Mechanical an Useful arts. Colouring Matters of Madder.

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Dr. Schunck concludes a long paper, read to the British Association, on this subject, with the following practical deductions: -

Few subjects connected with the arts have raised so much discussion as the nature of the process of Madder-dyeing. The investigation of Robiquet on this subject, instead of clearing it up, seemed to add to its complexity. He considered his alizarin as the substance mainly concerned in the production of madder colours. This has been denied by others, though I think on insufficient grounds. A remarkable discovery in regard to madder-dyeing, was the fact that lime is very essential in this process. It was found that madder, if not grown on calcareous soil, is incapable of producing fast colours, but that if in this case chalk be added to the madder during dyeing, of if calcareous water be employed, the desired effect is produced. This, again, has given rise to endless discussions. It was found by Persoz that the minutest quantity of lime added to alizarin impaired its colouring power during dyeing, and the effect of lime in madder-dyeing apperared to be an inexplicable mystery. I will not enter further into the disputes on this subject, but shall state at once my own views. It seems to me that former investigators have erred in supposing that madder contained only one colouring matter, whereas I think I have proved that there are two, perfectly distinct and definite, alizarin and rubiacin, which perform distinct functions during the process of dyeing. I have found, as I stated above, that of the two colouring matters, alizarin and rubiacin, the former is the only one that is capable of dyeing when in a free state, and further, that the brown precipitate produced by acids in a watery extract of madder contains the whole of these two colouring matters in a free state. If, then a piece of mordanted cloth be dyed with this brown precipitate, after being freed from all excess of acid, the whole effect is produced by the alizarin contained in the brown precipitate. If, however, a small quantity of lime, chalk, soda, or any alkaline base, either caustic or carbonated, be added to the brown precipitate before dyeing, then its power of dyeing is very much increased. In order to prove this, I took six pieces of mordanted cloth, all of the same size. Nos. 1, 2, and 3 were mordanted in the usual way with acetate of alumina, and Nos. 4, 5, and 6 with acetate of iron. Nos. 1 and 4 were dyed with a certain quantity of the brown precipitate; Nos. 2 and 5 with the same quantity of the brown precipitate, to which, however, there had previously been added a very small quantity of lime water; Nos. 3 and 6, lastly, with the same quantity of brown precipitate, and a large excess of lime water. The dyeing was performed each time in the same vessel with the same quantity of water, and for the same length of time. Now I found at the conclusion that No. 2 exhibited a far darker, fuller, and more brilliant shade of red than No. 1, and No. 5 a much more intense purple colour than No. 4, whereas Nos. 3 and 6 showed hardly any colour at all. Now I can offer only one explanation of these differences. When a small quantity of lime is added to the brown precipitate, it combines exclusively with the rubiacin, or is transferred during the process of dyeing exclusively to the rubiacin. The first effect of the dyeing is the combination of the alizarin with the alumina and peroxide of iron of the mordants. These compounds then attract and combine with the lime compound of rubiacin contained in the fluid, by which means a greater intensity of colour is produced. I repeated this experiment with the pure colouring matters. I took two pieces of mordanted cloth of the same size, and dyed the one with pure alizarin, and the other with the same quantity of alizarin to which rubiacin, combined with lime, was added, and I found that the latter was much darker than the former. I therefore conclude that madder colours are always double compounds of alizarin, rubiacin, alumina, and an alkaline base, or of alizarin, rubiacin, peroxide of iron, and an alkaline base.

It follows from this that the maximum of tinctorial power in madder is produced when the alizarin is in a free state, and the rubiacin is in combination with lime or some alkaline base. If an excess of lime be added, then the alizarin also combines with it, and is thus rendered incapable of attaching itself to the alumina and peroxide of iron of the mordants. A slight excess of lime exists in the root when grown on a calcareous soil; for if a quantity of madder which has dyed as much cloth as it is capable of doing, and is seemingly quite exhausted of colouring matter, be treated with sulphuric acid, and the acid be carefully removed by washing, it is found that after being so treated it is capable of again dyeing almost as much mordanted cloth as it did before, - a fact long known in practice. I may state, in addition, that the colours produced by the brown precipitate to which a small quantity of lime has been added, resist the action of soap and acids, &c., to which all madder colours must be subjected in order to heighten them, much better than if no lime had been added. I therefore conclude, that though the possibility in general of dyeing with madder is due to alizarin, the solidity and brilliance of madder colours must be ascribed to rubiacin.

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