Raddle
There are numerous deposits of red ironstone, in the state of fine earth, where the operations of grinding and levigation have, to a considerable degree, already been carried out by Nature. These deposits form the mineral which, under the name of raddle, is often used as a pigment for ordinary paints. It may be considered to have originated in the transformation of red ironstone, by the natural forces that can everywhere be seen disintegrating rocks, namely water and frost, into a fine powder, which has been transported, often over long distances, by water, and has finally settled down.
In places where the process has been carried out in this manner, the raddle will be in a condition, as regards fineness of division and beauty of colour, that leaves nothing to be desired, and the material itself is ready for use as a very valuable pigment. Large deposits of this kind, however, are of rare occurrence; but there are plenty in which the ferric oxide is associated with varying quantities of clay, sand, and sometimes lime.
The conditions here are on all fours with those of clay, which, too, has been formed in a similar way. Pure clay, the so-called kaolin, is a highly valuable material, whereas ordinary loam—highly contaminated clay—is only of low value. In judging the quality of raddle as a pigment, the presence of impurities is of less account than their nature; and in some cases a very highly contaminated raddle may be worth far more, as a pigment, than one containing only very small admixtures of extraneous substances.
As stated above, the ordinary impurities in raddle are clay, lime and quartz sand. An admixture of clay, even if fairly large, is no great drawback, since the material can be used in its natural state, and also be toned by burning. Lime is less favourable, for though a calcareous raddle can be used as it is, the lime parts with its carbon dioxide on calcination, becoming changed into caustic lime and imparting to the product qualities which preclude its employment for a number of purposes, especially for mixing with delicate organic colours.
The presence of quartz sand is immaterial when the raddle is to be burned, inasmuch as sand is unaltered by calcination. But it constitutes a drawback because it makes the fine raddle gritty and unsuitable for fine paint work. The only way to eliminate this impurity is by levigation—an expensive operation which should, as far as possible, be avoided for these native ferric oxides, because they must be sold very cheaply, and have to compete with the large quantities of oxide obtained as a by-product of the chemical industry.
The suitability of a given specimen of raddle for use as a pigment may be easily ascertained by weighing out exactly 100 grams and heating to about 120° C. The loss of weight will give the amount of water in mechanical retention. The residue is suffused with strong vinegar, and left for several days, being stirred at frequent intervals. The carbonates of lime and magnesia present will dissolve in the acid, the ferric oxide remaining untouched. The liquid is decanted, and the residue washed several times with water and dried, the diminution in weight being a measure of the carbonates in the sample. If the vinegar has turned a yellow colour, the presence of ferric hydroxide in the mineral is indicated, this hydroxide being readily soluble in acetic acid. If the residue feels gritty, it contains quartz sand, the amount of which can be found with sufficient accuracy by levigating the mass and weighing the sandy residue after drying.
Deposits occur, in many places, of a mineral similar to raddle, but formed under peculiar conditions. Thus, there are found, in the vicinity of brown-coal deposits that are rich in pyrites, earthy masses which are occasionally of a handsome red colour and consist of a variety of minerals admixed with a considerable proportion of ferric oxide.
These masses probably originated in fires in the coal seams, whereby the pyrites became transformed into ferric oxide and basic ferric sulphate; and where the deposits are of sufficient size, they may be advantageously utilised in the production of cheap reds. In most cases, however, the minerals must be levigated, owing to the frequency with which they contain large proportions of extraneous minerals in a gritty condition.