‹‹‹ Vitr. 2.8.18 | Table of Contents | Vitr. 2.8.20 ›››
19To judge of such burnt bricks as are fit for the purpose is not at first an easy matter; the only way of ascertaining their goodness is to try them through a summer and winter, and, if they bear out through these undamaged, they may be used. Those which are not made of good clay are soon injured by the frost and rain; hence if unfit to be used in roofs they will be more unfit in walls. Walls built of old tiles are consequently very lasting.
19With regard to burnt brick, nobody can tell offhand whether it is of the best or unfit to use in a wall, because its strength can be tested only after it has been used on a roof and exposed to bad weather and time—then, if it is good it is accepted. If not made of good clay or if not baked sufficiently, it shows itself defective there when exposed to frosts and rime. Brick that will not stand exposure on roofs can never be strong enough to carry its load in a wall. Hence the strongest burnt brick walls are those which are constructed out of old roofing tiles.