Introduction: Acid/Brass/Stone Washing a Knife

For my first Instructable, I thought I would lay out a step-by-step guide to creating a one of a kind look for any knife. This will work for any knife (actually any item made of steel!), so long as you have large enough containers to fit them in.

Step 1: Safety Notes!

DISCLAIMERS: Don't do this kind of stuff at home if you don't have a basic understanding of high school chemistry. You will hurt yourself BADLY, and I won't be held responsible. Legalese out of the way; onward!

Safety first, you are working with dangerous chemicals.
1. Goggles, eye protection: wear them.
2. Gloves: get some. No vinyl, acetone will melt it. Nitrile is good, rubber is better.
3. Respirator if you have one, the fumes from this procedure can be quite irritating. You may want to complete this outside if you do not have a well ventilated room. Do not purposely inhale the fumes. Don't quote me, but I have read the fumes may contain chlorine gas amongst other things, and chlorine will kill you in high enough doses. Be smart.
4. All bowls, containers, measuring tools, and funnels must be glass or plastic. Metal will corrode!

Another IMPORTANT note, this will void any warranty you have. I'm not responsible if you attempt this and dissolve your blade. Pay very close attention to the timing of your acid bath, and don't etch screws. The acid will eat the threads, and then you're the one who's screwed.

You will also need to sharpen your blade when all is said and done with your desired sharpening methods. These are like fingerprints and snowflakes, extremely individualized, so I won't touch on sharpening much.

Step 2: Required Chemicals & Must Haves

Gather your chemicals. You will need:
1. Muriatic/hydrochloric acid. This is available at almost any hardware store, it is used to clean concrete, and adjust pH of pools.
2. Hydrogen peroxide, the 3% from your local pharmacy is fine. Higher percentages will work faster, and are available at any cosmetic supply store or online.
3. Acetone, used to clean and remove oils from all metal surfaces.
4. Nail polish. This is used as an acid resist, color doesn't matter but I do recommend black or red as it is easier to see and thus remove later.
5. I use the reinforcement stickers as stencils for cleaner lines when applying resist.
Not pictured but also required:
6. Baking soda, this will be used to neutralize the hydrochloric acid etchant.
7. A plastic/glass measuring device, and plastic funnel. Do not use metal as they will corrode immediately.
9. Tumbling "media" and tumbler. Here you will need some type of plastic jar or bottle, I choose to use a 32oz Gatorade jug. You can also use a mayo or peanut butter jar. For media, you will need either a.)7-12 thumb-sized smooth rocks (these can be from a local brook/river or the craft section at Wally World), or b.) +/-50 pistol brass cases, in this case 9mm.
10. WD-40/PB Blaster (any aerosolized petroleum-based oil will probably work) or water and dish soap.

Step 3: Disassembly

Starting with a bone stock knife, you will need to disassemble it completely. A Spyderco Tenacious and Kershaw Chive are both used in this Instructable.
Be sure to separate all screws, standoffs, springs, etc into a secure container. In one photo, you can see I have already begun applying my "resist", the nail polish.

Step 4: Cleaning Parts to Be Etched.

Once disassembled, put on your gloves and glasses, and clean all parts thoroughly with acetone. You must remove all oils from the parts or it will result in a poor etching! From this point on, do not touch any parts without gloves.

Step 5: Applying Resist.

Change your gloves at this point if using nitrile. Nitrile has a "fair" resistance to acetone and at this point is probably at breakthrough.
Apply nail polish/resist to all parts you do not wish to be etched. I recommend using some type of stencil, such as the reinforcement stickers, to keep your work more professional. As you can see, I did not use them on the Chive, and I feel the results suffered.
You will want to pay very close attention to the detent, lockbar surfaces on the liner and blade, the pivot and thrust areas, and all screw holes. Make sure all threads and hole edges are thoroughly painted, even using 2-3 coats. The tolerances on most high quality knives are so close that even the microns removed by the acid can result in changes to the knife's geometry, resulting in "lock rock" and poor action. Change gloves as necessary to avoid getting any "resist" on areas it shouldn't be.

