95. The application of potash conjointly with an oxide of iron, is stated by Dr Bancroft to have been practised at Manchester for almost half a century. The iron was usually, but improperly, dissolved in aquafortis, as this acid acts with great energy on the fibres of linen and cotton, and soon destroys their texture.
96. The colours produced by iron, though very dura ble, are liable to be affected by the infusion of tea, arid other substances which contain the gailic acid. The spots occasioned by these may, however, be removed by an oxymu•iate of potash or lime.
97. Dr Bancroft has proposed to employ the oxides of copper as substantive colours, though the advantage to be derived from their use in this respect is extremely doubtful. lie mentions that he had produced acciden tally, and fixed permanently upon calico, a brownish red oxide of copper, nearly resembling the ruby copper ore in colour, which withstood repeated washing with soap, and six weeks exposure to the atmosphere, without al teration. 1 le thinks that this colour may prove useful by simple topical application, in calico priming; but he has not examined it sufficiently to give a decided opinion on the subject.
98. The green colour exhibited by most of the pre parations of copper arises from the absorption of oxy g•n, for which the oxides of this metal have a strong affinity. It does not appear that any of the solutions of copper or its oxides, can be applied simply to linen or mutton, so as to resist the action of soap, though they suffer but little change front exposure to the sun and air. It liqi ud ammonia be saturated with copper, and thick ened with gum, it may, by simple topical application, be fixed upon linen or cotton, when, by an evaporation of :a part of the volatile alkali, and the absorption probably of both oxygen and carbonic acid, its blue colour will be changed into a green resembling that of verdigrease, or rather that of the malachite, w hich will sufficiently re sist the impression of sun and air, and bear a considera ble number of washings with soap. without being affect ed thereby. It may therefore, says Dr Bancroft, be usefully employed in this way, especially upon line mils lins, by reason of the great delicacy of its colour, and the facility of its application.—Perm. Col. i. p. 320.
99. The oxides of various other metals have been pro posed to be used in dyeing; but it does not appear that any of them can be usefully or extensively employed as colouring matters upon cloth.
Of Adjective Colouring Matters.
100. The most important and valuable colouring mat ter belonging to this class, is the coccus cacti of Lin met's, or cochineal. \Vc have already described the methods which are employed for rearing this insect, and shall therefore confine ourselves at present to an account of its qualities as a dye. See CocurNEA r..
101. Two kinds of cochineal arc employed in dyeing: they arc distinguished in Mexico by the Spanish terms grana firm, and grana sylvestra. The first variety, be ing of a larger size, and yielding a greater quantity of colouring matter, is reckoned the most valuable ; the other is smaller, and covered with a downy substance, resembling cotton, which increases its weight, without being of any use in dyeing.
102. Cochineal which has been properly prepared, and carefully kept, is of a greyish colour, inclining to purple. The greyish colour is occasioned by a powder, which covers the insect in its natural state, and part of which still adheres to it : the purple shade is owing to the colouring matter extracted by the hot water in which the insect has been killed. Cochineal retains its pro perties for a very long time if it be properly kept. Ifel lot says, that lie made some experiments with a quantity of it which was 130 years old, and that its properties as a dye were unchanged.
103. The colouring matter of cochineal may be ex tracted by water. The decoction is of a crimson colour, inclining to violet. A little sulphuric acid being added to it, causes the liquor to assume a red colour, inclining to yellow, and forms a small quantity of a beautiful red precipitate. Muriatic acid produces nearly the same ef fect, but no precipitate. A solution of tartar converts the liquor into a yellowish red, and slowly produces a small quantity of a pale red precipitate. The superna tant liquor retains a yellow tinge, which is changed into purple by tire addition of an alkali. The precipitate is quickly dissolved by the alkali, and the solution becomes purple. A solution of alum heightens the colour of the infusion, renders it redder, and produces a crimson pre cipitate. A mixture of alum and tartar produces a bright lively colour, inclining to a yellowish red. A solution of muriate of soda, (common salt), renders the colour somewhat deeper, but does not make the liquor turbid. Muriate of ammonia gives a purple tinge, without any precipitate. Sulphate of soda produces no change. The acetate of lead forms a purple, inclining to violet. Ni trate of lead, a delicate lively colour, between red and cinnamon, but inclining most to the former. The sul phate, nitrate, 'muriate, and acetate of iron, produce a dark violet, and even a full black colour, when they arc used in sufficient quantity. The salts of copper and mercury debase the colouring matter of cochineal. Ni trate of zinc gives a lively bright blue colour, approach ing to purple. Muriate of zinc, a colour like the last, but more of a purplish tinge. The salts of bismuth pro duce indifferent colours ; and the same remark may be applied to the salts of cobalt, nickel, tungsten, antimony, and manganese.