4In buildings of rubble work it is of the first importance that the sand be fit for mixing with the lime, and unalloyed with earth. The different sorts are these; black, white, deep red, and bright red. The best of each of these sorts is that which, when rubbed between the fingers, yields a grating sound. That, also, which is earthy, and does not possess the roughness above named, is fit for the purpose, if it merely leave a stain or any particles of earth on a white garment, which can easily be brushed away.
2If there be no sand-pits where it can be dug, river sand or sifted gravel must be used. Even sea sand may be had recourse to, but it dries very slowly; and walls wherein it is used must not be much loaded, unless carried up in small portions at a time. It is not, however, fit for those walls that are to receive vaulting. In plastered walls, built with sea sand, the salt which exudes destroys the plaster;
3but plaster readily adheres to and dries on walls built with new pit sand, and vaulting may safely spring from them. If sand have been dug a long time, and exposed to the sun, the moon, and the rain, it loses its binding quality, and becomes earthy; neither when used does it bind the rubble stones together so as to prevent them sliding on their beds and falling out: nor is it fit to be used in walls where great weights are to be supported. Though pit sand is excellent for mortar, it is unfit for plastering; for being of a rich quality, when added to the lime and straw, its great strength does not suffer it to dry without cracks. The poorness of the river sand, when tempered with beaters, makes the plastering as hard as cement.