ABSTRACTThe formula, % Al2O5= 0.85 ×% SiO2+ 0.85 (100 – 2.15 ×% SiO2) – 5.1, is presented for use in estimating the per cent of All08 of diaspore and burley clays. The silica determination is quick enough and cheap enough to make control testing of carload shipments practicable. The method applies to the lowest‐grade burley as well as the highest‐grade diaspore. The results of 381 actual analyses are presented showing that the method is accurate to 1% in 65% of the tests and to 2%, in 89% of