Hand-Forging.—Two rates of heat figure in this operation. If the object is merely a smoothing of the surface of the iron, the “cherry-red heat,” so-called, is the rate wanted. The work of smoothing is performed by striking lightly and evenly with the hand-hammer, until the desired condition is secured. The same degree of heat is employed where hammer-hardening the iron is one of the objects to be obtained; in this case the blows with the hammer must be heavier than in the case first named.
If the forging is to extend to a material change in the shape of the iron, the rate of heat must be much higher; it must be increased to what smiths call the “white flame heat.” The hammering must be much heavier, of course; if the piece is large a sledge-hammer must be brought into requisition. But the gunsmith will seldom have work heavy enough to demand the aid of an assistant with a sledge-hammer.
Welding.—The “welding” or “sparkling heat” is required in this process, which is a higher degree of heat than either named in the forgoing. In securing this heat, the metal is brought nearly to a state of fusion; which condition is made known by its sparkling, and presenting the appearance of being covered with a glaze, or a fresh coat of varnish. So soon as the two pieces of iron to be welded together have both attained to this necessary degree of heat, they are taken from the fire with the utmost dispatch, the scales or dirt which would hinder their incorporation scraped off, placed in contact at the heated point, and hammered until a union has been effected, and no seam or fissure remains visible. If the first effort fails to unite them sufficiently, they must be reheated and rehammered until the desired end is secured.
The fire for welding should be free from sulphur; and the iron, while heating, should be taken out now and then and sprinkled over at the point of greatest heat with powdered glass, or with powdered borax. A small proportion of sand or powdered clay is sometimes mixed with the borax. These applications tend to prevent the iron from running or burning, and they are supposed to assist the adhesion when the two pieces are brought together in the act of welding.
Hardening Iron by Hammering.—Iron may be hardened to the character of a pretty fair spring metal by simply hammering it thoroughly while in a cold state. Many of the cheap spiral springs in use, as those attached to small bells for the purpose of imparting a vibratory motion, are hardened or stiffened in this way. They are first cut from soft sheet-iron and then hammered into the required hardness. Some heat to a cherry red and hammer to and after entire coldness.
Case-Hardening.—The various parts of gun mountings, such as guards, heel plates, etc., and the different parts of locks, such as hammers, tumblers, triggers and plates, as received by the gunsmith from the manufacturer or dealer in such articles, are generally in the rough or partially finished condition. Many gunsmiths, particularly those in the country, where there is more or less a class of cheap work, finish up these parts with a file and a little hand polishing, and when the work is put together hand it over to the customer. Not only tumblers and triggers, but even sears and tubes are finished up in this manner. As these parts are almost always made of soft iron, the result is they soon wear and have to be repaired.
The gunsmith who does good work will thoroughly case-harden the parts when they are fitted and finished, and by so doing will turn out a really good piece of work that will wear as well as hardened steel. Why the majority of the trade do not case-harden their work cannot very well be explained, unless they are ignorant of the process or do not care to be put to the trouble of doing it. It is true it may be made a tedious job or a quick and easy one.
Some gunsmiths, when such work is finished, heat it red hot, smear it with prussiate of potash or cyanide of potassium, and while hot, plunge it into cold water, letting it chill. This produces a superficially hardened surface that is not “skin deep,” and as soon as this surface becomes abraded will wear away rapidly.
If the case-hardening of the expert manufacturer be examined, it will be observed that the surface of such work has a fine grayish appearance, and in many places mottled with colored tints that are pleasing and beautiful to the mechanical eye. It will be further observed that the hardening is of such depth that it will wear for a long time. In fact it will wear better than hardened steel. The condition of the material is that of a hardened steel surface stretched over and shrunk upon the iron body of the work. It is stronger than steel, for it has the tenacity of iron for its interior. It has the advantage of steel, inasmuch as it may be bent when cold to a limited degree, and when so hardened will not break as readily as steel. This property of bending is not confined to all articles, as they may case-harden entirely through, and then they will be very brittle and easily broken, but by drawing them to temper after hardening, in the same manner as a tool is drawn to temper, they may be of any hardness desired.
A good way to Case-Harden.—The easiest and perhaps the best way to case-harden gun work is to have a number of short pieces of common gas pipe, such as will be adapted to the size or quantity of the work, and have one end of these pieces securely plugged or closed. One way will be to heat the pipe and close by flattening the end with a hammer on the anvil, but it is a “slouchy” way of doing it. A neater way is to have a gas fitter cut a thread in the pipe and then screw in a plug, such as are used to close ends of gas pipe; if such cannot be obtained, drive in a cast-iron plug and upset the end of the pipe so that it will not readily come out. In these pieces of pipe place the work, packing it well with good, fine bone-dust, such as is used by farmers for fertilizing land. Be careful to so pack that the different pieces of work will not touch each other. Stop the open end of the pipe with a cover, but in such a manner as to be readily opened; place the pipe and its contents in a good fire, letting it remain at a red heat for fifteen minutes or more, dependent upon the thickness of the articles or the depth they should be hardened. Remove from the fire and quickly empty the contents of the pipe into a pail of cold water.
