chemical compounds are used the expected effect is not obtained furthermore it is necessary to follow the order indicated for the three stages and none thereof may be omitted in that case an effect is obtained which is greater than that which might be expected from each of the stages taken separately thus aluminum oxides or hydrates might be used u s pat no 4 063 956 or silica might be precipitated with boron oxide or hydrate u s pat no 4 784 761 but the results obtained are markedly worse a physical mixture of a zinc borate and of a bismuth vanadate pigment treated in a known way with silica u s pat no 3 649 172 might also be prepared in proportions 10 90 to 1 99 the degree of resistance to heat obtained according to the process of the invention is never achieved in this case use may be made as boron source of alkali metal metaborates and tetraborates mainly sodium and potassium tetraborates na 2 b 4 o 7 10h 2 o k 2 b 4 o 7 4h 2 o or na 2 b 4 o 7 5h 2 o and boric acid h 3 bo 3 use may be made as zinc source of soluble zinc salts for example acetate ch 3 coo 2 zn 2h 2 o chloride zncl 2 nitrate zn no 3 2 6h 2 o sulfate znso 4 7h 2 o as well as oxide zno provided that it is dissolved before hand in the.