For a quantitative determination, a small quantity--usually 1 grm.--is weighed out, treated with a corresponding amount of hydrochloric acid, and the solution filtered into a glass. The residue on the filter is washed with distilled water, the washings being united to the acid solution. This solution is treated with ammonia so long as a precipitate of ferric hydroxide continues to form, this being collected on a tared filter and dried at 110° C. The precipitate may be regarded as pure ferric hydroxide, and its weight will indicate the proportion of hydroxide in the ochre with sufficient accuracy for technical purposes. In reality, however, it is not pure ferric hydroxide, but contains in addition all the oxides that are precipitable by ammonia, lime being always carried down as well. It is therefore desirable to dissolve the precipitate with a little hydrochloric acid, and reprecipitate with ammonia. CALCINING (BURNING) OCHRE In many places ochre is only put through a very simple mechanical preparation before being sold for pigment, namely left to dry in the air so that most of the uncombined water evaporates. No matter how this drying process is protracted, however, it is impossible to get rid of all the water in this way, a certain proportion being retained by the hygroscopic action of the ferric hydroxide, and to expel this the mass must be heated to above 100° C. Drying is usually succeeded by pulverising and sifting the loose earthy mass, which is then ready for sale. When the ochre contains sand or stones, this treatment is not sufficient, and levigation is necessary. No particular trouble is involved, the mineral being fairly heavy as the result of its content of ferric hydroxide. A simple method of treatment suffices to improve the value of the ochre considerably, and enables a grade that is not particularly bright-coloured in its natural condition to be converted into products of very handsome tone and various shades. This treatment consists in heating the raw ochre to a definite temperature, during which process the colour changes progressively, and any desired tone can be obtained by suddenly cooling the hot mass. The reason for this phenomenon is that the higher the temperature, the larger the amount of water driven off from the ferric hydroxide, until finally, when a very high temperature has been reached, the whole of the water is expelled, and the ferric hydroxide is transformed into ferric oxide. The hydroxide is brown, whereas the oxide, provided the temperature has not been raised too high, exhibits the characteristic colour known as “iron red.”