If pieces of gas pipe cannot very well be obtained, thimbles from old carriage hubs may be used instead. Plug up the small end, fit a cover to the large end and use as if it were gas pipe. As these thimbles are made of cast iron they will not bear the rough usage nor the heat that wrought iron will withstand. Common cast malleable iron makes the best receptacles to contain work for case-hardening.
Articles of malleable iron and cast iron are as easily case-hardened as wrought iron. A poor quality of steel is benefited by the operation, as the metal imbibes carbon in which it was before deficient.
Material for Case-Hardening.—For case-hardening, bone-dust is the article most readily obtained and it is clean and neat to use; but it will not produce the mottled tints that charred or burned leather will give. The leather may be prepared by cutting up old shoes or boots, putting them in an old pan and setting the mass on fire. Let it burn until it is a charcoal that will readily crumble in pieces by using a little force. Grind this charcoal to a fine powder by pounding in a mortar or by running it through an old coffee or spice mill. Pack the work with the powder, the same as bone-dust. Bone-black may be used the same as bone-dust, but it is not very satisfactory in its results. It is also dirty to use and to have around a shop. Ivory dust will also answer the same purpose as bone-dust. Gun guards, straps, and long pieces of work will become shorter by case-hardening, and it is best not to fit such pieces into the stock until after they are hardened. If it be desired to have a portion of the work left soft and the other parts hardened, securely cover the places to be left soft, with a coating of moist clay, and this will prevent the hardening material from coming in contact, and, consequently, it will have no opportunity to absorb carbon and harden when put in the cold water.
It may also be observed that articles that are case-hardened will not rust so readily as those not so treated.
If the articles be quite thin and there be danger of their cracking by sudden chilling, the water may be warmed a little, or a film of oil may be spread on the water which will tend to prevent a too sudden contraction of the articles while cooling.
If it be desired to have the work present the colors or mottled tints as seen on some kinds of case-hardened gun work, the surface of the work before being put in the receptacles containing the burnt leather, must be nicely polished and then buffed or burnished. The higher the finish the more brilliant will be the colors.
In using prussiate of potash to case-harden, the potash must be finely powdered, the work heated and dipped in, or if the work be large the potash must be spread over it. The work must be hot enough to fuse the potash, and if it become somewhat cold by removing from the fire it must be reheated, removed quickly from the fire and quenched in cold water.
Another way to Case-Harden.—Collect such articles of animal origin as cows’ horns, or hoofs of either cows or horses, or leather trimmings from about the shoe-shops, or old cast off boots or shoes, and burn them until sufficiently charred to admit of being easily pounded into a powder. Having finished up the article to be hardened, ready for the final polish, place it in an iron box, and surround it completely on all sides by a packing of the powder. Pour into the box, until the powder is made moist, a saturated solution of common salt in urine. Next close the box and seal it until airtight, with wet and well-worked clay, then put it into the furnace and blow up gradually until heated to a cherry red. Don’t run the heat any higher, but hold it at that about five minutes, then take out and plunge at once into the slack-tub.
By this means a piece of soft malleable iron is rendered as hard as hardened steel. Some workmen contend that the salt solution is of no particular importance—that just as good results will come of packing in the animal charcoal alone. The iron box, though very convenient when a good deal of case-hardening is to be done, is not an absolute necessity. If the article, surrounded by the animal charcoal, is incased in a ball of stiff and well-worked clay, and then exposed to the proper heat and slacking, the results will be the same as if heated in an iron box.
Another Formula.—In earlier times, when guns were more in use than either agricultural or mechanical implements, and there was a gunsmith’s shop at almost every cross-road, they had a way of case-hardening that was much more simple than either of the foregoing, and yet quite effectual. Scraps of old leather, as cut from old boots or shoes, were tightly wrapped and tied around the piece of iron to be made hard, to the extent of several thicknesses. Around this was placed a layer of sand and salt in equal proportions, to the thickness of half an inch. The sand and salt was dampened with water to make it stick together. A layer of plastic clay, an inch in thickness, was worked around the whole, and the ball, so made, was exposed to heat at about the cherry-red degree, sufficiently long to consume the leather, when it was dropped suddenly into the slack-tub.
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.