say most of my projects end up that way, but this one was the rare bulls-eye.
You may be asking yourself something to the effect of What happened to that knife? or Why would I dip a perfectly good knife in acid?
I'm glad you asked!
There are many who prefer carbon steel in their knives. It tends to be easier to sharpen and can stay sharper longer than stainless, because of chemistry/science stuff.
The downside is that carbon steel is more prone to rust if not maintained properly, and let's face it, a lot of us throw things in a drawer and forget it's there--even things we shouldn't. That's one of the reasons so many things are now made with stainless steel (or were, but now we have plastic. I digress).
However, even if cared for properly, carbon steel (and other metals) can develop a discoloration or pattern over time, called a patina. Think of what an old penny looks like compared to a new one. That's a patina.
When it comes to steel, think of a patina as a form of good rust. The blue- or blackish patina is a more stable form of oxydation that can help prevent that bad, bad, red rust. How well a patina protects from rust is, well...above my pay grade. I just wanted the cool factor.
Inspired by this forum post on acid-etching, I set out to strip the factory coating from my ESEE Izula and customize it to look like it had survived an apocalypse--and I think I hit the mark. Note that acid is not necessary to force a patina, but I wanted the textured look.
One of the elders at my church asked me to share how I did it, so I figured I might as well post for all to see, but I want to make sure to give credit where due, because that's where I started.
Ferric Chloride Acid Etchant (Follow all safety protocols & do your homework: I am not an expert): This is used to make motherboards, so if you can't find it, check Radio Shack or similar.
Windex (to counter the acid)
Nail Polish & Remover (protects the naked steel where necessary); alternatively, you may use melted candle was, etc.
Container to safely hold the Etchant: I used a Nalgene bottle. Again, I am not a scientist.
Paint stripper, sandpaper (if your knife is coated)
Mustard, Ketchup, Vinegar, etc. to force the patina. Use Google to view various results.
The process is to dip the knife into the acid, which will slowly eat away at the metal until it has reached the look you want. Windex is then used to neutralize the acid.
For myself, I first dipped the knife with the factory coating in order to etch the logos deeper into the blade; then, I stripped the coating and etched the entire knife to give it a textured look.
Step by Step:
Step 1: Prepare to Dip
After gathering the materials, disassemble any handles/scales/parts that won't be dipped from the blade and apply nail polish to any part of the knife you don't want the acid to touch (most importantly, the edge).
If I did this again, I'd be sure to coat the edge two or three times--parts of the edge were eaten away a tiny bit, and it took some sharpening to correct.
A quick word on laser-etched logos/text, especially on Izulas: only cover with the polish if you don't want them etched deeper into the blade. If you don't want to keep any logos/etc., skip to Step 3. If your blade doesn't have a coating, go to Step 4.
Note that on my blade, I covered part of the laser-etching with polish. I wanted a simplified result, and by protecting the ESEE KNIVES USA etching with the polish, I deleted it from the finished product. You can just barely see it in the sanded-down picture, and it vanished on dip #2.
Step 2: Acid Trip
Perhaps it should go without saying, but plan ahead for how you will remove the blade from the acid--you don't want to be splashing around trying to find the thing. I used a coat hanger to fish through the lanyard hole.
Alternatively, you could lay the blade down in a shallow container with the acid, but I wanted full immersion so I used a Nalgene bottle (remember to use something the acid will not eat through!).
Check the knife periodically, using Windex to neutralize the acid off of the knife and rub/rinse any residue off the blade. Spent acid can stick to the blade and prevent further progress.
Neutralize again when done (or when checking results, if necessary). Time will vary based on what you want; mine stayed in for an hour or two for the logo dip.
Step 3: Go to the Strip
For blades with coatings, it will be necessary to strip them off in order to acid-etch the whole blade. Sanding will work, but I didn't have the patience so I went with a heavy-duty paint stripper (Crown Tuff Strip, for the record). Following the directions, I pretty much just sprayed the knives down (modified a Becker BK-2 at the same time) and the paint just sloughed off pretty fast.
On the BK-2 that was all that needed done, but the Izula had some kind of matte finish, maybe a bead blast, so I used some high-grit sandpaper to bring it down to the metal. Your results may vary.
Step 4: Let 'er Rip
Don't forget to protect your edge again*, and then dip it as before. Neutralize with Windex when done. As for cleanup with the acid, I just left it in the Nalgene bottle in case I ever need it again--note that bits of metal remain in the acid (if you poured the whole thing in a bottle & immersed like I did), at some level anyway, and make the acid less effective the more you use it.
*Note that there is an opportunity here: with the coating gone, you can now protect any portion you wish to remain un-etched with the nail polish. Some people paint on a flame pattern or their initials or whatnot. I intentionally slathered the nail polish onto the edge kind of heavily to give some contrast.
Step 5: Take a Sip
...of your favorite beverage and pat yourself on the back for making it to the home stretch. Some food might be in order as well, since we're essentially playing with food now and you might be getting hungry.
To force the patina, use whatever acid-based food item you wish: common ones include vinegar, mustard, ketchup, A1, and many others (Google is your friend). These allow you to sort of pattern or design what the patina will look like. Alternatively, you can simply stab the knife into an orange, apple, potato, or what have you and see what happens, but unless you want to acid-etch or sand it off you could be stuck with the results for a while, so be warned.
I played around with mustard and vinegar until I found patterns I liked. Some of the mustard patterns began to rust quickly, and I rubbed it away with vinegar and tried again. Play with how much oxygen the blade has--I tried wrapping the BK-2 in a mustard patterned towel and enjoyed the results.
With the Izula, I tried a little mustard and then soaked an old hamburger bun in vinegar and sandwiched it in betwen for a while. I think my results were pretty unique. In the end, just have fun with it.
When done, I resharpened and applied some mineral oil to the blades, which gave them an interesting sheen. Sure, the patina is supposed to protect the steel, but why not be safe? The edge should have some, at any rate.
Step 6: Wear it on your Hip
....or wherever, it's your knife. I just got caught up with the rhymes. Hope you have fun reading, and just maybe making your own custom-finished knife.