Over the last six years I have been working out a suitable mixture to give a good result on most firearm steels. The following recipe has been developed and has proved very acceptable with all of my clients. Two other smiths have now had equally good results after getting my recipe.

The following are by volume:

AMMONIUM NITRATE 33.3%
SODIUM HYDROXIDE 33.3%
WATER 33.4%

Mixing of the components should be done in the following order and at all times protective goggles, gloves and Ammonia Mask should be worn. (Ammonia Mask Insert can be bought from Safety Equipment Suppliers to fit Standard 3M 6000 series Respirators).

(1) Measure out the water and add to tank then very slowly add the Ammonium Nitrate till the total 33.3% has been added.

(2) Now add the Sodium Hydroxide (Caustic Soda). The adding of this component MUST BE done SLOWLY as the caustic reacts with water and forms a lot of heat and the Ammonium Nitrate gives off Ammonia Gas which may cause it to boil over if the caustic is added too quickly. It may take one hour or more to add this last component to the average forty to fifty litre tank.

Once all the above three are mixed the mixture can be heated and checked for boiling temperature, which should be around 302 deg F. If mixture boils below this temperature add some more caustic soda or if it is boiling too high add some water. (Do this very slowly as it will spit back if added too quickly). After the mixture boils once it gives off no more Ammonia smell.

While the tank is still boiling add one litre of Sodium Nitrite to the average size tank.

Polished steel parts hung in this boiling mixture should colour up in approx five to ten minutes and obtain full colour and density in twenty to thirty minutes.

For people who live in areas that have fluoride in the water supply I suggest that distilled water be used and source of this can be obtained from anyone who has a Dehumidifier.

Sodium Hydroxide (Caustic Soda), Ammonium Nitrate and Sodium Nitrite can be obtained in bulk bags from Chemical Suppliers in most large cities.