Rings and such like can be rammed tight with ash free from carbon without any adhesive, and tubular articles can also be filled with ashes in the same way. Only the parts to be hardened should be in contact with the hardening material, and if this is borne in mind, then very satisfactory results are easily obtainable. At the same time, considerable practice is necessary to make an expert operator, and a good many points can only be learned by actual practice.
When the boxes are ready they must be placed in an oven in such a way as to allow the heat to circulate all round them, and when raised to the necessary heat they must be kept steadily at that heat until it is judged that the necessary depth of hardening is secured. Small variations of heat will affect the results obtained, and it is safe to say that hardly any two ovens or operators give exactly the same result in a given time. Differences in the packing material will also cause some differences in the speed of the penetration of the hardening; but it will not otherwise differ much if the operator keeps his wits about him.
Loss of time and increased fuel costs are usually resultant effects of the use of inferior packing material and often increased costs result from the use of badly-arranged ovens or furnaces, especially where the hardening boxes heat unequally.
After the boxes are placed in the oven, they should be raised to the required heat steadily and quickly, and the heat should be tested from time to time - not too often, however - by means of the test wires. The progress of the hardening can also be tested if the wires are of steel, and are quenched in cold water on withdrawal from the boxes. On breaking the wires so treated, the depth of the hardening can be readily ascertained; but this necessarily only refers to a moderate depth of hardening. Large articles to be deeply hardened can only be worked by time, the effect being noted in practical working. Some articles will require packing and firing more than once, and in regard to these experience is the only guide.
When the boxes are opened, the usual thing is to dump the whole of the contents into a rather deep water-bosh, say one a couple of feet deep, and then to extract the hardened articles when they are practically cold, when they should be almost glass-hard on the surface. In some cases running water is used, and various additions are made in other cases - solutions of salt, cyanide, prussiates and other things being used; the chief apparent object being to increase the coldness of the water, the effects of the chemicals on the metal being nearly if not quite ignored.
Some things, and particularly the cyanides of potassium or potassium and iron, appear to affect iron and steel more than is generally thought, greatly increasing the hardness of a thin layer on the surface of the metal, and for this reason should always be used with some articles; but where tempering is to be done this is not necessary.
After the articles are cooled off they should be dried and thoroughly brushed with wire brushes to leave them clean; and if to be tempered, the surface should be brightened. Any fine grinding should be done before tempering; but the final polish should only be put on after the whole of the fire-work has been performed.
Tempering is done in the usual manner, and most articles which are casebardened and tempered should be finally quenched in hot water, covered with a 1/4 in. layer of some fairly thick oil, as this to some extent prevents cracking. At the same time, no hard-and-fast lines can be laid down in the matter, owing to differences in individual practice; but it is as well to remember that casebardened goods differ somewhat from those made.of solid steel. The same tempering colors apply as with solid steel, however, and in this point there is no material diffeience. The real difference is in the fact that, instead of solid steel you have a soft metal coated with hard steel, and that these two jayers do not contract so equally as an article of uniform content.
Coming to the surface hardening of copper and bronze, the surface has to be alloyed with, say, tin; and about the easiest method of doing this is to have a bath of molten tin ready at hand in which to plunge the article to be hardened; and after heating the article to as high a heat as it will stand short of fusion, to plunge and hold it in the tin until of the same temperature, and then after wiping off surplus tin, to allow the article to cool. A second or even a third heating may be necessary in some cases, and a coating of borax or boracic acid may often be of advantage; but usually one heating and plunging will be found enough for practical purposes, this rendering the surface almost too hard to file. In using arsenic the articles are packed with ordinary arsenious acid and subjected to a red heat for some hours, and the same applies to carbonate of manganese; but in general use these things are not so good as tin, according to the writer's experimental work.
Of course, the whole thing depends on the absorptive alloying of the article to be hardened, and in using arsenic or manganese these materials would be in powder and be mixed with bone-ash, or some other inert substance to reduce cost. The heat should in no case exceed 500° Fahr., or bright red; otherwise there is danger of fusion, and this is quite high enough for any absorption process of alloying. Even a lower temperature will be found better in the majority of cases, as copper and tin will alloy at a dull red, as in the case of soldering bits when they become burnt.
The materials needed for iron and steel are either raw or carbonized bone - not bone-ash - leather cuttings, or some specially prepared casehardening material with which to pack the articles; possibly some cyanide or prussiate of potassium, proper boxes and clay luting; a good oven or furnace, and fuel which will maintain a regular heat without serious fluctuations during the time the process lasts. Possibly a mixture of gas-coke and hard furnace-coke broken to the size of eggs would give the most regular heat in some furnaces, while in others hard coal would have to be used, each furnace having its own peculiarities. For small work, gas furnaces answer well: but they usually consume a sufficiently large quantity of gas to become expensive in working, although against this can be placed the entire control of the heat, which is so necessary with fine work.
The waste from the hardening boxes should be collected and dried, as in this will be found partly consumed carbonaceous material, as well as ash, and both of these have their uses; the partly consumed material for use in backing up the new material in packing, and the ash for annealing and for coating and packing parts of articles which have to be left soft. For annealing, many articles, if they are packed with the dead ash in close boxes and kept at a full red heat for some hours, it will be found that if allowed to cool in the ash before opening the boxes, they will be softer and less oxidized than if annealed in the usual way - a point which is often of some advantage with fine work.
In using leather waste, oak and chrome tanned leather should be kept apart if possible, as the resultant hardening would be probably more pronounced with the latter form. Necessarily, if the waste is carbonized before use there can be no selection, and probably the results would be practically the same; but where raw leather is used there is a difference. It must always be remembered that carbon is not the only thing that can be got into iron and steel by absorptive alloying and with very little experiment many persons would be able to get very useful results for particular purposes. At present a good many disadvantageous results occur in places where metal is kept in contact with heated fuel and with gases of different kinds, these results being quite of an unintentional character; but if experimentalists would pay more attention to the subject, in many cases they would find it profitable.
The writer has designedly left the question of ovens or furnaces in this paper, as each one must be designed and constructed to meet the requirements of the particular work which has to be done, and except in large works, probably no general size or shape would be useful. Probably down-draught furnaces are the best, as with these you hold the heat on the floor of the furnace better. But this is a matter on which furnace-builders do not always agree, as each builder is in favor of his own particular ideas on the subject. Anyhow, efficient heating with economy of fuel is the chief thing, and when this is secured little else is to be desired.-" Mechanic and World of Science."