One great difficulty you would find if you had much tempering to do, viz., that the articles bend under the operation, some more than others. Try this: Take a thin knitting-needle when the owner is not looking, and run off with it;—it is all in the cause of science! Heat it red-hot, and with a pair of pliers take it up and drop it sidewise in a basin of water. It will bend like a bow. Heat again, straighten it, re-heat, and this time pop it in lengthwise—endwise, point first—I mean (don’t you see that a round needle has no sides, and puts me into a perfect quagmire of difficulty). However, you will understand this, and will find the needle not bent nearly so much as before, but still it is not straight. As I explain most things as I go on, I may as well explain why this bending occurs before I tell you how to straighten your work again. All metals expand with heat, and contract with cold. I am sure I contract terribly in the winter until I have had plenty of hot soup, and hot roast-beef, and plum-pudding; and I know my temper improves, too, when I get expanded and warm. Well now, when you dropped your sister’s knitting-needle all hot on its side into the water, that side contracted before the other, and consequently the needle bent; but when you put it in the water, end on, it was cooled all round at once, and if you could but cool a piece of metal equally all over, inside and out, at once, all parts would shrink equally fast, and the article would remain straight.
But there is, unfortunately, another cause of this bending, which is, that all articles are not of such form that the same quantity of metal is on all sides of the axial line. Take a half-round file, for instance; one side is flat, the other curved, so that taking these two surfaces into consideration, one contains a great deal more metal than the other, and will not cool at the same rate. These articles are far more liable to bend than those whose sides are parallel. Another result of the hot mass being cooled most quickly on the outside is, that cracks are produced in the latter, because, so to speak, the skin is contracted, and can no longer contain all the expanded metal within it. Hence, to make a mandrel for a lathe, it is common to bore it out first, before hardening, to remove this mass of metal, and to allow the water to touch it inside as well as out. Such mandrels seldom crack or bend.
The only way to straighten articles which have warped by hardening, is by what is called hacking or hack-hammering, which is nothing more than hammering the concave or hollow side with the edge of the steel pane of a hammer. This spreads the metal upon the hammered side, and, by expanding it, straightens the tool, for the hollow side, remember, is that which was too much shrunk or contracted. This is not an operation you will have to do, especially if you only harden the extreme points of the drills and little tools you make.
There is another way of hardening, not steel, but iron, called “case hardening,” because it puts a case of steel over the surface of the metal. Obtain a salt called prussiate of potash. It is yellow, like barley-sugar, but is poison. Heat the iron red-hot, and well rub it upon this salt, and then cool it in water. You will find that now a file will not touch it, its surface being as hard as glass. It is carbonised on its exterior, and made into hard steel. This can be done in another way, as gun-locks, snuffers, and many other things are case hardened. They are enclosed in an iron box, with cuttings of leather and bone-dust, and the box is luted about with clay and put in the fire. All the pieces get red-hot, and the leather chars and blackens, and some of it combines as before with the hot iron, and makes it into steel. And our friend oxygen is considerably at a loss in this case to find his way in, or he would make black scales again and spoil the work; or combine with the carbon (or charcoal) and make it into gas. Probably, however, as we shut up a little oxygen with the contents of the box, this change DOES take place, but just as the gas rises the iron seizes it, and holds it fast.
And now, boys, I find it necessary to lay down the pen, which I see has almost run away with me, and written a good many more pages than I at first intended. Since I began to write I have visited the workshops at King’s College, and seen a sight to gladden my eyes. Boys carpentering, boys turning, boys filing; engines of real use, with single and double cylinders, finished, and in course of construction, and all these the work of schoolboys, whose hands and brains are alike engaged in this delightful branch of industry. Let no one, therefore, pretend that boys are not capable of executing good work of this kind in a masterly manner, or that what they do is always child’s-play, or I shall take up the cudgels in their behalf. I have also seen, in the Working-Men’s Exhibition, a very neat little engine, made by a boy only twelve years of age, which makes me hope and believe that the few hints upon wood and metal work which I have here thrown together will neither be unacceptable nor useless to those whom I address in these pages. In this hope I take my leave, and sign myself, with gratification and pride—
The boy mechanic’s faithful friend,
THE AUTHOR.
