[CHAPTER XX.]
USEFUL NOTES AND TABLES.
Preparation of Lakes.—Brazil-wood Lake.—Cochineal Lake.—Lac Lake.—Madder Lake.—Orange Lake.—Yellow Lake.—Artificial Ultramarine.—Twaddell's Hydrometer.—Dalton's Table showing the proportion of Dry Soda in Leys of Different Densities.—Table of Strength of Caustic Soda Solutions at 59° F.—Table showing the Specific Gravity corresponding with the degrees of Baumé's Hydrometer.—Table of Boiling Points of Alkaline Leys.—Table showing the Quantity of Caustic Soda in Leys of Different Densities.—Table showing the Quantity of Bleaching Liquid at 6° Twaddell required to be added to Weaker Liquor to raise it to the given Strength.—Comparative French and English Thermometer Scales.—Weights and Measures of the Metrical System.—Table of French Weights and Measures.—List of Works relating to Paper Manufacture.
Preparation of Lakes.—These are prepared by either of the following processes:—1. By adding a solution of alum, either alone or partly saturated with carbonate of potassa, to a filtered infusion or decoction of the colouring substance, and after agitation precipitating the mixture with a solution of carbonate of potash ("salt of tartar"). 2. By precipitating a decoction or infusion of the colouring substance made with a weak alkaline ley, by adding a solution of alum. 3. By agitating recently precipitated alumina with a solution of the colouring matter, prepared as before, until the liquid is nearly discoloured, or the alumina acquires a sufficiently dark tint. The first method is usually employed for aciduous solutions of colouring matter, or for those whose tint is injured by alkalies; the second for those that are brightened, or at least uninjured, by alkalies; the third, those colouring matters that have a great affinity for gelatinous alumina, and readily combine with it by mere agitation. By attention to these general rules, lakes may be prepared from almost all animal and vegetable colouring substances that yield their colour to water, many of which will be found to possess great beauty and permanence.
The precise process adapted to each particular substance may be easily ascertained by taking a few drops of its infusion or decoction, and observing the effects of alkalies and acids on the colour.
The quantity of alum or of alumina employed should be nearly sufficient to decolour the dye-liquor, and the quantity of carbonate of potassa should be so proportioned to the alum as to exactly precipitate the alumina, without leaving free or carbonated alkali in the liquid. The first portion of the precipitate has the deepest colour, and the shade gradually becomes paler as the operation proceeds.
A beautiful "tone" of violet, red, and even purple may be communicated to the colouring matter of cochineal by the addition of perchloride of tin; the addition of arseniate of potassa (neutral arsenical salt) in like manner gives shades which may be sought for in vain with alum or alumina. After the lake is precipitated it must be carefully collected, washed with cold distilled water, or the purest rain-water, until it ceases to give out colour.