The catalytic acids include phosphoric acid, sulphuric acid, alumin-
ium chloride and bromide, and various fluorides. These are
occasionally used with clay catalysts. The activated clays have a cer-
tain amount of residual acidity, mainly as aluminium and iron
sulphate, or, in Germany, chloride; some clay catalysts in Germany
have been activated with fluoric acid. The activation process lends
itself to a great variety of products, for not only can the amount of
acid be varied, but its concentration, the time and temperature of
digesting, and the extent of lixiviation; apart from the choice of
acid or acids, there are also the selection and pretreatments of
the clay.