Step 6: Acid Etching

Using a bamboo skewer and some wire (a chain of plastic coated paperclips is better, they won't be dissolved like the wire), fashion a hanger that will allow you to place the parts to be etched in a glass container. Here, I am using an old food jar, but you can also use a flower vase or other glass container. In the photos you will see I have placed the jar in a larger bowl, and surrounded it with baking soda. Baking soda will neutralize any spills.
Using a plastic/glass measuring device and funnel (metal will corrode immediately), add two parts hydrogen peroxide, and slowly add one part muriatic acid to your etching container, and always remember AA: Add Acid. Do not add H2O2 to HCl. The reaction is exothermic (produces heat) and can be violent if you do it wrong. Always add the acid to the hydrogen peroxide! The solution will begin to turn green as it contacts the steel.
Pay attention! The reaction is fast, check every minute or so till it reaches the desired darkness. In the case of the Kershaw, I only etched it for five minutes, the Spyderco was etched for a timed twenty minutes. In reality, with this etchant, more than ten minutes is probably unnecessary.
Something to note: different steels will react to the acid in their own ways. Some will darken more, some cheaper steels will pit and nearly dissolve completely. Heat treats can affect how the steel takes color, with raw untreated steels typically staying lighter. This will affect many fixed blades, as some do not quench the tang, giving it different properties. Pay close attention to unmarked or lower quality knives so you don't destroy them.

Step 7: Neutralizing Acid.

Once the desired time and/or darkness has been reached, I recommend placing the parts immediately into near boiling water. This is a step I use while acid/rust bluing gun parts and results in a darker etch, and helps to "lock" it in. After 30 seconds to one minute, remove the parts from the water and neutralize any remaining acid with baking soda. Any acid will bubble just like the volcano you built in middle school for the science fair. Once neutralized, rinse with clean water and dry thoroughly. VERY IMPORTANT: You must neutralize the acid using baking soda (or ammonia/Windex if you can monitor pH to a neutral 7.0). Failing to do so will cause rusted blades, chemical burns, and general stupidity. Maybe your first born will have a tail, I don't know...

Step 8: Clean Up.

At this point I recommend starting your cleanup. The etchant you mixed can be stored in a plastic container and used indefinitely. It can be renewed by simply adding air using an aquarium pump. Please mark your container well and cover tightly to avoid accidental ingestion or other mishaps. All jars, bowls, and measuring tools must be neutralized with baking soda and rinsed thoroughly!

Step 9: Completing Your New Look.

After neutralizing, you need to decide your next step. You can oil your blade and leave it as is with your new, darker finish, or take it a step farther.
Shown here are both methods I have used. The methods are the same, the only difference is in the media used. You will place your parts one at a time (one part per cycle, very important as the steel on steel can result in damage!) in your "tumbler" and add your media. You can use stones or brass cases. Once this is done, give everything inside a very liberal spray of WD-40/PB, or a mix of water and dish soap (Tablespoon total?). Close up your container and tape it shut.

Step 10: Tumbling

Two options here.
1. Shake the jug until your arm falls off (may result in a more uneven finish), or
2. Wrap your jug in a towel, tape it up (picture is really overkill, one wrap around will do), and place in a clothes dryer. Tumble on Fluff/No Heat setting for 10-20 minutes. Ten minutes results in a light pattern, twenty obviously will be heavier.

Step 11: Cleanup and Assembly

All pieces must now be cleaned of any remaining resist, and grime from your tumbling media. Skipping this will leave you with a very "gritty" knife, one that doesn't open properly. Begin by rinsing thoroughly with soap and water, cleaning off the oils and dirt. Using fresh gloves and acetone (don't NEED gloves at this point, but acetone can be very drying to your skin), remove all traces of nail polish using paper towels, Q-tips, whatever you have handy. Go ahead and reassemble the knives. Check your pivot for that gritty feel, if it's there, take it apart and clean again. Once you have it all cleaned up, I recommend threadlocker (medium/blue?) on all screws, but some will say it's sacrilegious. Adjust your pivot screw to your preferred opening tension, and verified blade is centered. Allow the correct amount of time for the threadlocker to set.

Step 12: Sharpen and Enjoy!

Obviously acid and tumbling will kill your edge, so at this point it will need to be refreshed. You can choose to do this before assembling the knife, entirely up to you. Your bevel should still be solid, so a few quick passes at a lower grit are all that's necessary to break through the finish, followed by higher grits and so forth. I use a Gatco Professional diamond system followed by stropping on canvas, rough leather, and smooth leather (50 laps each) to get that hair popping edge. You can use your preferred method of course. You can protect your new finish with electrical tape if you're using a clamp system like the Gatco, Lansky, Apex, etc.
Finally, it's time to enjoy having the coolest knife on the block! Unless of course you have awesome neighbors who like knives as much as you...