5Having treated of the different sorts of sand, we proceed to an explanation of the nature of lime, which is burnt either from white stone or flint. That which is of a close and hard texture is better for building walls; as that which is more porous is better for plastering. When slaked for making mortar, if pit sand be used, three parts of sand are mixed with one of lime. If river or sea sand be made use of, two parts of sand are given to one of lime, which will be found a proper proportion. If to river or sea sand, potsherds ground and passed through a sieve, in the proportion of one third part, be added, the mortar will be better for use.
2The cause of the mass becoming solid when sand and water are added to the lime, appears to be, that stones, like other bodies, are a compound of elements: those which contain large quantities of air being soft, those which have a great proportion of water being tough, of earth, hard, of fire, brittle. For stones which, when burnt, would make excellent lime, if pounded and mixed with sand, without burning, would neither bind the work together, nor set hard; but having passed through the kiln, and having lost the property of their former tenacity by the action of intense heat, their adhesiveness being exhausted, the pores are left open and inactive. The moisture and air which were in the body of the stone, having, therefore, been extracted and exhausted, the heat being partially retained, when the substance is immersed in water before the heat can be dissipated, it acquires strength by the water rushing into all its pores, effervesces, and at last the heat is excluded.
3Hence, limestone, previous to its burning, is much heavier than it is after having passed through the kiln: for, though equal in bulk, it is known, by the abstraction of the moisture it previously contained, to lose one-third of its weight by the process. The pores of limestone, being thus opened, it more easily takes up the sand mixed with it, and adheres thereto; and hence, in drying, binds the stones together, by which sound work is obtained.