3When arched ceilings are introduced, they must be executed as follows. Parallel ribs are set up, not more than two feet apart: those of cypress are preferable, because fir is soon injured by the rot and age. These ribs being got out to the shape of the curve, they are fixed to the ties of the flooring or roof, as the case may require, with iron nails. The ties should be of wood not liable to injury from rot, nor age nor damp, such as box, juniper, olive, heart of oak, cypress, and the like, common oak always excepted, which, from its liability to warp, causes cracks in the work whereon it is employed.
2The ribs having been fixed, Greek reeds, previously bruised, are tied to them, in the required form, with cords made of the Spanish broom. On the upper side of the arch a composition of lime and sand is to be laid, so that if any water fall from the floor above or from the roof, it may not penetrate. If there be no supply of Greek reeds, the common slender marsh-reeds may be substituted, tied together with string in bundles of appropriate length, but of equal thickness, taking care that the distance from one ligature to another be not more than two feet. These are bound with cord to the ribs, as above directed, and made fast with wooden pins. All the remaining work is to be performed as above described.
3The arches being prepared and interwoven with the reeds, a coat is to be laid on the underside. The sand is afterwards introduced on it, and it is then polished with chalk or marble. After polishing, the cornices are to be run along the springing: they are to be as slender and light as possible; for, when large, they settle by their own weight, and are incapable of sustaining themselves. But little plaster should be used in them, and the stuff should be of uniform quality, such as marble-dust; for the former, by setting quickly, does not allow the work to dry of one consistence. The practice of the antients, in arched ceilings, is also to be avoided; for their cornices are dangerous, from their great projection and consequent weight.
4Some cornices are of plain, others of carved, work. In small private rooms, or where fire or many lights are used, they should be plain, to allow of being more easily cleaned; in summer rooms, and exedræ, where the smoke is in such small quantity that it can do no injury, carved cornices may be used; for white works, from the delicacy of their colour, are always soiled, not only with the smoke of the house itself, but also with that of the neighbouring buildings.
5The cornices being completed, the first coat of the walls is to be laid on as roughly as possible, and, while drying, the sand coat thereon; setting it out, in the direction of the length, by the rule and square; in that of the height, perpendicularly; and in respect of the angles perfectly square; inasmuch as plastering, thus finished, will be proper for the reception of paintings. When the work has dried, a second and afterwards a third coat is laid on. The sounder the sand coat is, the more durable will the work be.
6When, besides the first coat, three sand coats at least have been laid, the coat of marble-dust follows; and this is to be so prepared, that when used, it does not stick to the trowel, but easily comes away from the iron. Whilst the stucco is drying, another thin coat is to be laid on: this is to be well worked and rubbed, and then still another, finer than the last. Thus, with three sand coats, and the same number of marble-dust coats, the walls will be rendered solid, and not liable to cracks or other defects.
7When the work is well beaten, and the under coats made solid, and afterwards well smoothed by the hardness and whiteness of the marble-powder, it throws out the colours mixed therein with great brilliancy. Colours, when used with care on damp stucco, do not fade, but are very durable; because the lime being deprived of its moisture in the kiln, and having become porous and dry, readily imbibes whatever is placed on it. From their different natures the various particles unite in the mixture, and, wherever applied, grow solid; and when dry, the whole seems composed of one body of the same quality.
8Stucco, therefore, when well executed, does not either become dirty, or lose its colour when washed, unless it has been carelessly done, or the colour laid on after the work was dry: if however executed as above directed, it will be strong, brilliant, and of great durability. When only one coat of sand and one of marble-dust are used, it is easily broken, from its thinness; and is not, on that account, capable of acquiring a brilliant appearance.
9As a silver mirror, made from a thin plate, reflects the image confusedly and weakly, whilst from a thick solid plate it takes a high polish, and reflects the image brilliantly and strongly; so plastering, when thin in substance, not only cracks, but soon decays. On the contrary, that which is well covered with plaster and stucco, and closely laid on, when well polished, not only shines, but reflects to the spectators the images falling on it.
10The plasterers of the Greeks thus not only make their work hard, by adhering to the above directions, but, when the plaster is mixed, cause it to be beaten with wooden staves by a great number of men, and use it after this preparation. Hence, some persons, cutting slabs of plaster from the antient walls, use them for tables; and the pieces of plaster so cut out for tables and mirrors, are, of themselves, very beautiful in appearance.
11If stucco be used on timber partitions, which are necessarily constructed with spaces between the upright and cross pieces, and thence, when smeared with clay, liable to swell with the damp, and when dry to shrink, and cause cracks, the following expedient should be used. After the partition has been covered with the clay, reeds, by the side of each other, are to be nailed thereon with bossed nails; and clay having been laid over these, and another layer of reeds nailed on the former, but crossed in their direction, so that one set is nailed upright, and the other horizontally; then, as above described, the sand and marble coats and finishing are to be followed up. The double row of reeds thus crossed on walls prevents all cracks and fissures.