MORDANTS .

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clear that the cow-dung owes its good effedt to the fadt that it contains sub-stances capable of masking, or precipitating by means of double decomposition,the mordants which get detached. To the saturating effedt of the alkalinephosphates contained in the dung is due its power of fixing upon the goods’ alarger proportion of the oxides than would be left by a simple cleansing withwater. We should not overlook the utility of the insoluble substances sus-pended in water. These particles play a chemical as well as a mechanicalpart as regards the saturation due to phosphates and alkaline-earthy carbonates,and take up the alumino-ferric precipitates, thereby preventing them frombecoming fixed to the white portions of the tissue.

Since in adtual pradtice substitutes have been found for dung, chiefly of asaline nature,— e.g ., the phosphates of soda and lime, silicate of soda andothers, about which hereafter, all of which possess the power of saturatingany as yet undecomposed mordant, while they also adt as precipitants for anyportions which may become detached,—we have, by analogy, a right to attri-bute to the phosphates contained in the dung an adtive part.

The dung-bath is commonly made up of 30 litres of cow-dung to from 1600to 2000 litres of water : dung from animals fed upon green forage, and espe-cially upon beet-roots, is less useful and advantageous than the dung excretedby animals fed with dry fodder. Powdered chalk is usually added to the cow-dung-bath, especially for acid mordants and heavy patterns. It should benoted that the mordants acquire in the dung-bath a peculiar olive-greencolour. This appears to be a true dyeing process, chlorophyll and some of thebile pigments being deposited. It is difficult to state the number of pieces ofgoods which can be passed through a given bulk of such a bath with goodeffedt. The number depends on the acidity of the mordants and the nature ofthe designs, and therewith varies from 20 to 60. The duration of the immer-sion has to be modified according to the concentration of the mordants andthe kind of thickening used, and these considerations also regulate the tem-perature to be applied. When starch or farina has been used the temperatureshould be higher than when gum is employed. It varies from 45 0 to ioo° C.Some kinds of prints, especially if starch or farina have been used as thick-eners, require two dungings and a washing between. A strong and acidmordant is more difficult to dung and cleanse well than a neutral mordant,and this applies especially for madder work. A too high temperature and atoo large quantity of dung are injurious to weak mordants, and especially torose-reds and pinks.

Great care is required in passing the goods, regularly stretched and withoutfolds or creases, and also as rapidly as possible, through the dung-bath,because from the moment the hot water penetrates the cloth the acetic acidleaves it, and therefore if the immersion took place slowly, and fold by fold,the acid and the uncombined mordant would spread and have time enoughleft to dissolve the alumina which had already combined, the consequence ofwhich would be inequality and deterioration of the tints to be afterwardsbrought out in the dye-beck. After leaving the dung-bath the goods areseveral times thoroughly washed, in order thereby to eliminate the last portionsof the thickeners.

Among the many substitutes for dung we enumerate the following:—thedouble phosphate of soda and lime, the arsenites and arseniates of soda, andthe silicates of soda. These have lattely almost entirely displaced cow-dung.