Vitruvius[2412] gives elaborate instructions about the preparation of the clay for sun-dried bricks, and counsels in the first place a careful choice of earth, avoiding that which was sandy or stony or full of loose flints, which made the bricks too heavy, and so liable to split and fall out when affected by rain; it also prevented the straw from binding properly. Clay which was either whitish or decidedly red (from a prevalence of ochre) was preferred, and that combined with coarse sand (_sabulo masculus_) made light tiles, easily set. The process of manufacture was a very simple one. The clay was first carefully cleaned of foreign bodies, and then moistened with water and kneaded with straw. It was then moulded by hand or in a mould or frame of four boards, and perhaps also pressed with the foot.[2413] The bricks were then dried in the sun and turned as required, the usual process also adopted in the modern brickfield. Some bricks actually bear the marks of the feet of animals and birds which had passed over them while the clay was soft, and there is one in the Shrewsbury Museum with the imprint of a goat’s feet. Others at York and Wiesbaden show the nails of a boy’s shoes.[2414] These impressions of feet (where human) may also be referred to the practice of using the feet to knead the bricks. The bricks were then ready for use, but were kept for two years before being employed, otherwise they were liable to contract, which caused the stucco to break off and the walls to collapse. At Utica, Vitruvius tells us, they had to be kept five years, and then could only be used if passed by a magistrate. Altogether, much care was taken in their preparation, and it was generally considered that spring and autumn were the most favourable times for making them, probably because they dried more slowly and were less liable to crack during the operation. In summer the hot sun baked the outer surface too fast, and this appeared dry while the interior was still moist, so that when the inside dried the outside contracted and split.[2415] It was also, of course, advisable to avoid seasons of rain and frost. But the bricks could not be properly tested until they had undergone some exposure to the weather, and for this reason Vitruvius recommends the employment of old roof-tiles where possible in building walls.[2416]