THICKENERS.

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hooks, c, lie upon four inclined planes fixed on the ring, D. The adlion is asfollows:—On turning d to either hand the plate, b, which is covered with asuitable straining cloth or tissue of very fine wire gauze, is brought close upto the end of the cylinder, and if this is then filled with the colour to bestrained, and the piston, M, worked by the pulley, e, the wheels, f, g, h, i, k, andthe rackwork, L, presses it through the filter. It is possible that an adaptationof the Needam filter press might answer the same purpose.

The subjeft of thickeners has been already discussed on pp. 17, 66, and 75,to which the reader is, therefore, referred. A few additional remarks are,however, needed. Certain thickenings, such as flour and starch, are insolublein cold water, others, like the gums, are soluble in cold water, while certainmixed thickeners are partially soluble only. These distinctions are highlyimportant as to their action. A smaller weight of the insoluble thickeningswill produce as high a degree of tenacity as a larger of the soluble class.The following quantities may be regarded as respectively equivalent:—

Starch

Flour.

Gum tragacanthCalcined farina

.. 20 ozs. per gal.

.. 22 ,, „

10 to 12 „ „

8 to g lbs. ,,

}»

As a rule the smaller the quantity of thickening needed the darker is theresulting colour. What may be called the penetrating power of differentthickeners is also a point of importance. Generally speaking, the more thecolours can be kept to the face side of the cloth, and prevented frompenetrating into the thickness of the tissue, the greater is the economy, andthe finer and clearer the effects produced. Great judgment is required incombining various thickeners together for any desired effect. Combinationsunskilfully made often separate, and in consequence work badly. Chemicalcombination between the thickening on the one hand, and the colours ormordants on the other, is to be avoided as far as possible. It produces loss,and may sometimes totally spoil a colour. Saccharine matters cannot besafely added as thickeners to colours containing metallic salts, since theyhinder the combination which would otherwise take place between the mordantand the metallic colouring matter. Starch, likewise, if used to thicken veryacid colours, may, on standing for a few days, become partially converted intostarch-sugar or glucose, and may then have the effect of masking the mutualreactions of the various ingredients. As an interesting instance of such“masking,” we may mention that a sample of nitrate of iron has been knownto produce no black colouration when mixed with logwood or gall-liquor. Astrong smell of bitter almonds explained the mystery. The nitric acid usedin preparing the nitrate of iron was mixed with a residual product from themanufacture of nitro-benzol, and a small quantity of the latter body effectuallyprevented the reaction of the nitrate of iron and the tannin.

From such instances it will be seen that no collection of receipts, howevervaried, complete, and valuable, will suffice to make a colour-mixer. Extensivechemical knowledge, long experience, a habit of minute and accurate observa-tion, and great fertility in resources are needed.

The styles of calico printing are roughiy grouped under eleven heads.“ Roughly,” we say, since each of these has its subdivisions and its modifi-cations, and since many effects are produced by a combination of processes

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