Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/597

575 DYEING as follows: 20 Ib of tin solution, containing about 20 ounces of metallic tin dissolved in nitric acid, with the addition of a little common salt, are used to 100 It) of woollen cloth. Of the 20 lb of mordant, 13 lb are taken and mixed with a solution in water of 8 lb of crude tartar, and about 8 ounces of cochineal are added to enable the dyer to form a judgment of the progress of the mordanting. The ingredients having been boiled for a couple of hours, the cloth is rinsed in clean water and placed in another boiler, containing the residual 7 lb of mordant and 6 tt&amp;gt; of ground cochineal, which are sufficient to dye up a full scarlet colour ; but if the scarlet is required to be very bright, or what is called &quot;fiery&quot; coloured, a further quantity of tartar is added ; this has the effect of somewhat reducing the depth of colour, and at the same time giving it a yellowish or orange hue, which for certain purposes is much desired. Use of yellow in scarlet. It appears that Bancroft, who wrote nbout the end of the last century, was the first to suggest that the bright fiery scarlet, which the dyers found they could best obtain by using a large quantity of tartar, might be produced more cheaply by adding some yellow colouring matter to the cochineal, or by first dyeing the cloth a light yellow ; he tried the yellow from quercitron bark, and succeeded as far perhaps as was possible with that material. At any rate from his time it has been customary for dyers who do not aim at the highest degree of excellence in the scarlet colour to use a purified preparation of quercitron bark, commercially known as flavine, in conjunction with cochineal; other yellow colouring matters, such as fustic and turmeric, are also used. An admixture of these substances cheapens the cost of the colour, which can be made nearly equal in appearance to that obtained with cochineal alone, but it does not stand wear so well, and is more readily stained by various influences. The best scarlets are still dyed exclusively with cochineal. Scarlets on wool from lac-dye. The colouring matter of lac-dye is in its chemical properties and composition very similar to, if not quite identical with that of cochineal. As it is imported into this country from India, it is, however, less pure than average qualities of cochineal; and it is probably on account of its impurities that the dyer cannot obtain quite so good results as the best cochineal colours, although if skilful he may approach them very closely. Having been submitted to a preliminary treatment with acid to free it from alumina and other earthy matters used in its prepara tion, it is then applied exactly in the same way as cochineal. It is extensively used for a second class scarlet, and is believed to be somewhat more durable and stable even than cochineal. The red cloth so much used for military dress is reputed to be prepared mainly with lac-dye. Crimson red on wool. This colour is also dyed with cochineal, but with a mordant of alum instead of tin. It is a far less important colour than the scarlet, and compared with it is dull and flat ; it is, however, rich and durable, and combines excellently with other colours. The mordanting of cloth by means of alum, an operation of capi tal importance for a large series of colours derived from all varieties of dye-stuffs, must now be noticed. Aluming of wool. The method of mordanting with alum, generally called aluming, is practically a simple process, but the chemical principles involved have given rise to much debate amongst experimenters. The aluming is usually performed by boiling the wool for one or two hours in a solution of common alum mixed with tartar ; a certain portion of alumina, or, it may be, of some compound of aluminium, becomes thus intimately combined with the wool, and forms a basis upon which a coloured lake may be produced with solutions of colouring matters. The chemical conditions are somewhat different here from those which obtain in the case of mordanting with tin; for the disposition of tin salts in dilute solutions to decompose even spontaneously is so manifest that it may readily be supposed that some action on the part of the wool takes place which induces the formation of oxide of tin. The great apparent stability of alum caused the explanation of its iction given by Thenard and Roard to be for a long time accepted. They held that it was absorbed whole or unchanged by woolj which retained it by some undefined power, so that it could not be removed by cold water, and required to be heated twenty times with boiling water to dissolve it out. In the light of modern researches this explanation may be safely rejected as erroneous. What appears to be the true state of the case was mainly brought out by experiments of Havrez, suggested by the celebrated Belgian chemist Stas, and supported by further knowledge of the properties of alum discovered by Tichbourne and Naumann. In fact, alum contrary to what was formerly thought, is particularly liable to decomposition, even when not in contact with fibrous matters which might possibly have an influence upon it. Naumann has shown that by simply heating a solution of alum, saturated in the cold to its boiling point, an insoluble basic compound is soon produced so that, after prolonged heating, as much as 25 per cent, of the alumina is precipitated, and the liquid is found to have become id. Beyond this fact it is proved that wool when placed in a .oiution of alum, containing p ure sulphuric