by most chemical agents, and hence has great advantages over the glazing formed of lead, which is apt, by the dissolving of the lead, to communicate a poisonous quality to the liquids contained in the vessel.

Where colors or pictures are to be imprinted on the ware, it must be done, in most cases, previous to the glazing. In China, Europe, and America, if elaborate pieces of workmanship are to be produced, the design, or drawing, is executed beforehand, with the pencil. But in the common figured white ware, the designs are first engraved upon copper, from which an impression is taken, on thin paper, with a color formed of some metallic oxide. This paper is then moistened, applied closely to the unglazed surface, and rubbed in, so that the coloring matter may be absorbed. The paper is

then washed off, leaving the printed figure transferred to the sides of the vessels.*

Chinese porcelain differs from the finer kinds of pottery merely as to its materials, which are two varieties of feldspar, abounding in China, but not found in Europe or in this Country. Materials, however, of nearly the same kind, have been discovered, and porcelain is manufactured in Philadelphia, which, in respect to the elegance of its forms, and the beauty of the designs which are executed upon it, excels that of China. But in point of hardness, durability, transparency, strength, and the permanency of the glaze, the Oriental manufacture has not yet been equalled.

It must be evident to the reader, on the slightest reflection, that the arts described in this and the preceding Chapter, are most intimately allied with chemical science. Their progress has been proportioned to the intelligence and scientific skill which has been applied to them. It is less than seventy-five years, since, in England, the manufacture of pottery was limited to the coarsest kinds of crockery ware, and to small quantities even of these. Like our own country, it was depen

* Transferring, as it is called, is on much the same principle.

dent for such articles on Holland, Saxony, and China. Now, England exports, annually, near forty millions of pieces of earthen ware, to all parts of the world; and many of these are of a very fine quality. Much of this improvement is to be attributed to the increased resources, furnished by machinery and a knowledge of chemistry. Mr. Wedgewood, whose name has already occurred in this Chapter, and who may be said to have created the manufacture of fine pottery in England, not only was himself a man of science, but employed, in his experiments, the constant aid of a distinguished chemist. In his attempts to improve the art, he was not content to grope in the dark, in the hope that he might blunder upon his object; but he sought in the nature of his agents, carefully observed, for rules to guide his experiments and processes. The consequence is seen, in the abundance and beauty of the important fabric with which he enriched and benefited his country. Had he, and his successors in the same kind of manufacture, been satisfied to tread in the old paths, or to adventure improvements without the light of science, they might have left that manufacture where it was fifty years since, and we might now be compelled to use an inferior article, and yet pay for it a vastly higher price.

CHAPTER XII.

COPYING, INCLUDING CASTING, PRINTING, ENGRAVING, ETC.

ONE very important object in the arts is, to produce copies of an article; or, in other words, to produce a great number of articles which are exactly similar in form, size, and finish. To do this by hand would be not only difficult, but impossible. Hence the method of copying, by casts and superficial impressions, has been adopted, an expedient which strikingly resembles machinery, in the facility and cheapness with which it

produces the most finished specimens of workmanship. We have already described the manner in which copies are taken, in the manufacture of some kinds of pottery and glass, and also in the metallurgic arts; printing, engraving, and stamping, furnish us with other familiar examples.

We propose, in this Chapter, to make a few remarks, I. on the patterns, from which the articles are made, and II. on the methods of taking the impressions.

