CHAPTER IV.
On forming Vaults in Concrete, to be impressed with Enrichment: when the Centerings are to be removed, and how to mix the Plaster.
§ 29. The Construction of enriched Stucco Vaults.
When walls have reached the point where the arches of brick or light stone or tufa have to spring, it is necessary to turn a centering with planks in a close circle, over the framework of struts or boarding. The planks are fitted together according to the form of the vault, or in the shape of a boat, and this centering for the vaults must be fixed with strong props in whatever mode you wish, so that the material above does not strain it by its weight; and afterwards every crevice, in the middle, in the corners, and everywhere, must be firmly stopped up with clay so that when the concrete is spread the mixture shall not filter through. This finished, above that surface of boards they make caissons of wood, which are to be worked contrariwise, with projections where a hollow is wanted; in the same way let the mouldings and details that we wish to make be worked by opposites, so that when the material is cast, it may come, where (the mould is) hollow, in relief; where in relief, hollow, and thus similarly must all the members of the mouldings be arranged. Whether the vault is to be smooth or enriched, it is equally necessary to have shapes of wood, which mould the desired forms in clay; with this clay also are made the square panels for such decoration, and these are joined the one to the other on the flat or by mouldings or enriched bands, which can be made to follow the line of this centering. Having finished covering it all with enrichments of clay, formed in intaglio and fitted together, as was said above, one must then take lime, with pozzolana earth or sand riddled finely, mixed liquid and mostly lime, and of that lay evenly a coating over all, till every mould is full. Afterwards, above this coating make the vault with bricks, raising or lowering them according as the vault turns, and continually adding till the arch be closed. This done, it must all be left to set and get firm, till the work be dry and solid.[[132]] Then when the props are removed and the vault is left free, the clay is easily taken away and all the work remains modelled and worked as if done in stucco, and those parts that have not come out well are gone over with stucco till they are complete. In this manner have been executed all the works in the ancient edifices, which had afterwards stucco enrichment upon them. This the moderns have done to-day in the vaults of St. Peter’s, and many other masters throughout Italy have done the same.
§ 30. Stucco made with Marble Dust.
Now let us show how the stucco is mixed.[[133]] Chips of marble are pounded in a stone mortar; no other lime is used for this stucco save white lime made either of marble chips or of travertine; instead of sand the pounded marble is taken and is sifted finely and kneaded with the lime, in the proportion of two thirds lime to one third pounded marble. The stucco is made coarser or finer, according as one wishes to work coarsely or finely. Enough now of stuccoes because the rest will be said later, when I shall treat of them in connection with Sculpture. Before passing to this subject, we shall speak briefly of fountains which are made for walls and of their various ornaments.