the sun, in plaster vessels of an absorbing nature, or in placing it over furnaces, as practised in England, in order to give it the solidity required for its manipulation.

10. But, in the manufacture of pottery, either white or of a superior quality, the preparation of the clay is an operation both delicate and complicated. After having at first merely mixed several kinds of clays, potters have, by degrees, introduced into the paste, various matters which at first sight did not appear to be intended for the purpose. Thus it was that rocky substances, such as flint, quartz, felspar, and granite; and organic matters such as bones, sands, carbonates of lime and baryta, were successively used for the purpose of giving to the wares whiteness, density, or transparency. Now every one of these substances has advantages or defects of a peculiar nature. For instance, flints, quartz, and sand, whilst they give whiteness, destroy the property of cohesion, and cause brittleness in the ware. Felspar and granites, whilst they give transparency, render the ware more apt to lose its form by the action of fire, and so on with the other substances. We may thereby conceive what care and constant study are necessary in the use of those various substances, in order that each one of them may enter into the mixture only in the proportion which is strictly required.

11. For the purpose of rendering useful this category of substances, each of which is without any plasticity, it is necessary to calcine them at first by means of red heat, so as to render them more friable, then they are ground down to a great state of division. The grinding is managed by the help of powerful mills propelled either by water-wheels or steam. The matters are mixed up in a certain quantity of water, and, under the continued action of large blocks of hard stone wheeled on the bottom of the pan, they are reduced to a very fine pap. Each one of the substances is ground down separately. The person who makes up the mixtures has only then to mix together the different slips in the proportion which he considers the best, being careful, however, to take as the basis of the composition a clay, containing alumina in sufficient quantity so that the mixture

may be worked. When the mixture is complete, it is strained off three times through a fine sieve, called lawn, so as to purify it, and is afterwards placed on a furnace, by which the remaining water is evaporated, and the required consistency of the body is attained. The body thus prepared is ready to receive the form intended for it, and this is effected by three different processes.

12. The first consists in making use of the wheel ; and this process, which is the most ancient, is particularly applicable to wares of a round form without any ornament in relief. The wheel is a round platform made of wood, mounted on a pivot, put in motion either by the foot of the workman pressing on another wheel placed lower down and larger, acting like a fly-wheel; or by a strap connected with another vertical wheel put in motion by a winch. With both hands the workman manipulates the lump of clay placed on his platform, the wheel being put in rapid motion; and when he has thus roughly prepared the form with his hands, he completes the exact outline or contour by means of a profile made of horn, copper, or slate. A very short time suffices to transform by these means a shapeless lump of clay into elegant and graceful vessels. The invention of the wheel is traced to the remotest antiquity. The Greeks ascribed it to Talus, son of Didulus, who lived 1200 years before the Christian era, and they valued the discovery so much, that they raised a statue in honour of the inventor, and even struck medals to perpetuate his fame: 500 years later the Scythian Anacharsis presented to the Athenians a wheel much improved. Of all nations the Greeks have shown themselves the most skilful in the management of this mechanism so simple in its action, and their vessels still serve us as models for producing the most beautiful forms.

13. The second process used in giving a form to clay is by moulding. It is practised for the manufacture of wares which are not perfectly round, or of those, which being round, are covered with ornaments in relief, or hollowed out on their surfaces. This operation requires first of all the preparation of a model executed by a sculptor or modeller. A plaster mould is then cast on that model

(the Romans used to make this mould with clay which they lightly baked). When the plaster mould is dry, it is covered over inside with a crust of soft clay; then the workman, holding in his hand a damp sponge, presses in the clay, in order to make it penetrate all the cavities and sinuosities in the mould. As moulds made with plaster absorb water rapidly, it follows that the clay duly pressed down dries up very quickly, and that the ware can be soon taken out of the mould without any injury to its form. In addition to this all ornaments of high relief, such as handles and spouts, are moulded separately, and, before the drying up of the ware, are fixed to it by means of the same clay in a more liquid state, acting here as glue does with objects of a different nature. The compression of soft clay on a hard substance called a mould, constitutes, therefore, the operation of moulding. In England, for all wares of certain dimensions, even when round, moulding is preferred to the use of the wheel, because as moulding favours the drying up of the ware the fabrication proceeds more rapidly, and, moreover, the workman is not occupied with the form, since it is produced by the mould. It follows that, if the economical results are greater, the workman loses something of his importance, on account of the facility with which his art can be acquired.

