stone buildings; and is performed by diluting sulphuric acid, (oil of vitriol,) with water, and adding fluid-ochres, &c. of the required tint.

When stucco is washed over with this mixture, the affinity existing in the iron of the cement ceases; and the acid and colour suspended in and upon the stucco are fixed. When dexterously managed, the surface assumes the appearance of an ashlar bond of masonry.

Scagliola is a distinct branch of plastering, discovered or invented, and much used in Italy, and thence introduced into France, where it obtained its name: the late Mr. H. Holland, who introduced it into England engaged artists from Paris, some of whom, finding a demand for their labour, remained in this country, and instructed the natives in the art.

Columns and pilasters are executed in this branch of plastering in the following manner: A wooden cradle, composed of thin strips of deal, or other wood, is made to represent the column designed; but about two inches and a half less in diameter than the shaft is intended to be when finished. This cradle is lathed round, as for common plastering, and then covered with a pricking up coat of lime and hair. When this is quite dry, the artists in scagliola commence operations, by imitations of the most rare and precious marbles, with astonishing and delusive effect; indeed, as the imitation takes as high a polish, and feels as cold and hard as the most compact and solid marble, nothing short of actual fracture can possibly discover the counterfeit.

In preparing the scagliola, the workman selects, breaks, and calcines the purest gypsum, and as soon as the largest fragments, in the process of calcination, lose their brilliancy, withdraws the fire, and passes the calcined powder through a very fine sieve, and mixes it, as required for use, with a solution of glue, isinglass, &c. In this solution the colours required in the marble to be imitated are diffused; but when the work is to be of various colours, each colour is prepared separately, and afterwards mingled and combined, nearly in the same manner as a painter mixes on his palette the primitive colours to compose his different tints.

When the powdered gypsum is prepared, it is laid on the, shaft of the intended column, over the pricked-up coat of lime and hair, and is then floated with moulds of wood, made to the requisite size: the artist uses the colours necessary for the imitation during the floating, by which means

they mingle and incorporate with the surface. To obtain the glossy lustre, so much admired in works of marble, the workman rubs the work with one hand with a pumice-stone, while with the other he cleans it with a wet sponge: he next polishes it with tripoli, charcoal, and a piece of fine linen; afterwards with a piece of felt dipped in a mixture of oil and tripoli, and finally completes the work by the application of pure oil. This imitation is, certainly, the most complete that can be conceived; and when the bases and capitals are made of real marble, as is the common practice, the deception is beyond discovery. If not exposed to the weather, it is, in point of durability, little inferior to real marble, retains its lustre full as long, and is not one-eighth of the expense of the cheapest kind.

There is another species of plastering, used in the decorative parts of architecture, and for the frames of pictures, looking-glasses, &c. which is a perfectly distinct branch of the art. This composition, which is very strong, and, when quite dry, of a brownish colour, consists of the proportion of two pounds of powdered whiting, one pound of glue in solution, and half a pound of linseed oil, mixed together, and heated in a copper, and stirred with a spatula, till the whole is incorporated. When cool, it is laid upon a stone, covered with powdered whiting, and beaten till it assumes a tough and firm consistence; after which it is covered with wet cloths, to keep it fresh, till required for use.

The ornaments to be cast in this composition, are modelled in clay, as in common plastering, and afterwards a cameo, or mould, is carved in box-wood. This carving requires to be done with the utmost care, otherwise the symmetry of the ornament which is to be cast from it will be spoiled. The composition, when required for use, is cut with a knife into pieces of the requisite size, and forced into the mould; after which it is put into a press, worked by an iron screw, and still further compressed. When the mould is taken from the press, the composition, which is generally cast about a foot in length, is dislodged from the mould, and the superfluous parts pared off with a knife, and cast into the copper for the next supply.

The ornaments thus formed, are glued upon wooden, or other grounds, or fixed by means of white lead, &c.; after which they are painted or gilt, according to the purposes for which they are intended. This composition is at least 80 per cent. cheaper than carving, and, in most cases, equally calculated to answer all the purposes of the art.

It is much to be wished, that the art of plastering could be restored to its ancient perfection; for the Romans possessed an art of rendering works of this kind much more firm and durable than can be accomplished at the present time.

The specimens of ancient Roman plastering still visible, which have not been injured by force, are found to be firm and solid, free from cracks or crevices, and as smooth and polished on the surface as when first applied. The sides and bottoms of the Roman aqueducts were lined with this plastering, and endured many ages.

At Venice, some of the roofs of houses, and the floors of rooms, are covered with a sort of plaster of later date, and yet strong enough to endure the sun and weather for several ages, without either cracking or spoiling.

