458

DYEING AND CALICO PRINTING.

properties, hardness, specific gravity, the quantity and nature of the foreignmatters it contains, either naturally present or fraudulently added, all theseconditions have to be studied when a sample of indigo has to be reported on,and the chemical assay of the ware is preceded by a physical examination, forwhich purpose several pieces are taken and attentively examined. First, a9regards the recent fraCture, it should be seen whether the different cakes con-tained in a box are alike, and whether the fragments broken off from one andthe same lump exhibit the same hue. The porosity of the sample is examinedsimply by applying the moist tongue to the recently fractured lump ; thegreater the adherence to the tongue the greater also will be the porosity of theindigo. The coppery lustre of the drug is determined by rubbing it with thenail, and the specific gravity is taken by the well-known method of weighingwith the iooo-grain bottle. By these determinations, aided by practice, someidea of the value of the ware may be obtained, and a great many dyers do notapply any other tests at all; but even the most experienced men are liable toerrors, and it is therefore better that the quantity of indigotin should be esti-mated, but this estimation, however valuable in a sense, is also not sufficientby itself. When the percentage of indigotin in two samples is the same, thepreference should be given to products which are light and soft, and for theapplication to indigo-vats it is best to take the violet-red in preference to thebright blue kinds.

For the quantitative estimation of indigotin many methods have been pro-posed, but some of these are not sufficiently accurate, while others are toocumbrous and lengthy, and hence not available when a large number ofsamples have to be analysed simultaneously. We shall here only quote thereally useful methods, and those which are actually applied or capable ofbeing so. A weighed quantity of indigo is dried at ioo°, and next exhaustedby boiling water, weak acids and alkalies, and alcohol (indigo is soluble tosome extent in the latter liquid when boiling and very concentrated, but is re-deposited on cooling); the residue left after these operations, deducting fromthe weight thus obtained the quantity of ash as determined by the ignition ofa separately weighed sample, is considered to be pure indigotin. The pre-sumption is that this body is insoluble in all the liquids applied, while theforeign matters are withdrawn.

M. Pungh proposes the following method, which, although accurate, takestoo much time, and is hence impracticable where several samples have to beassayed :—30 grms. of previously dried indigo are mixed with lime and sul-phate of iron (see “White Indigo”); the clear liquid is decanted, and thesediment submitted to another reduction process. The joint liquids areoxidised and precipitated by an acid; the indigotin which is thrown down iscollected on counterpoised filters, washed, dried, and weighed. A certainweight of woven tissue or yarn may be also dyed by this process, and thequality of the indigo judged from the intensity of the colour.

M. Mittenzwei* reduces the indigo by means of an alkali and a proto-salt ofiron, pours on the liquid a layer of light petroleum oil, takes—by means of acurved pipette—a known volume of the clear liquid, and introduces this into agraduated tube containing a known measured bulk of oxygen gas standing ina mercurial trough; from the quantity of gas absorbed the quantity of indigotin

* “Journal fur Praktische Chemie,” cxi., 8r.