liquid will enter the boiler through the pipe A. A continual circulation is thus established, which is not only satisfactory as a means of heating, but also keeps up a constant agitation of the contents of the bath, and mixes the impoverished lighter portions with the richer and denser ones. A small quantity of water, or mercury, introduced into the upper branch of the tube S, to prevent the escape of the steam.

is

Large pieces, like cast-iron kitchen utensils, are simply cleansed and rinsed, and dropped into the bath with a few fragments of zinc. It is, however, preferable to use spirals of zinc, since, having fewer points of contact with the articles to be tinned, they lessen the risk of spotting them. On an average, the surface presented by the zinc introduced into the bath should be the thirtieth of that of the tinned articles.

Fig. 85.

If, on the other hand, we wish to tin very small ob jects, such as pins, nails, hooks, etc., we dispose them in layers, say, about an inch thick, upon perforated trays zinc, which allow of the circulation of the liquid, and

of

which are provided with a rim to prevent the objects from falling off. These trays (Fig. 85) are suspended in the bath from chains, one of which carries a numbered tag, by which each one may be identified and removed from the bath in its proper order.

From, time to time, the surfaces of these zinc trays must be scraped and cleaned to remove the white incrustation which forms upon them, and which prevents metallic contact with the articles to be tinned.

The length of the operation may vary from one to three hours. Then all the objects are removed, and 25 parts of pyrophosphate, and as much of fused protochloride of tin, are introduced into the bath..

During the time the solution of these added salts is taking place, the large articles are scratch-brushed, and the small ones moved about with a suitable fork in order to change the points of contact. The objects are then again steeped in the bath for at least two hours. These two immersions, and this length of time, are necessary for good tinning. Finally the large pieces are again scratch-brushed, and the small ones rendered bright by tumbling by one of the methods previously described. The operation is ended by drying them in dry and warm sawdust.

Should it be observed that the tin deposit is gray and dull, although abundant, the bath should be recharged once or twice, with the crystallized protochloride of tin. If, on the contrary, the deposit is very white but blistered and without adherence or thickness, the acid salt must be replaced by the fused one. In this latter case, the proportion of tin salt may be diminished, and that of the pyrophosphate may be increased.

As much of the success of the operation depends upon the quality of the pyrophosphate, we shall indicate, in

the brief treatise on chemistry at the end of this work, the process of obtaining it in satisfactory form.

When this tinning bath has been worked for a long time, the liquid should be decanted to separate the pyrophosphate of zinc that has formed. And when, after several years, the bath is entirely spent, from the profound alteration of its salts, it should be put aside in suitable vats provided for the purpose and called preserving vats, so called because it is customary to temporarily preserve in them, after cleaning, the articles to be tinned.

Zinc is well tinned in a similar bath, but the proportions should be modified as follows:

Distilled water

Pyrophosphate of sodium.

Protochloride of tin (fused)

300 parts.

5

66

1 part.

With this, a thin coating of tin may be obtained by simple immersion, as noticed under that heading, and one of any thickness by the aid of the battery.

F. Weil's process (Chemical News, xiii. 2) consists in preparing a solution of a tin salt in strong alkali (soda or potassa), in which he immerses the articles in contact with zinc. This bath is used hot, and may be used for tinning on copper, and also on iron and steel. In order to obtain a heavier deposit free from zinc, with which that above named is apt to be contaminated, the same inventor recommends placing in the vessel containing the tin solution, a porous cell containing the alkaline lixivium and the metallic zinc. The article to be tinned is suspended in the outer liquid, and connected with the zinc in the porous cell by means of a conducting wire.

Heillier (Moniteur Scient., July 15, 1869) employs a solution made by dissolving 1 part stannous chloride in 10 parts of water, to which he adds a solution of 2 parts caustic soda in 20 of water. The solution is used hot,

the articles to be tinned being placed on a perforated plate of block tin in the hot solution, and moved about with a rod of zinc until sufficiently coated.

Another method of general utility consists in immersing the well-cleansed metallic articles in a bath composed of

Fragments of zinc are then introduced into the bath, and the articles receive a very uniform coating of tin.

To tin articles of zinc, Hess (Winckler, Handb. d. Metallüberzügen) allows them to remain for a quarter to a half hour, in a boiling solution, composed of—

In this bath, some fragments of pure zinc are likewise placed.

Prof. Fz. Stolba (Dingler's Polytechnisches Journal, excviii. 308) uses, for tinning, a 5 or 10 per cent. solution of tin salt (stannous chloride) to which a small pinch of cream of tartar (potass. bitartrate) has been added. The well-cleansed article is first rubbed with this solution, and then with zinc powder; then washed and polished.

Urquhart (Electro-Plating, 207) recommends the operator to prepare a saturated solution of the double chloride of tin and sodium in water, and to dilute this with twice its bulk of water. The articles to be tinned should be placed in a tray or basket of zinc and immersed in this solution, or otherwise immersed in contact with zinc.

CHAPTER XIII.

GILDING GILDING BY SIMPLE IMMERSION-PREPARATION OF SIMPLE IMMERSION GILDING BATHS-COLORING PROCESS.

Gilding.

GILDING, in the arts, may be executed by two very distinct methods: The wet and the dry way.

The first method appears to be the only one needed in this work, but the recent demands of the trade, which often requires the two kinds of gilding upon the same piece, render it necessary for us to give some information upon what are called the "old processes."

Gilding by the wet way is effected either by means of simple chemical reactions, and receives then the name of immersion-gilding; or with the aid of electricity, when it is called gilding by the battery, electro-gilding, or gold electroplating.

Gilding by dipping, or simple immersion, has great analogy with the old processes called gilding by stirring, with the rag, with the cork, etc., according to the mode of operation, all of which result in producing mere films of gold without either firmness or lasting properties.

Electro-gilding, on the contrary, corresponds with the old process of gilding by fire, or mercury. The thickness of the deposit is entirely in the control of the operator; but it must be admitted that, notwithstanding certain advantages possessed by the method, electro-gilding is, even at the present day, far inferior to fire-gilding in respect to firmness, the preservation of the subjacent sur