14. The last process used in giving a form to wares, and which exclusively belongs to modern art, is casting. It consists in pouring the mixture, when still in a liquid state, into moulds made of plaster. The plaster, dried previously, rapidly absorbs the water, and the portion of the mixture which has yielded its water to the mould, forms in the interior a solid crust. When the thickness of this crust is considered sufficient, the remainder of the mixture is thrown away, and the ware is thus moulded by absorption, instead of being moulded by compression. There are certain clay-pastes which can receive a form by means of this process only. The pastes, for instance, which formerly were used in the manufactory at Sèvres, and which, being almost entirely composed of substances previously vitrified, had lost the plastic property necessary for their manipulation.

LESSON V.

POTTERY (continued). FIRING AND GLAZING. "Hath not the potter power over the clay, of the same lump to make one vessel unto honour and another unto dishonour?"

1. EARTHEN vessels prepared by any of these three processes are so brittle, before they are baked, that none but experienced hands can touch them. It is therefore necessary to give them the property of resistance required for their intended use. This resisting property is attained by exposing the vessels to an intense heat in ovens of a peculiar construction. The action of heat on all kind of ware should, however, be understood.

2. We have seen that in order to prepare clays for manipulation, it is necessary to dilute them with a certain quantity of water, and thereby form a paste of a consistency similar to that of the dough prepared by bakers for making bread. When, therefore, a vessel is prepared and exposed to the action of the atmosphere, the water evaporates out of it, and as the moisture escapes, it follows, according to the law in physics, whereby bodies of a similar nature mutually attract each other, that the earthy particles approach each other, and as this effect is simultaneously produced through every portion of the clay, it causes a considerable diminution in the bulk of the ware. When the ware appears dry we must not suppose that it has lost all the water contained in it, because those clays impregnated with a great quantity of alumina retain water for a long time even at a high temperature. The vessels are therefore exposed to a great heat, and the effect of this rapidly disengages the water remaining, connects still more the earthy particles together, and, when these particles are so brought sufficiently near they enter into combination; that is to say, new bodies are formed of a nature different from the heterogeneous elements which entered into the preparation of the paste.

3. There are two very distinct periods in the firing of potteries. In the first, which is the firing at a low temperature, the greatest portion of the water is evapo

rated, and the earthy particles become sufficiently connected with each other to give sound to the wares, but they still remain very porous, and the alumina, not being incorporated into a substance sufficiently vitrified, still absorbs the water with great energy. This porous or absorbing property is a proof that no combination has taken place between the various elements of the paste. 4. In the second period, the firing at a high temperature, vitrification takes place, that is to say, the bases, such as lime, oxide of iron, potash or soda, produce with the silica glass more or less perfect; whilst the alumina gives consistency to the whole substance, and allows the wares to preserve their shape. Vessels thus vitrified are fit to contain liquids without absorbing them, and they belong to the higher class of potteries, such as stone and porcelain wares.

5. To understand the process of firing, let us imagine a round tower furnished at its base with a certain number of feeders whose mouths convey to the interior of the tower the productions of the fire. The tower is completed at its top by a vault pierced with as many holes as may be necessary to create a good draught. In the empty place within the potteries for baking are placed, care being taken to enclose them previously in round earthen boxes called saggers. By this precaution the wares are not in immediate contact with the flame or the ashes, and a great quantity of them can likewise be accumulated in one oven, without injuring by their weight those which are placed below them. The saggers placed inside the oven are piled over one another in the form of columns, of the height of the tower inwardly. When the oven is filled the entrance is bricked in, and heat is produced at first by means of a slow fire, in order that too rapid a disengagement of the water may not cause the wares to crack. Then, as the oven gets warmer, the draught increases, and after a certain time the fire is no longer checked, but as much fuel is allowed as the draught will consume.

6. The duration of the firing and the temperature both depend on the nature of the wares in process of baking. When the required degree of heat has been