Since, on account of the cost of pure hydrochloric acid, crude acid will always be used, it will be necessary to remember that this crude acid always contains ferric oxide in solution—this being the cause of its yellow colour. If the amount of acid taken is barely sufficient to combine the whole of the lime, leaving the latter slightly in excess, the ferric oxide—which would otherwise tinge the clay yellow—will be precipitated.
If, on the other hand, the acid is in excess, the clay is obtained free from all constituents soluble in the acid. The purified clay must then be freed from the calcium chloride, formed by dissolving the lime, by a thorough washing, since the clay would otherwise always remain moist on account of the hygroscopic properties of the chloride in question. Moreover, any small residuum of free acid would constitute a drawback on the clay being mixed with other colours.
Calcium chloride is very soluble in water, and therefore can be completely removed from the clay by washing. The purified clay is left to settle down as completely as possible, and after drawing the liquid off from the sediment, the latter is suffused with pure water and left to settle once more. As a rule, two such washings will cleanse the clay of calcium chloride and free acid sufficiently to render the product suitable for any purpose.
When large quantities of clay have to be treated in this manner, considerable amounts of calcium chloride solution will be obtained, which can be advantageously utilised for the production of precipitated chalk, all that is necessary being to collect the liquor in a large tank and treat it with a small quantity of slaked lime, to transform the surplus free acid into calcium chloride and precipitate the ferric oxide present in solution. At the end of a few days the liquor in the tank will consist of a very pure solution of calcium chloride which will furnish an excellent precipitated chalk when treated in the manner already described under that heading.
Barytes, or Heavy Spar
This mineral—chemically, barium sulphate, BaSO4—occurs native, as extensive deposits, in many places—England, Bohemia, Saxony, Styria, etc. It sometimes forms handsome tabular crystals, but more frequently compact masses, which may be pure white, grey yellow, etc., in colour, and are distinguished by high specific gravity (usually 4·3–4·7), to which the mineral owes its name. This high density also limits the application of the mineral, and it cannot be used as a pigment, in the true sense of the term, being only suitable as an adjunct to artificially prepared colours.
The employment of barytes in the colour industry is often regarded as adulteration, which, however, it is not when the case is considered from the right point of view. For instance, the only preparation which can properly be termed white lead consists of basic lead carbonate. This, when pure, is a rather expensive pigment, whereas, for certain purposes, the consumer requires a product that can be obtained at a low price. In order to satisfy this demand, the only course open to the colour-maker is to mix the white lead with a cheap white substance, which enables him to turn out different grades of white lead, which, although low in price, are far inferior to the pure article in covering power. Pure white lead being itself a very heavy substance, the only bodies suitable as adjuncts are such as are also of high specific gravity; and of all the cheap pigments known, heavy spar is the only one endowed with this property. Consequently, this substance is extensively used in making the cheaper grades of white lead and the pale kinds of chrome yellow.
The only cases in which the addition of heavy spar to a colour can be regarded as an intentional fraud on the consumer is when he is sold, as pure white lead, chrome yellow, etc., a product really composed of a mixture of such colour and barytes. Moreover, the presence of barytes in white lead can be easily detected by a simple examination, pure white lead readily dissolving, with considerable effervescence, in strong nitric or acetic acid, whereas barytes is insoluble in all acids, and therefore remains, as a heavy white powder, at the bottom of the vessel. In this way both the presence and amount of barytes contained in a sample of white lead or chrome yellow can easily be ascertained.
The preparation of barytes for the purposes of the colour-maker is entirely a mechanical operation. The barytes, which though fairly hard is easily reduced, is crushed with stamps, ground in a mill and finally levigated, it being impossible to obtain a sufficiently fine powder even by repeated grinding.
Native barytes must not be confounded with the artificial barium sulphate sold as permanent white or blanc fixe, which is an extremely finely divided barium sulphate obtained by precipitating a solution of a barium salt with sulphuric acid or a soluble sulphate, and is a painters’ colour that is highly prized for certain purposes. Both the native sulphate and the artificial variety have the property of remaining completely unaltered by exposure to air, and they can therefore be mixed with any kind of pigment without fear of the colour deteriorating.