Alumina is not affected by sulphuretted hydrogen, and must thus be regarded as the most suitable oxide for the preparation of lakes. It is generally applied in the form of alum, in the selection of which great care is necessary if fine colours are to be produced. Commercial alum frequently contains ferric oxide; when the alumina is precipitated simultaneously with the colouring matter the ferric oxide is also thrown down, and is mixed with the lake, its dark colour influencing the shade of the lake greatly to its disadvantage. The effect of the ferric oxide upon the colour of the lake is so important that it is impossible, for example, to obtain a pale red lake with alum containing iron. To avoid the bad results given by alum containing iron it should be examined before it is used, and if it is found to contain any considerable quantity of iron it should be rejected for this purpose. After alum a solution of stannic chloride is most commonly used to precipitate lakes, but it must be free from stannous chloride; it produces as a rule darker lakes than alum. Care must be taken that the stannic chloride solution is free from iron. The ordinary process for making lakes is very simple: a clear aqueous solution of the colouring matter is obtained; to this alum or stannic chloride solution is added in proportion to the amount of colouring matter dissolved by the water; the metallic oxide is then precipitated by an alkali. Sodium or potassium carbonate or caustic alkalis may be used; ammonia is very suitable for this purpose, since it is free from iron. The precipitant must be cautiously added, it is introduced in drops when the greater part of the colouring matter has been precipitated; an excess of the alkaline solution would have considerable effect on the shade of the precipitate. The precipitated lake rapidly settles to the bottom of the almost colourless liquid; the settling is particularly rapid when stannic chloride has been used. The liquid is then drawn off, the pasty residue brought upon a strainer, washed several times with water, and dried in the air or in stoves. When dry, a properly prepared lake forms a mass of little coherence, and can be readily ground to a soft powder, which may then be ground with oil or gum solution to produce oil or water paints. At the same time the materials are added to the pure lake which are used to shade its colour; white pigments are generally employed for this purpose—the paleness of the shade is proportional to the amount of white pigment introduced. Certain lakes are shaded by small quantities of other colouring matters; for example, the admixture of a small quantity of a blue pigment with a pure red lake produces a much deeper colour inclining to purple. If a small quantity of a red or blue colouring matter is added to a yellow pigment there is produced a deeper yellow inclining to orange, or a yellow inclining to green, in proportion to the quantity of the added colouring matter.