acid, has the property of absorbing more acid than alum ; this unequal absorption is attri buted by Havrez to a kind of dialysis, which, together with the ten dency of the alum to decompose, sufficiently explains the deposition of alumina upon the wool. The action of tartar in aluming, accord ing to the same authority, is that of an acid salt, and its addition in fluences the nature of the mordant deposited in the same way ns if an excess of alum were present, or as if other acid bodies, such as sul phuric acid, oxalic acid, &c., were added. The insoluble aluminous compound which separates from solution of alum on prolonged boiling in a glass flask could not act as a mordant, being indifferent or passive to colouring matters ; when deposited on fibrous matter it does not adhere, but can be washed off, or when dry may be shaken off like dust ; this, therefore, is not the alumina mordant, nor do the researches of Havrez really point out what the alumina mordant is, though they are valuable and suggestive as showing under what conditions either a basic or an acid aluminous deposit is formed. With the former, which is unfavourable for dyeing, a blue colour is given with logwood, and a purplish red with Brazil wood; with the latter, the wool dyes up a violet with logwood, and a purer red with Brazil wood. The basic state of the aluming results, it is supposed, from the deposition of hydrate of aluminium upon the wool, and is caused by having too little alum or too much water, by boiling for too long a time, or by the use of salts which have a neutralizing action upon the alum. It is easily induced, when the weight of wool is more than 15 times greater than that of the alum. In other circum stances the acid state results, in which the wool is said to fix first hydrate of alumina, and also hydrated sulphuric acid from the sul phate of alumina. These conclusions of Havrez cannot, however, be accepted as final or satisfactory ; and there is still much to learn upon the principles of aluming and mordanting generally. The wool being successfully alumed acquires a crimson colour by dyeing in cochineal, but, as before stated, this shade is not of much value. The shades of red between scarlet and crimson reds proper, or cherry reds, are also dyed with tin mordant and cochineal in nearly the same way as the scarlet ; but in order to avoid a yellowish tone, the natural cochineal may be mixed with the manufactured or modified material known as ammoniacal cochineal. Ammoniacal cochineal. This is made by treating ground cochi neal with concentrated aqueous ammonia for several days ; the colouring matter undergoes important changes by this process, an amide is formed, and the effect upon the colouring matter is that tin mordants give with it no longer a scarlet, but rather a violet tone. Ammoniacal cochineal is much used in fine dyeing for pinks; and according to the proportion in which it is added to ordinary cochineal, the normal scarlet shade is gradually brought over to the red and even to the crimson. Pink or rose colour upon wool. This shade is obtained from ammoniacal cochineal, mordanting previously in a mixture of tin solution, alum, and tartar ; the quantity of tin mordant used is small, the alum being the essential basis. Other red colours upon ivool. The colours mentioned above are from cochineal or its congener lac-dye ; there are several reds obtain able from other colouring matters, which, though less important, are still worthy of mention. Madder red upon wool. This colour is wanting in brightness, but it is valuable for its stability, and has at times been largely used for common red military cloth. As a basis for browns, chocolates, and other dark colours, it is very suitable when its com paratively high cost is not an objection. To obtain madder red, the wool is boiled for two hours with a mixture of alum, tartar/ and tin salt, 3 lb alum, 1 lb tartar, and 4 ounces of the tin solution being taken for 10 lb of cloth; after boiling, the cloth is rinsed in water to remove uncombined mordant, and then dyed with madder, or preferably its derivative garancin, with addition of a portion of tartar ; the dyeing may be accomplished in an hour, the depth of colour varying with the amount of colouring matter used. Artificial alizarin on wool. By employing artificial alizarin somewhat better shades of colour can be obtained, and even pink colours of much solidity produced. A process for obtaining a fast red on woollen yarn, from alizarin, is as follows : boil 10 lb wool for an hour and a half with 1J lb sulphate of alumina and 4 RJ tartar ; rinse in water, and then dye with 6 to 7 ounces of artificial alizarin paste containing 10 per cent, of dry matter ; commence the dyeing cold, and gradually heat to boiling. Alizarin can be used as a basis for producing fast brown shades, by adding fustic and extract of indigo after the red has been developed, and if necessary, a further quantity of sulphate of alumina and tartar. Red colours can also be obtained by using Brazil wood or other red woods instead of madder ; they are, however, of a low class and sel dom employed. Archil alone, without mordant, can yield a full crim son upon wool, but it is not very stable, and is, moreover, expensive. Aniline reds upon wool. There are several artificial red dye-stuffs, which may be used for wool, but none possesses great excellence. The only one which resembles cochineal in its qualities is the recently discovered cosine ; this, with an alumina mordant, gives upon wool a very good imitation of cochineal scarlet, but an imita-