I. The patterns are formed of wood, metal, or plaster, according to the degree of pressure which is to be applied to them, and to the temperature at which the copy must be taken. If great pressure is necessary, plaster would not be sufficiently strong, and hence the pattern must be formed of metal, or, where the pressure is less severe, of wood. So, again, if the material is applied to the pattern, when very hot, the latter must evidently be composed of some substance, not liable, on the one hand, to take fire, nor, on the other, to crack, and fall to pieces. In casting iron, and other metals, the pattern is usually made up in sand, and is, in fact, merely a copy, from another pattern, which has been previously executed in wood or metal. This first pattern, being placed in a mass of soft sand, sufficiently damp to adhere a little, is made, by pressure, to communicate its form, and is then removed. Into the cavities, which it leaves in the sand, is poured the melted metal, which, on cooling, will assume, of course, the same figure with the original pattern. Plaster is generally used where no great pressure is needed in taking the impression, and when the pattern has to be taken directly from the object. Thus, in taking busts of the human face or form, plaster, when moistened, will, from its plastic nature, adapt itself exactly to the object, and is therefore peculiarly fitted for this purpose. It is removed in separate pieces from the face, and then reunited, to form the mould for future copies. In stamping, printing, and engraving, the pattern is almost always formed of metal; since it is better fitted

both to take a high polish, and to bear great pressure. It may be interesting, to be informed of the method pursued in forming engravers' plates and printing types, which, of all patterns, are the most useful.

1. The plates on which the engraver draws his designs are generally metal, though in some cases he employs wood and stone; and hence the two species of engraving, called wood-engraving and lithography. The lines are drawn, in some cases, by mechanical, and in others, by chemical, agents. In mechanical engraving, the artist uses a sharp instrument called a graver, with which he either cuts out the parts which are to appear black or shaded in the picture, or leaves them raised by removing the intermediate parts. In the finest kinds, called line-engraving and stippling, the shaded parts are cut out; in mezzotinto, which resembles painting in India ink, and also in wood cuts, they are raised. In the first of these, (mezzotinto,) the whole surface is roughened with a toothed instrument, called a needle, and the part corresponding to light is then rubbed down. In wood-engraving, the part left light is cut out.

In chemical engraving, the artist, in order to produce lines, avails himself of the chemical affinity which subsists between some of his materials. The most familiar examples of this are, etching and lithography; to which we may now add the art of galvanic engraving. In etching, the engraver having prepared his plate, as in common engraving, proceeds to cover it, throughout its whole surface, with a thin coating of varnish, made of wax, mastic, and asphaltum,-sometimes, of rosin and animal oil. This varnish is blackened by the smoke of a lamp, in order that the operator may see the progress and state of his work. He then takes a needle, and, supporting his hand on a ruler, makes his drawing in the coat of the varnish, being always careful to penetrate to the copper. Having completed his design, he pours a quantity of diluted nitric acid over the plate, which is prevented from es

caping, by a wall of soft wax, surrounding the plate. By the action of the acid, all the lines or points, where the copper is exposed by the strokes of the needle, are bitten in, as the artist terms it; that is, impressions are made, as if by cutting. When the acid has acted for some time, it is poured off, and those parts which are to be most lightly shaded are stopped out (or covered) with varnish, to protect them from further action. New acid is poured for the second shades, which are next stopped out, and so the processes are repeated, alternately, till the piece is finished, and the varnish is cleaned off. Such engravings must evidently be coarse.

In lithography, a smooth slab is selected, of a species of marble, which will imbibe water, so that the ink used in printing will not adhere. On this, the lines to be copied, whether of writing or of a picture, are drawn with a composition, which has the double property of adhering to the stone, and of attaching to itself the printing ink. This composition, which is made of soap, tallow, white wax, shell lac, and lampblack, is used in a liquid form, so as to be applied with a pen, or a delicate brush; or it is solid, and moulded into the form of a crayon, or pencil. The writing or figure on the stone must be reversed; and the beauty of the copy will correspond to the delicacy of this drawing. When the stone is thus prepared, the impression is taken in a manner somewhat similar to that of copperplate printing. The stone being wet with a sponge, the ink is applied over the whole surface, but adheres only to the composition, leaving the intermediate parts perfectly clean a sheet of paper is immediately placed upon it and pressed forcibly, and thus receives a perfect transcript of the figure on the stone.

In galvanic engraving, the shaded lines are raised, being deposited from a solution of copper on a plate of the same metal, by means of galvanic electricity.*

* A description of the process will be found in Appendix, VIII.