Still Another Formula.—Make a powder of pulverized prussiate of potash, sal-ammoniac and saltpetre in equal parts. Heat the iron to cherry-red and sprinkle thoroughly on all sides with the powder, then immediately plunge into the slack-tub.
Some smiths contend that the pulverized prussiate of potash, used in the same way, is entirely effectual without the other ingredients.
To Chill Cast Iron.—Make a powder by pulverizing together, salt, 2 lbs.; saltpetre, ½ lb.; alum, ½ lb.; ammonia, 4 ozs., and salts of tartar, 4 ozs. Heat the iron to cherry-red, sprinkle thoroughly with the powder and then plunge into cold water.
Another Mode.—Make a solution by dissolving in 10 gallons of soft water, salt, 1 peck; oil vitriol, ½ pint; saltpetre, ½ lb.; prussiate of potash, ¼ lb., and cyanide of potassium, ½ lb. Heat the iron to cherry-red and plunge at once into the cold solution. This makes cast iron hard enough to cut glass, and is the method usually resorted to for hardening the cheap cast-iron glass cutters, now so common on the market.
To Soften Wrought Iron.—Heat the iron with a slow blast to a dark-red, then pour upon the burning coals half a pint of fluoric acid. Keep up the blast gently, without increasing the degree of heat, until all sign of the acid has disappeared, then lay out the iron to cool gradually of itself.
Alloy for Filling Holes in Iron.—Melt together nine parts lead, two parts antimony and one part bismuth. Pour into the hole while in a molten state, or drive in while the iron is somewhat hot. This alloy possesses the peculiarity of expanding as it cools, consequently the plug tightens as its temperature falls.
To Harden Iron for Polishing.—Pulverize and dissolve the following-named articles in one quart of boiling water: blue vitriol, 1 ounce; borax, 1 ounce; prussiate of potash, 1 ounce; charcoal, 1 ounce, and common salt, ½ pint. Add to this 1 gallon raw linseed oil. Having finished up the article ready for polishing, heat it to a cherry-red, and plunge into the mixture; a rapid stirring of the mixture should be going on at the time when the plunge is made.
This preparation hardens the iron to such a degree that it takes and retains polish almost equal to the best of steel.