The decoction of cochineal is difficult to filter. Paper cannot be used, because the pores are so rapidly stopped up that new filters would be continually required. Fine silk is the most serviceable; it should not be washed with soap—the alkalinity of the small quantity of soap the fabric would retain would affect the colour. The decoction is made in a well-tinned copper pan, all other vessels should be of glass or porcelain, which are most easily cleaned, and great cleanliness is the prime essential to the success of the process. The greatest care is required to prevent the contact of the liquid with iron during the whole process, the smallest trace of this metal would result in a discoloured product. The essentials of the manufacture of carmine on the large scale are that the colouring matter is dissolved in water, and precipitated by the addition of an aluminium salt, generally alum, absolutely free from iron. The more slowly the carmine separates the finer is its colour. It is generally observed that the last portions to precipitate show the brightest shade. This is because the foreign substances occurring with the carminic acid in the decoction are thrown down with the first portions of the precipitate. In the process of Frau Cenette, famous for the beauty of the product, a solution is made from which the whole of the carmine is separated in about three weeks. During this long time the majority of the substances which have been dissolved in the water together with the carminic acid are decomposed; the liquid acquires an unpleasant smell, and is covered with mould. In the author’s opinion so good a product is obtained by this process because the greater part of the impurities is decomposed: the nearer the product approaches the pure compound of carminic acid and alumina, the purer and brighter will be the shade of the carmine. It is known that light has considerable influence on the beauty of carmine. During dull winter days it is quite impossible to produce so fine a product as in summer. Instead of alum a tin solution may be used to precipitate the colouring matter; the shade of the product is different to that of alumina carmine. CENETTE’S METHOD.—1 kilogramme of finely powdered cochineal is boiled with 75 litres of water for two hours; 90 grammes of saltpetre are added, the liquid boiled for three minutes, then 120 grammes of salt of sorrel (acid potassium oxalate) are added and the liquid again boiled for ten minutes. The liquid is then completely clarified by standing, drawn off from the residue by a syphon, and brought into shallow glass dishes which are placed, protected from dust, in a bright light in a uniformly warm place. During several weeks the carmine separates, the last portions being always brighter than the first. The addition of potassium oxalate has the object of assisting the separation of the carmine, for acid salts separate the carmine from solutions; the saltpetre may reasonably be omitted.