If, however, these solutions were mixed together in a crude state, the resulting product would be of only low value as a pigment, being of a yellow tinge and never pure white. This is due to the fact that the impure lime salts, being waste products from chemical works, frequently contain fairly large amounts of ferric oxide, and the soda also is often so high in that impurity that the colour of the precipitated chalk is considerably impaired.
Fortunately, there is no difficulty in eliminating this ferric oxide by chemical means, and obtaining a product of superior colour to the best native chalk. This is effected by treating the perfectly neutral lime-salt solution with calcium carbonate, which causes the precipitation of the iron, a corresponding amount of lime passing into solution.
In order to eliminate the ferric oxide from the lime-salt solution so completely that not even the most delicate chemical test known will be able to reveal any trace remaining, the solution is placed in a vat and stirred up with finely powdered chalk. If the solution contains any free acid, effervescence, due to the liberation of carbon dioxide, will take place; and in such event the addition of chalk is continued until the free acid is all neutralised, and the added chalk sinks to the bottom undissolved. The chalk should be in slight excess, so that a decided sediment is visible at the bottom of the liquid when at rest.
This deposit is stirred up again at intervals with the liquid for several days. When ferric oxide is present, the colour of the deposit will gradually change to a yellowish brown, through the precipitation of ferric hydroxide by the chalk; and in this way the final traces of iron can be removed.
The liquid is then carefully drawn off, without disturbing the sediment, and the soda solution is run in so long as a precipitate of calcium carbonate continues to form. The completion of the reaction can be ascertained by pouring a small quantity of the liquid into a tall, narrow glass, leaving it to clarify, adding a little more soda solution and observing whether any further precipitate is produced. On the other hand, it may be that an excess of soda has already been added in the precipitating tank; and this can be determined by testing a sample with turmeric paper—blotting-paper soaked in a solution of the colouring-matter of turmeric root—which is turned brown by alkaline reagents. Even in very dilute solution, soda will give this colour change, and the test is therefore very accurate. The complete precipitation of the lime in the solution can be ascertained by passing a small quantity through blotting-paper and treating it with a little acid potassium oxalate solution, which, if lime be present, will at once produce a strong crystalline precipitate of calcium oxalate, which is only very sparingly soluble in water. If the oxalate gives merely a slight turbidity, the residual amount of lime is so small that the process may be regarded as complete.
Since carbonate of soda is usually much dearer than the lime-salt liquor, it is preferable to leave a small quantity of the lime unprecipitated. Given sufficient care in effecting the precipitation, and especially when fairly strong solutions are used, a brilliant white precipitate of calcium carbonate is obtained, which is in such a finely divided state that the minute constituent crystals can only be detected under a high magnifying power.
This precipitated chalk being already in an extremely fine condition needs no further preparation, and, when washed, is ready for immediate use, forming a handsome pigment with excellent covering power.
When precipitation is ended, the deposit is allowed to settle down, and the clear supernatant liquid is carefully drawn off so as not to disturb the delicate sediment, which is then stirred up thoroughly with clean water, left to subside, washed again, and then spread out to dry on cloths which are suspended by the four sides. The surplus water drains away and the residue gradually assumes the consistency of paste, in which condition it can easily be moulded to any desired shape. If left long enough to dry completely, it forms a very delicate powder, furnishing a pigment of excellent quality.
If this precipitated chalk be moulded into prisms for sale, the blocks are laid on one of their broad sides until firm enough to turn over on to one of the narrow faces, slabs of gypsum being used as the supporting material, in order to ensure uniform drying. The gypsum absorbs water with avidity and thus dries the prisms evenly.
A defect of these prisms is their great fragility; but their strength may be improved by mixing a little very weak solution of dextrin to the mass after the last washing-water has been completely removed. In drying, the dextrin binds the material of the prisms sufficiently to keep them from breaking except under the influence of a fair degree of force.