I've read many articles on various gun forums regarding gun bluing, most often cold bluing but seldom any information on how to properly hot blue. I'll share this information for those that would have a notion they might like to attempt this at some time. Please remember safety and never attempt mixing salts indoors unless you have some extremely good ventilation. Also remember that you should not do this unless you are competent in fully disassembling the firearm to be blued. NEVER PLACE AN ALUMINIUM PART IN BLUING SALTS. Also, post 64 Winchester lever action receivers will not blue without a special activator.
The process is not as difficult as it may sound and with practice, can be done exceptionally well without a lot of equipment costs. The key to any bluing job is the polishing done prior to it being blued or if you prefer a matte finish, it is bead blasted prior to bluing. Preferably, practice using a gun that isn't worth a lot.
USE EYE PROTECTION AT ALL TIMES
Of course you can purchase pre-mixed salts or you can do as myself, make your own. The following recipe is as, if not more, durable than any mfg that I have tried and the color is what you would typically find on older higher end pistols/revolvers.
USE EYE PROTECTION AT ALL TIMES
Salt recipe: 1)Sodium Hydroxide (e.g. lye, caustic soda) can be bought at any chemical supply and some cleaning supply houses. 2) Ammonium nitrate which you can buy at most farm supplies. 3) Distilled water. The mixture is 2:1 sodium hydroxide/ammonia nitrate. For small tanks/pans, 2lbs/1lbs mixture should do it and in a standard long tank, 5lbs/2.5lbs will do. When the two are first mixed and just enough distilled water added to moisten the chemicals, there will be an extreme chemical reaction reaching boiling temp and creating a very powerful ammonia smell that will subside within about 5-10 minutes. However, it is IMPORTANT that when first mixing, you do this outside in a very well ventilated area. After the reaction occurs, the salts will cool and the fumes will be minimum and you then add more distilled water until you have about 2" of water above the salts.
This solution can be used numerous times. Just keep it covered when not in use
Metal (Steel, NOT CAST IRON OR ALUMINIUM) pans large and deep enough to completely submerge part in leaving about 2" from top of pan. For long gun barrels, etc, a bluing tank can be purchased from brownells for reasonable cost. I've used commercial grade stainless steel rectangle shaped pans like you find on a food serving line and they work as good as any and have a nice lip all the way around.
metal wire for suspending parts and small baskets made from steel window screen to place screws etc in.
A heat source that you can control the amount of heat. Gas burner(s) or electric hot plate, old stove, etc
Thermometer that will register to at least 300*f. A good cheap method is to buy a glass candy thermometer.
4x steel wool
Dawn dishwashing detergent.
WD-40 or the cheap Wal-Mart brand will work fine.
Aluminium Oxide automotive grade sand paper in various grits up to 1000 grit.
After disassembly of the weapon to be blued, place all parts such as springs and aluminium parts into a container (zip lock will work) and put aside.
Polishing. Here is where patience comes in. The best bluing jobs are and always will be hand polished. If you choose to do this with a buffing wheel (not to be confused with what you would find on a Dremel tool), I use Felt Wheels (Brownells) and have never seen anything much better than utility grade jobs coming from Muslin wheels. Also, if you use wheels, avoid the lettering, screw/pin holes and sharp edges on the piece being polished. Here's what I recommend. Depending on the condition on the metal, start with coarse Aluminium Oxide paper in the 100 range. Using wood or padded blocks, place the piece in a vice snug enough to hold it, using a shoe shine method, begin polishing the rounds (barrels, top of frame/slide, etc) until it is uniform. Depending on the condition of the metal, you may notice what looks like fine black marks/pits in the metal. If so, continue polishing until the steel is uniform and free of imperfection. Progressively work up using finer grits of paper to polish. When you get to the 500 grit range, you can stop as the finish will at least rival anything on what was once seen on older S&W's. For Python like finish, work up to 1000 grit. For just a good looking finish, stopping in the 300 grit range will be superior to most any weapon sold on the market today.
For the flats, I use a flat table and a smooth marble 18"x18" tile. Lay the paper on the tile and work the piece across the paper keeping it flush and progress to the grit paper you desire.
For a matte finish, plug all chambers and barrel (I use spent cases in the chambers and lead bullets to tap into the ends of barrels: remove before placing in bluing) and use a good grade of masking tape to cover any areas you do not wish to matte. Blast with a fine grade of glass bead until uniform.
Bluing: ALWAYS USE EYE PROTECTION & DO NOT PLACE ALUMINIUM IN SOLUTION, IT WILL DESTROY IT. Keep distilled water handy and add to mixture as needed for evaporation loss. Add it between batches and let solution get back to proper temp before continuing.
Heat needs to be constant. Going above 315*f, 310*f on some, will almost always result in "exotic colors" such as the often seen purple (plum), reds, and greens.
Heat bluing salts to 295*-305*f, stir the mixture with a steel ladel to insure heat is continuous throughout solution. Suspend piece(s) completely submerged in tank for 20 minutes. Remove and place in a pan of water. Using 4X steel wool and dawn detergent, scrub piece completely, examine and if it isn't perfect, rinse and place back in solution for another 20 minutes and clean again with steel wool. After scrubing and the peice is satisfactory, spray with a heavy coat of WD-40, set aside and finish other parts. When finished, for best results, boil all parts in clean water with 1/2 cup baking soda per gallon of water (I know both baking soda and sodium hydroxide are bases, but it works) and while rinsing, use a toothbrush in any crevis. Dry and spray/wipe down with a quality lube, Breakfree CLP works fine. Re-assemble your weapon.