Each eide of the eteel plate must be equally decarbonated, to prevent it from epringing or warping in hardening. The safest way to heat the plates ie to place them in a vertical poeition. The best steel is preferred to any other for the purpose of me.king plates, and more especially when such plates are intended to be decarbonated. The steel is decarbonated solely to render it sufficiently soft for receiving any impression intended to be made thereon ; it ie, therefore, necessary that, after a piece of steel has been so decarbonated, it ehould, previously to being printed from, be again carbonated, or reconverted into steel capable of being hardened. In order to effect this recarbonization, or reconversion into steel, the following process is employed. A quantity of leather is converted into charcoal, by exposing it to a red heat in an iron retort until most of the vaporizable matt,er is driven off. The charcoal is reduced to a very fine powder ; a box made of cast-iron of sufficient dimensions to receive the plate which is to be reconverted into steel, eo that the intermediate space between the sides of the box and the plate may be about 1 in., is filled with the powdered charcoal. Having covered it with a well-fitting lid, it is placed in a furnace similar to those used for melting brass, wbere the heat is gradually increased until the box is somewhat above a red heat ; it is allowed to remain in that state till all the vaporizable matter is driven off from the charcoal. The lid is removed from the box, and the plate is immersed in the powdered charcoal, taking care to place it so that it may be surrounded on all sides by a stTatum of the powder of a nearly uniform thicknees. The lid being replaced, the box, with the plate, remaine in the degree of heat before described for 3-4 hours, according to the thickness of the plate so exposed ; 3 hours are sufficient for a plate in. thick, and 5 hours when the steel is in. After the plate has been exposed to the fire for the proper length of time, it is taken from the box, and iramediately plunged into cold water. It is found by experience that the plates, when plunged into cold water, are least liable to be warped or bent when they are held in a vertical position, and made to enter the water in the direction of their length. If a piece of steel, heated to a proper degree for hardening, be plunged into water, and suffered to remain there until it becomes cold, it ie very liable to crack or break, and, in many cases, it would be found too hard for the purposes for which it was intended. If the steel cracks, it is spoiled. Therefore, to fit it for use, should it not be broken in hardening, it is the common practice to heat the steel again, in order to reduce or lower its temper. The degree of heat to which it is now exposed determines the future degree of hardness, or temper, and this is indicatdd by a change of colour upon the surface of the steel. During this heating, a euccession of shades is produced, from a very pale straw-colour to a very deep blue. It is found that, on plunging the steel into cold water, and allowing it to remain there no longer than is sufficient to lower the temperature of the steel to the same degree as that to which a piece of hard steel must be raised to temper it in the common way., it not only produces the same degree of hardness in the steel, but, what is of much more import ance, almost entirely does away with the risk of its cracking. The proper temperature arrived at, after being plunged into cold water, can only be learned by actual observation, as the workman must be guided entirely by the kind of hissing noise which the heated steel produces in the water while cooling. From the moment of its first being plunged into the water, the varying sound will be observed ; and it is at a certaiu tone, before the noise ceases, that the effect to be produced is known. As a guide, take a piece of steel which has already been bardened by remaining in the water till cold, and by the common method of again heating it, let it be brought to the pale yellow or straw-colour, which indicates the desired temper of the steel plate to be hardened. By this experiment, as soon as the workman discovers the colour to be produced, he will be able to judge of the precise time at which the steel ehould be taken out. Immediately on withdrawing it from the water, the steel plate must be laid upon or held over a fire, and heated uniformly until its temperature is raised to that degree at which a smoke is perceived to arise from the surface of the steel plate after having been rubbed with tallow ; the steel plate must then be ag,ain plunged into water, and kept there until the sound becomes somewhat weaker than before. It is taken out, heated a eecond time to the same degree as before, a third time plunged into water till the sound becomes again weaker than the last, exposed a third time to the fire as before, and for the last time returned into the water and cooled. After it is cooled, the surface of the eteel plate is
cleaned by heating it over the fire. The temper must be finally reduced by bringing on a brown, or such colour as may suit the purpose required. The engraving is effeoted by graving and etching like copper ; for biting-in, a mixture of 1 part pyroligneous acid, 1 nitric acid, and 3 water is used ; it is run off from the plate in less than a minute; the plate is rinsed in running water, and dried quickly. Stronger acid is used when a deeper tint is required.