The method of making the Venetian composition is not known in England; but such might probably be made by heating the powder of gypsum over a fire, and when boiling, which it will do without the aid of water, or other fluid, mixing it with resin, or pitch, or both together, with common sulphur, and the powder of sea-shells. If these be mixed together, water added to it, and the composition kept on the fire till the instant of its being used, it is not improbable that the secret may be discovered. Oil of turpentine and wax, which are the common ingredients in such cements as are accounted firmest, may also be tried as additions; as also may strong ale wort, which is by some directed to be used instead of water, to make mortar of lime-stone of more than ordinary strength.

SLATING

This branch of ouilding, which is principally employed in the covering of roofs, is not unfrequently combined with. that of plastering. The slates chiefly used in London are brought from the quarries at Bangor, in Caernarvonshire, which supply all parts of the United Kingdom. Another kind of slate, of a pale blue-green colour, is used, and most esteemed, being brought from Kendal, in Westmoreland, called Westmoreland slates. These slates are not large; but of good substance, and well calculated to give a neat appearance to a roof. The Scottish slate, whieh assimilates in size and quality to a slate from Wales, called ladies, is in little repute.

Slaters class the Welsh slates in the following order:

The doubles, are made from fragments of the larger kinds, and derive their name from their diminutive size. Ladies are similarly obtained. Countesses are a gradation above ladies; and duchesses above countesses.

Slate, like most other stony substances, is separated from its bed by the ignition of gunpowder. The blocks, thus obtained, are, by the application of wedges, reduced into layers, called scantlings, from four to nine inches in thickness, and of any required length and breadth, which are afterwards sawn to the respective sizes by machinery. The blue, green, and purple, or darker kinds of slate, are, in general, found capable of being split into very thin laminæ, or sheets; but those of the white or brownish free-stone kind, can seldom be separated or divided so fine; consequently, these last form heavy, strong, thick coverings, proper for buildings in exposed situations, such as barns, stables, and other out-houses.

The instruments used in splitting and cleaning slates are, slate-knives, axes, bars, and wedges; the three first being used to reduce the slates into the required thicknesses, and the last to remove the inequalities from the surface.

Imperial slating is particularly neat, and may be known by having its lower edge sawn; whereas all other slates used for covering are chipped square on their edges only.

Patent slate was first brought into use by Mr. Wyatt, the architect; but a patent was never obtained. It derives its name from the mode adopted to lay it on roofs; it may be laid on a rafter of much less elevation than any other, and is considerably lighter, by reason of the laps being less than is necessary for the common sort of slating. This slating was originally made from Welsh rags; but is now very frequently made from Imperials, which render it lighter, and also somewhat neater in appearance.

Westmoreland slate, from the experiments made by the late Bishop of Landaff, appears to differ little in its natural composition from that obtained from Wales. It must, however

be remarked, that this kind of slate owes its lightness, not so much to any diversity in the component parts of the stone, as to the thinness to which it is reduced by the workmen; consequently, it is not so well calculated to resist violent winds as those which are heavier.

Slates, when brought from the quarry, are not sufficiently square for the slater's use; he therefore picks up and examines the slates separately, and observes which is the strongest and squarest end; then, seating himself, he holds the slate a little slanting upon, and projecting about an inch over, the edge of a small block of wood, which is of the same height as his seat, and cuts away and makes straight one of its edges; then, with a slip of wood, he gauges, and cuts off the other edge parallel to it, and squares the end. The slate is now considered prepared for use, with the exception of perforating through its opposite ends two small holes, for the reception of the nails which are to confine it to the roof. Copper and zinc nails, or iron nails tinned, are considered the best, being less susceptible of oxidation than nails made of bar iron.

Before we proceed further with the operations necessary in the slating of building, we shall give some account of the tools used by this class of artificers.

Slaters' tools are very few, which sometimes are found by the masters, and sometimes by the men. The tool called the saixe, is made of tempered iron, about sixteen inches in length, and two inches in width, somewhat bent at one end, with a handle of wood at the other. This tool is not unlike a large knife, except that it has on its back a projecting piece of iron, about three inches in length, drawn to a sharp point. This tool is used to chip or cut all the slates to the required sizes.

The ripper is also of iron, about the same length as the saixe; it has a very thin blade, about an inch and threequarters wide, tapered somewhat towards the top, where a round head projects over the blade about half an inch on each side: it has also two little round notches in the two internal angles at their intersections. The handle of this tool is raised above the blade by a shoulder, which enables the workman to hold it firm. This instrument is used in repairing old slating, and the application consists in thrusting the blade under the slates, so that the head, which proiects, may catch the nail in the little notch at its intersection, and enable the workman to draw it out. During this