MORDANTS.

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of Turkey red), rose or pink, black, violet, and lilac of the highest brilliancy,whether separately or conjointly, consists of the following separate manipu-lations :— (t). Bleaching the calico so as to render it perfedtly white.(2). Printing the mordants dissolved in a proper vehicle, and suitablythickened. (3). Fixation of the mordants—that is to say, the precipitatjonof the oxides—on and in the pores of the textile fibres. (4). Dyeing operation( garancage ). (5). Avivage, or purification of the colours produced in the dye-

beck : this may perhaps be designated as the process of modification of thecolours by the introdudtion thereinto of a certain quantity of a fatty substancefrom the soap, and of oxide of tin from the salt of tin.

A. Bleaching.—We have previously entered into the details of this opera-tion. What is especially required for this purpose is, that the operation shallbe so perfe&ly performed that no foreign substance left in the tissue may, or!the fabric being placed in the dye-beck, absorb so much of the colouringmatter as to render the elimination thereof afterwards either impossibleor impracticable.

B. Printing and Fixing the Mordants.—The operation of printing is apurely mechanical process, performed by highly complex machinery, thedescription of which is entirely beyond the scope of this volume. The sub-stances used as mordants, and the methods whereby they are fixed, requireour particular attention.

Mordants for Red-Pink, Rose-colour.

The substance used for this purpose is either a hydrate or a sub-salt ofalumina. This material has to be intimately united with the fibre.

Alum is composed of sulphate of alumina and sulphate of potassa combinedwith water. There are two kinds of alum used in the arts. The greater partof the alum now met with in commerce, excepting what is yet made in thePapal States near Rome, is ammonia-alum. For the uses to which this salt isapplied it is quite indifferent which of these two kinds is taken : the ammonia-alum is slightly more advantageous in strength. There are some cases whereammonia-alum cannot be applied, and where alum absolutely free fromammonia is required for use. By external appearance there is no possibilityof distinguishing ammonia- from potash-alum; but if the sample is mixed upwith caustic soda or slaked lime, ammonia-alum gives off the well-knownpungent smell of ammonia. Alum in large crystalline lumps is more likely tobe pure than that which is sold in powder, which often contains a small per-centage of mother-liquor in excess, and more frequently traces of iron. Iron isthe only impurity likely to cause trouble in alum : sometimes it is present insuch quantities as to cause serious injury. As a test for iron, logwood liquor,tin&ure of galls, or a solution of ferro- and ferri-cyanide of potassium may beused. M. Persoz recommends, as a reliable test, the addition first of an excessof tartaric acid to the alum, and then caustic potassa in excess, when, a dropof sulphide of ammonium being added, the minutest trace of iron gives ablack colour.

Potash alum (K0,S0 3 +Al 2 0 3 ,3S03-f 24HO) consists of potash, 9-95 percent; alumina, io' 83 ; sulphuric acid, 33'7r ; water, 45-5^ At 60° C. it losesr8 equivalents of water; fuses at g2° in its water of crystallisation, but is

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