.Engraving Steel Cylinders.—A cylinder of very soft or decarbonized steel is made to roll, under great pressure, backward and forward on the hardened engraved plate, till the entire impression from the engraving is seen on the cylinder in alto-relievo. The cylinder is then hardened, and made to roll again backward and forward on a copper or soft steel plate, whereby a perfect fac simile of the original is produced of equal sharpness.
On Stone.—Lithography, or engraving on stone, depends upon the following principles :— (1) The facility with which calcareous stones imbibe water ; (2) the g-reat disposition they have to adhere to resinous and oily substances; (3) and the affinity between oily and resinous substances, and their power of repelling water. Hence, when drawings are made on a polished surface of calcareous stone, with a resinous or oily medium, their adhesion is so great that only mechanical means can effect their separation ; and while the other parts of the stone take up the water poured upon them, the resinow or oily parts repel it. When, therefore, a coloured oily or resinous substance is passed over a stone prepared in this manner, it will adhere to the drawings, but not to those parts of the stone which have been watered.
The Stones.—The stones used in lithography come principally from Germany, but it is said that the Bavarian quarries are exhausted of the best kind, and that the finest stones are now obtained from Bruniquel, Tar-a, and Garonne, in France, and a somewhat inferior quality from Vigan. A bed of lithographic stone 12 yd. thick has recently been discovered at Longinowka, in Galicia.
Stones are prepared for chalk drawings by rubbing two together, with a little silver sand and water between them, taking care to eift the sand from large grains, by which the surface would bc scratched. The upper stone is moved in small circles over the under one, till the surface of each is sufficiently even, when they are washed, and common yellow sand is substituted for the silver sand, end produces a finer grain. They are then again washed clean, and wiped dry. It will be found that the upper stone is always of a finer grain than the under one. To prepare stones for writing or ink drawings, they are rubbed with brown sand, washed, and rubbed with powdered pumice ; the stones are again washed, and each polished separately with a fine piece of pumice, or water Ayr-stone. Chalk can never be used on stones prepared in this manner. The same process is followed in order to clean a stone that has already been used.
The Ink and Chalk.—Besides the inks described on p. 1172, the following may be used:— Tallow, 2 oz.; virgin wax, 2 oz. ; shellac, 2 oz. ; common soap, 2 oz.; lamp-black, oz. The wax and tallow are first put in an iron saucepan with a cover, and heated till they ignite ; whilst they are burning, the soap is thrown in in small pieces, one at a time, taking care that the first is melted before a second is put in. When all the soap is melted, the ingredients are allowed to continue burning till they are reduced one-third in volume. The shellac is now added, and as soon as it is melted, the flame is extinguished. It is often necessary in the course of the operation to extinguish the flame, and take the saucepan from the fire, to prevent the contents from boiling over ; but if any parte are not completely melted, they must be dissolved over the flre without being again ignited. The black is next added. When it is completely mixed, the whole mass is poured out on a marble slab, and a heavy weight is laid upon it to render its texture fine. The utmost care and experience are required in the making both the ink and chalk. Sometimes it la not sufficiently burned, and, when mixed with water, appears slimy : it must then be remelted, and burned a little more. Sometimes it is too much burned, by which the greasy particles aro more or less destroyed ; in this case, it must be remelted, and a little more soap and wax be added. This ink is for writing or pen-drawing ou the stone. The ink for transfers should have a little more wax in it. The chalk consists of 11 oz. common soap, 2 oz. tallow, 2i oz. virgin wax, 1 oz. shellac, oz. lamp-black, mixed in the same way as the ink.