‹‹‹ Vitr. 7.3.2 | Table of Contents | Vitr. 7.3.4 ›››
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.
3Having thus set the vaultings in their places and interwoven them, apply the rendering coat to their lower surface; then lay on the sand mortar, and afterwards polish it off with the powdered marble. After the vaultings have been polished, set the impost mouldings directly beneath them. These obviously ought to be made extremely slender and delicate, for when they are large, their weight carries them down, and they cannot support themselves. Gypsum should by no means be used in their composition, but powdered marble should be laid on uniformly, lest gypsum, by setting too quickly should keep the work from drying uniformly. We must also beware of the ancients’ scheme for vaultings; for in their mouldings the soffits overhang very heavily, and are dangerous.