Harper's new monthly magazine 259, DEC 1871
Among the problems connected with the art of dyeing, one of much importance is the best method of extracting aniline colors from cloths without injuring the fabric, especially those which it is desired to dye anew; and to solve this Dr. Reimann, an eminent authority on these subjects, publishes a detailed paper in his Dyer's Journal.
For this end several methods present themselves, the first referred to being the use of chlorine, which, however, is only applicable to cotton — this agent, whether in the form of gas, or of chloride of lime, being excluded when we have to deal with substances consisting principally or partially of wool. The simplest method of accomplishing the object in this instance consists in digesting the fabrics for a sufficient length of time in alcohol of 90 per cent., which usually completes the decolorization in a short space of time. The same alcohol can be used several times in succession, and can afterward be purified by rectification or redistillation, so as to involve but little loss. The work is best done in a well-covered copper kettle, which is to be set in boiling water. A little hydrochloric acid may be added if the articles are not too delicate, thereby increasing the solubility of the aniline colors.
Still a third method is based upon the fact that all the aniline colors pass into given uncolored combinations when brought into contact with hydrogen. Thus fuchsin red is almest immediately decolorized when hydrogen is developed in its solution — the same taking place with let, blue, and green. This priuciple has long been applied in the socalled etch printing, iu which the aniline colors are extracted in particular parts of the pattern by means of the hydrogen. This is done by laying on a sheet of metallic zinc, with water and the proper sizing. Water consists of hydrogen and oxygen; the metallic zinc takes the oxygen from the water, and the hydrogen developed renders the aniline tints colorless. After this it is only necessary to rinse out the fabric in order to extract they colorless combination
.Again, by saturating the substance to be deprived of its aniline dye with a feeble acid, such as vinegar or muchdiluted hydrochloric acid, and sprinkling the whole with puedered zinc, the color will be removed, especially if the fabric be slightly heated. This process is, however, much too complicated; and instead cif it we can better use liquids which will give off hydrogen, and tints have a reducing influence. Such a liquid we find in the solution of chloride of tin, usually known as the salt of tin. This must be of the very best quality to be efficacious, rind in external appearance should be of a white color, and composed of clear, dry, and tolerably transparent crystals. A solution of such a salt of tin should be placed in a stone vessel and diluted until it can not injure the fabric (about one to two degrees B. of strength), and some leaves of tin-foil placed at the bottom of the vessel. The fabrics, previeusly rendered perfectly free from dirt or grease, are to be placed in the solution and the vessel covered, the whole being then heated by immersion in boiling water.
As soon as the decolorization has been accomplished the cloth is to be taken out and rinsed in clean water, previously warmed. Generally a new fabric should be left in the hot solution from a quarter to half an hour, and the vessel then he set aside to cool; after which the color will be found to have vanished completely.
There still remain instances, however, in which Crell this efficient method does not entirely accomplish its object; and the last resort, which is absolutely certain and never-failing, is to the cyanide of potassium. This, however, is a deadly poison even in a very small quantity, and the utmost precaution must be adopted in using it. The operator must be certain that he has no sore or cut on the hand, fLi contact with the liquid in that case would be extremely dangerous, although while the skin remains perfectly sound no evil effect will be produced by contact. A stone vessel is to be selected, in which a small quantity of cyanide of potassium is to be introduced, and but water poured upon it, so as to make a solution of one-half to one degree B. Care must be taken not to inhale any of the vapor of the solution. The whole is to be stirred well with a long and strong glass rod, and the operation conducted in the open air, so that no harm may result from the condensation of the vapor. Tile fabric in question, previously well cleaned, is now placed itt the vessel, and pushed under the liquid with the glass rod, and the top of the vessel laid on.
It will he advisable to adopt some method to keep the solution warm, such as immersing the stone vessel in a wootleu tub properly supplied with steam or hot water. Should the vessel crack and the liquid leak out, it would in this instance become diluted with the surrounding water, and thus be less dangerous. After a short time the lid should be removed by taking it off at the end if a long handle, allowing the vapors to IlltSS of before the operator comes near. By means of the glass rod the cloth is to be lifted, and if not entirely white, is to be replaced and theprocess continued still longer. When finished the cloth is to be transferred by means of the glass rod to a large vessel containing hot water, and stirred around for a time, then removed and rinsed off. The solution of the cyanide of potassium can be used several times ithout losing its puwer, especially if a solution of sulphate of iron be mined in occasionally, producing a deposit of Berlin blue.
We give only an abstract of the article of Dr. Heimann, referring our reader to the original for further details. Throughout the whole paper injunctions are continually laid upon operators to avoid very carefully the inhaling of the fumes of the solution, or touching it in any way except through the intervention of the glass rod.
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