According to Wolff and A. Strecker the composition of these two substances is, Alizarine C H 6 0 6 Purpurine C is II 6 0 6 Purpurine furnishes, like alizarine, all the colors produced by madder. It may be separated from alizarine by a boiling solution of alum, in which it dissolves : with potash it gives a currant red solu tion, while alizarine gives a blue by re flected, and a purple by transmitted light. Both are converted by nitric acids into phtalic and oxalic acids. When madder is fermented by yeast only purpurine can be detected. The alizarine being con verted into that substance with the evo lution of carbonic acid and hydrogen. Schunck has demonstrated the presence of a bitter substance called by him it is not a coloring matter itself, but is in some way connected with the production of the coloring principles. It is a hard, dry, brittle, shining substance, like Finn soluble in water and alchohol. IN hen acted on by sulphuric and hydrodoric acids the products of the decomposition are :-1, alizarine; 2, verantine ; 8, ru biretine ; 4, rubianlue. Schanck believes that alizarine and verantine form with alumina a compound soluble in boiling water, which is identical with purpurine. It is a curious fact that madder grown on ground deficient in lime will not give much coloring matter, but when the ground has been well limed an abundance of red coloring matter is produced.
By digesting powdered madder in water, and acting upon the jelly like solution thus obtained, by boiling al cohol, an extract is afforded, which, at a subliming heat, yields the proper red coloring matter, or alizarine. Or ground madder may be treated direct ly with boiling alcohol; and to the solu tion dilute sulphuric acid is added, which precipitates the alizarin° in a copious orange precipitate. Another principle, xantdine, is obtained from a fawn-yellow matter, soluble in alcohol and water, by precipitation with oxide of lead, washing the precipitate with alcohol, and extract ing the color with sulphuric acid. It has an orange green tint, and a saccha rine taste. It is believed that xanthine prevails in the rose colored tints of mad der, and is absent in the violet.
The red mordants are prepared com monly in Alsace, as follows :—The crush ed alum and acetate of lead being weighed, the former is put into a deep tub, and dissolved by adding a proper quantity of hot water, when about one tenth of its weight of soda crystals is in troduced to saturate the excess of acid in the alum. The acetate of lead is now mixed in ; and as this salt dissolves very quickly, the reaction takes place almost instantly. Care must be taken to stir it for an hour. The vessel should not be covered, lest its contents should cool too slowly.
Much mordant should not be prepared at once, for sooner or later it will deposit some sub-acetate of alumina. This de composition takes place even in corked vials in the cold ; and the precipitate does not readily dissolve again in acetic acid. All practical men know that certain alu minous mordants are decomposed by heating them, and restored on cooling. Gay LUSSRC observed, that by adding to pure acetate of alumina, some alum or sulphate of potash, the mixture acquires the property of forming a precipitate with a heat approaching the boiling point, and of re-dissolving on cooling. The pre
cipitate is alumina nearly pure, accord ing to M. Gay Lussac ; but, by M. Kcech lin's more recent researches, it is shown to be sub-sulphate of alumina, containing eight times as much base as the neutral sulphate.
Madder dye.—On account of the feeble solubility of its coloring matter in water, we cannot dye with its decoction ; but we must boil the dye-stuff along with the goods to be dyed ; whereby the water dissolves fresh portions of the dye, and imparts it in succession to the textile fibres. In dyeing with madder, we must endeavor to fix as little of the dun matter as possible upon the cloth.
Dyeing on wool.—Alumed wool takes, in the madder bath, a red color, which is not so bright as cochineal red, but it is faster ; and as it is far cheaper, it is much used in England to dye soldiers' cloth. A mordant of alum and tartar is employ ed ; the bath of madder, at the rate of from 8 to 16 ounces for the pound of cloth, is heated to such a degree that we can just hold our hand in it, and the goods are then dyed by the wince, with out heating the bath more till the color ing matter be fixed. Vitalis prescribes as a mordant, one fourth of alum, and one sixteenth of tartar; and for dyeing, one third of madder, with the addition of a 24th of solution of tin diluted with its weight of water. He raises the tempera ture in the space of an hour to and afterwards he boils for 3 or 4 minutes ; a circumstance which is believed to con tribute to the fixation of the color. The bath, after dyeing, appears much loaded with yellow matter, because this has less affinity for the alum mordant than the red. Sometimes a little archil is added to the madder, to give the dye a pink tinge; but this is fugitive.
Silk is seldom dyed with madder, be cause cochineal affords brighter tints.
Dyeing on cotton and linen.—The most brilliant and fastest madder red is the Turkey or Adrianople. The common madder reds are given in the following way :—The yarn or cloth is boiled in a weak alkaline bath, washed dried and galled, by steeping the cotton in a decoc tion of bruised galls or of sumach. After drying, it is twice alumed ; for which purpose, for every four parts of the goods, one part of the alum is taken, mixed with 1-16th of its weight of chalk. The goods are dipped into a warm solution of the alum, wrung out, dried, and alumed afresh, with half the quantity. The ace tate of alumina mordant, described above, answers much better than com mon alum for cotton. After the goods are dried and rinsed, they are passed through the dye-bath, which is formed of lb. of good madder for every pound of cotton ; and it is raised to the boiling point by degrees, in the space of 50 or 60 minutes. Whenever the ebullition has continued a few minutes, the goods must be removed, washed slightly, and dyed a second time in the same way, with as much madder. They are then washed and passed through a warm soap bath, which removes the dun coloring matter. Holterhoff prescribes for ordinary madder red the following proportions :— 20 pounds of cotton yarn ; 14 pounds of Dutch madder ; 8 pounds of nut-galls ; 5 pounds of alum ; to which lpound of acetate of lead has been first added, and then a quarter of a pound of chalk.