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.
As a rule, barytes is first roughly crushed in edge-runner mills or stamps, and then ground to the extreme degree of fineness obtainable in ordinary mills. Even with the greatest care, however, it is impossible by this means to obtain sufficient fineness of division for mixing with fine colours, the only way in which this can be accomplished being by levigation.
Given a fairly pure white barytes to begin with, levigation furnishes a handsome white pigment that can be mixed with colours of any kind; but when used by itself in association with oil or varnish, its covering power is very low and the colour never perfectly white. Native barytes is therefore unsuitable, as such, for paint.
Varieties that are not pure white are sometimes corrected with ultramarine, added in the grinding-mill. If the yellow tinge is due to iron compounds, this can often be remedied by treating the finely ground material with hydrochloric acid, which dissolves them out, this treatment being followed by a thorough washing with pure water.
As already mentioned, white lead is most frequently mixed with barytes, this being usually added when the white lead is being ground, by feeding the two materials to the mill and grinding them together.
The crudeness of mechanical methods of reduction is clearly exemplified by comparing the most carefully ground and levigated barytes with that obtained by artificial means. The permanent white largely used in the production of wall-paper, and quite unalterable in air, is, chemically speaking, identical with native barytes, viz. barium sulphate. The two also seem to be identical in crystalline habit, as is usual in the case of one and the same mineral, whether native or prepared by artificial means. Artificial barytes is obtained by treating a soluble salt of barium with sulphuric acid, or a solution of sodium sulphate (Glauber salt), so long as a precipitate continues to form.
This precipitate is barium sulphate, which subsides completely on account of its extreme insolubility, this being greater than that of any other salt known. The rapid rate of deposition results in the formation of extremely small crystals, which, being colourless and reflecting the light completely, appear to be perfectly white. Even when permanent white is applied in very thin layers to any surface, its covering power is very considerable, by reason of the extremely fine subdivision of the material.
This behaviour of artificial barytes in comparison with that of the natural product, affords an important hint in connection with the preparation of earth colours, namely, that in order to obtain products of specially good quality, the endeavour should be to reduce the raw materials to the finest condition possible. This result is accomplished most securely by bestowing the greatest care on grinding and levigation; and it is therefore highly important that the manufacturer should select, from the various apparatus used in reducing the materials, those that are best adapted for the purpose.