The solution it is in when bought as a liquid etchant for copper PC boards is the stock solution. Dilute that 1:3 with water for etching knives.
If buying the crystal in dry form you have to be careful when handling and mixing them. The water you add them to will get very hot ( hot enough to melt a plastic container) and will emit chlorine gas. Done outside with care, this isn't a problem, but you have to know what will happen and prepare for it.
Here is my methods on using the granular form of FeCl:
Approx. 1 pint of water to each pound of FC granules makes the standard stock concentrate.
In use it is diluted 2 to 4 parts water to each part concentrate. I use 3 parts water and one part FeCl stock solution.
I make my stock a little less concentrated. For a gallon of concentrate, , I use 5# dry FC and a gallon of distilled water. Add the powder to the water slowly, outdoors. It will get hot, and give off chlorine. Wear safety gear....especially a face shield. Once mixed and cooled off to room temp, I store it in a heavy plastic jug that used to hold Muriatic acid. Obviously, I have re-labeled the jug.
For etching, I mix one part concentrate with two parts water. I store it in the etching tank. The tank is a 18" length of 3" PVC, with a flange foot plate fitting on the bottom (solvent welded in place), and a pipe cap slipped on and off for a top. These parts are cheap at Home Depot/Lowes/etc. The base flange makes a steady stand. The cap seals snugly. The solution stores in the tank harmlessly. It sits outside in the smithy year round.
There are two small "V" notches on the opposite sides of the tank top lip. This is to set a piece of stainless steel rod across to hang the blades from. Etching is much better if the blades are suspended in roughly the center, and not hung along the sides or resting on the bottom. I hang the blades on a long "S" hook made from 16 gauge stainless wire.
Many folks use a stronger solution than I do. I have found that for the best etch, use a weaker solution. Remove the blade often ,clean it off, return to tank for more etching. A slow long etch done this way is more even and deeper than a shorter etch in a stronger solution. When done, wash well with soap and water rinse then neutralize with ammonia water or TSP ( Windex works, too). Boiling for 30 minutes in water with a teaspoon of baking soda is done by some to "set" the darkened parts of the etch and assure all acid is neutralized. I don't normally do that, but several good makers do.
You can make several tanks for different concentrations, and in different sizes. I have a tall 36X4" tank for etching swords. Label them with "FeCl" and the concentration, e.g., "3:1", or, "STOCK".
Cost is almost nothing to make a storage/etching tank. An 18" tank costs about $10 to make, and lasts forever. They are also easy to pour in and out of.
Refresh the old solution with a little concentrate as it gets exhausted.
