The Future.

On the eve of finishing our descriptions of hydroplastic manipulations, let us say that the galvanoplastic art is open to an infinite number of applications which will be developed by the artist from time to time. Thus, by cutting an open pattern upon a sheet of copper, spreading it evenly upon another metallic plate, and dipping the whole into a bath of silver or gold, the empty spaces will be filled with the new metal, and the whole will form a kind of mosaic work which may be very rich and variegated. It is by an analogous process that, after having fastened by a thin coating of varnish precious stones, or other objects, upon a metallic plate, and the whole being steeped in the bath after metallization, it will be possible to set artificially different parts of an artistic production. A pattern may be hollowed out with a graver from a plate of ivory or mother of pearl, and the whole metallized and immersed in the galvanoplastic bath. After the entire surface has become covered with the metallic deposit, it is sufficient to grind and polish it until the reliefs of ivory or mother of pearl reappear, and the metal will form a variety of artistic incrustations. In fact, we may say that there is scarcely any industry in which the galvanoplastic art will not be found serviceable.

CHAPTER LXV.

TINNING BY IMMERSION IN MOLTEN TIN.

THIS method of securing upon metal surfaces, princi pally upon iron, and to a slight extent, also, upon copper, an adherent coating of tin of any desired thickness, is

the principal one employed in the arts for this purpose. The tinning of sheet-iron by this process is an industry of vast importance, and prodigious quantities of tin-plate -as sheet-iron thus coated is termed-are annually manufactured and consumed for a great variety of uses.

The objects sought to be accomplished by the tinning of iron, are, to protect the latter metal from oxidation, tin being very indifferent to atmospheric influences, and also to improve the appearance of the iron, and fit it for many uses for which in its ordinary condition it is quite unsuited.

The art of tinning by immersion in fused tin is a very ancient one. It is supposed to have been practised in Bohemia as early as the thirteenth century, from which country, which was then favored with the possession of rich mines of tin, it was introduced into Saxony in the year 1620. From Saxony, it was carried to Wales in 1670, in which country, in spite of some early vicissitudes, it has ever since flourished.

The question of the protective action of tin as a coating upon iron has some scientific interest.

It is a well-known fact that two dissimilar metals in contact with each other, in the presence of an exciting liquid, will form a galvanic couple; the conditions of the galvanic battery being thus practically established. Under these circumstances, precisely as in the galvanic battery, the electro-positive metal of the combination will be dissolved (corroded, oxidized) while the electro-negative metal will remain unaffected; and when thus coupled, the solvent or corrosive action of the liquid upon the electro-positive metal will be more energetic and rapid, than when it is exposed alone to its action. The energy of the galvanic action set up by two metals thus coupled, will be governed by their relative electro-chemical char

acters. For instance, between two metals standing near to each other in the electro-chemical series, the galvanic action will be exceedingly feeble, but this will progres sively increase, as they recede from each other in the electro-chemical series.

Applying these facts to the case of tin and iron, it will appear that they stand, relatively, considerably apart, and their union under the proper conditions will provide the elements for a galvanic current of some intensity. I such a combination, tin is the electro-negative element. and is therefore protected, and iron is the electro-positive element, and is therefore the metal which is chemically acted upon by the electrolyte or liquid excitant.

When exposed to atmospheric moisture which will play the role of the electrolyte, tinned iron must be perfectly coated with tin, in order that it shall not suffer rapid destruction. These facts are fully known, and the manufacturers spare no care and attention to coat the sheet-iron so perfectly that it shall be without a flaw. Wherever such a flaw exists, or wherever moisture finds its way between the two metals, galvanic action is at once set up, and the points in question become the foci from which the oxidation (or rusting) of the iron extends in all directions, like the continually-widening circles made by a stone cast into a pool of water.

So long, however, as the coating of tin remains intact. the tin by reason of its presence and its indifference to atmospheric influences, effectually protects the underlying iron, but it should not be forgotten that as soon as the continuity of the tin coating is destroyed, the rusting of the iron, when exposed to moisture, proceeds with greater rapidity than if it were uncovered. Where, however, the tin coating has been removed from a considerable space

by the oxidation of the underlying iron, the exposed

surfaces of the latter metal behave to all intents and purposes as though the tin were absent, since it is only at the points of contact of the two metals that the galvanic action is maintained.

The above remarks will serve to explain why tinned vessels, or sheets, exposed to the damp or allowed to re main wet for a comparatively short time, are rapidly eaten through by the rusting of the iron; and also why it is important to completely dry such vessels at once after using them, or to protect with a coating of paint or varnish tinned sheets that are exposed to the weather. The principles involved in the above considerations have, as we shall see, an equally important bearing on the behavior of galvanized (zinced) objects.

As the tinning of sheet-iron is the most important branch of the tinning industry, we shall proceed to describe it in detail.

Tinning Sheet Iron. (Tin-Plate.)-The rolled black plates as they come from the mill, cut up by the shears to proper size for the finished tin-plates, are first "pickled," to clear their surface of the black oxide with which they are covered. For this purpose they are immersed—a large number at a time-in a strong pickle of sulphuric acid contained in a wooden or lead-lined bath heated by steam. To facilitate the action of the acid, a little sawdust is placed between each sheet, and in about twenty minutes the operation will be finished. They are then removed to washing tanks, where they are well rinsed in fresh water.

The pickled plates are next piled up in air-tight iron boxes, capable of containing a large number of them, and transferred to a reverberatory furnace, in which they are annealed by heating to redness for about 10 or 12 hours. The object of this annealing is to bring the plates to the

right condition to take the proper polish or surface in the next operation of rolling. The plates are allowed to remain in the annealing furnace until they have entirely cooled off, when they will be found to have acquired a purplish color from a film of oxide on the surface, and to have become quite soft.

The quality of the finished tin-plates will depend very largely upon the manner in which the succeeding operation of cold-rolling" is done. This consists in passing them through several (usually three) pairs of polished steel rollers, to give them a smooth and even surface before they are tinned. They are next annealed a second time in the same manner as before described for a period of about 6 or 8 hours, to soften and toughen the sheets.

Before they are fit to accept the tin, the annealed plates must again be pickled, to remove the thin skin of oxide that has formed upon them during the last two operations of rolling and annealing. They They are, therefore, immersed again for about ten minutes in a warm pickle of sulphuric acid, but considerably weaker than that used at first. This operation is called "white_pickling," to distinguish it from the first, or "black pickling."

From this pickle, the sheets come out quite clean of scale; they are thereupon washed and scoured with sand, and placed in vats containing fresh water to protect them from further oxidation, until they are taken to the tin bath. We have drawn freely from various authorities in the following description of the apparatus and appliances which are necessary for tinning iron. These are termed a "set of pots," which are placed in brickwork with fireplaces below the ground, the flues from which work into a large open-mouthed chimney or stack, about 45 feet high (one for each set), which serves to carry off the