Newly dug clay is generally wanting in tenacity, and ware made from it is much more liable to crack than if the clay had been "weathered." Weathering, or exposure to the weather, will toughen the clay. The clay, when dug, is laid in heaps and occasionally turned over. The water and oxygen of the atmosphere and the influence of frost disintegrate, wash, and purify it, thus greatly improving its quality. Clay is also toughened by being well worked or kneaded. For modelling purposes there is nothing like old clay - that is, clay that has been repeatedly used; and consequently, when a mould has been made from a clay model, the clay is thrown back into the bin, becoming tougher and more ductile by this continual usage. Clay may also be toughened as follows: Spread out a small lump of it on a board. Mix together a tablespoonful of sulphuric acid and linseed oil, and spot this here and there over the exposed surface. Roll up the clay and well work it together.