CONTENTS
| CHAPTER I | |
| PAGE | |
| Introductory | [1] |
| CHAPTER II | |
| The Raw Materials for Earth Colours | [8] |
| (A) White Raw Materials and Pigmentary Earths | [11] |
| Limestone (Calcite, Limestone, Chalk) | [11] |
| Gypsum (Alabaster) | [18] |
| Barytes, or Heavy Spar | [19] |
| Talc, Soapstone, Steatite | [20] |
| Clay | [21] |
| (B) Yellow Earths | [23] |
| Brown Ironstone | [23] |
| Ochre | [25] |
| Yellow Earth | [26] |
| Terra di Siena | [27] |
| (C) The Red Earths | [27] |
| Red Ironstone | [28] |
| Bole | [31] |
| Alum Sludge | [32] |
| Mine Sludge | [32] |
| (D) Blue Earths | [33] |
| Azurite, or Ultramarine | [33] |
| Vivianite | [33] |
| (E) Green Earth Pigments | [34] |
| Green Earth | [34] |
| Malachite | [35] |
| (F) Brown Earth Pigments | [36] |
| Umber | [36] |
| Asphaltum | [37] |
| (G) Black Earth | [38] |
| Black Schist | [38] |
| Graphite | [38] |
| CHAPTER III | |
| The Preparation of the Colour Earths | [40] |
| Crushing Machinery | [43] |
| Crushing and Sifting | [77] |
| Calcining | [81] |
| Mixing and Improving | [81] |
| Moulding | [85] |
| CHAPTER IV | |
| White Earth Colours | [87] |
| Caustic Lime | [87] |
| Pearl White | [94] |
| Vienna White | [95] |
| Chalk | [98] |
| Precipitated Chalk | [107] |
| Calcareous Marl | [110] |
| Gypsum | [111] |
| Kaolin, Pipeclay | [112] |
| Barytes, or Heavy Spar | [119] |
| Carbonate of Magnesia | [123] |
| Talc | [124] |
| Steatite or Soapstone | [125] |
| CHAPTER V | |
| Yellow Earth Colours | [127] |
| The Ochres | [128] |
| Calcining (Burning) Ochre | [132] |
| Ochres from Various Deposits | [136] |
| Artificial Ochres | [138] |
| Ochres as By-products | [146] |
| CHAPTER VI | |
| Red Earth Colours | [151] |
| Bole | [152] |
| Native Ferric Oxide as a Pigment | [154] |
| Iron Glance | [154] |
| Hematite | [155] |
| Raddle | [155] |
| Burnt Ferric Oxide and Ochres | [158] |
| (a) Burning in the Muffle | [158] |
| (b) Caput Mortuum, Colcothar | [160] |
| (c) Calcining Ferric Oxide | [161] |
| Ferric Oxide Pigments from Alum Sludge | [164] |
| CHAPTER VII | |
| Brown Earth Colours | [168] |
| Terra di Siena | [168] |
| True Umber | [170] |
| Cologne Earth (Cologne Umber) | [173] |
| Asphaltum Brown (Bitumen) | [174] |
| CHAPTER VIII | |
| Green Earth Colours | [176] |
| Green Earth, or Celadon Green | [176] |
| Artificial Green Earth (Green Ochre) | [180] |
| Malachite Green | [181] |
| CHAPTER IX | |
| Blue Earth Colours | [183] |
| Malachite Blue (Lazulite) | [183] |
| Vivianite or Blue Ochre | [184] |
| CHAPTER X | |
| Black Earth Colours | [185] |
| Graphite | [185] |
| Black Chalk | [194] |
| CHAPTER XI | |
| The Commercial Nomenclature of the Earth Colours | [197] |
| White Earth Colours | [198] |
| Yellow Earth Colours | [200] |
| Red Earth Colours | [200] |
| Brown Earth Colours | [200] |
| Green Earth Colours | [201] |
| Blue Earth Colours | [202] |
| Grey Earth Colours | [202] |
| Black Earth Colours | [202] |
| Index | [203] |
EARTH COLOURS
CHAPTER I
INTRODUCTORY
Both from the chemical and practical standpoint it is necessary to divide pigments into clearly defined groups, the following classification being adopted on the basis and natural history of the substances concerned:—
(1) Pigments occurring native in a finished condition, and only requiring mechanical preparation to fit them for use as painters’ colours. (2) Pigments which are not ready formed in Nature, but contain some metallic compound as pigmentary material, which requires certain chemical treatment for its full development. (3) Pigments which, in contrast to these two groups, contain only organic, and no inorganic, constituents. This last class comprises all the natural vegetable pigments, together with the large group of colours obtained artificially from tar products, fresh groups of which are being continually introduced. Nowadays, there is no longer any strict line of demarcation between the natural and artificial organic colouring matters, it being possible to produce even those of the vegetable series, such as madder and indigo, by artificial means.
Whilst this group of colours exhibits the greatest variety, and is constantly being enriched and increased by the progress of colour chemistry, the case is different with the first group, the natural earth pigments. Here we have chiefly to do with the preparation of materials occurring in Nature, or with bringing about certain chemical results, so that, consequently, the range of variety is far more restricted, and there is little or no possibility of increasing the number of these colours by the manufacture of really new products. The earth colours nevertheless have a high technical and economic importance, on account of their extremely valuable properties, coupled, for the most part, with low cost.
If the term “earth colours” were strictly adhered to, the present work would have to be confined to a description of the physical and chemical properties of the various pigments, and of the various means by which they can be brought into suitable condition for use in paints.
However, of late, the term has found wider application than formerly, since it has been found practicable to modify (shade) certain of the earth colours by simple operations, and thus considerably increase the range of tones of the substances known as earth colours. The progress of chemical industry has also largely increased the number of the so-called earth colours, certain methods of chemical treatment having enabled substances that are of little use for other purposes, to be employed, in large quantities, as pigments. The application of these—usually cheap—by-products is still further facilitated by the fact that they can be transformed, by a simple chemical treatment, into pigments which are distinguished by their beauty of colour and at the same time possess the great advantages of durability and cheapness.
As an example of this, mention may be made of iron oxide, which occurs in Nature in the form of various minerals which can be made into pigments by mechanical treatment. In many cases, this treatment has already been carried out by Nature, and deposits of iron oxide are found in which the material has only to be incorporated with a vehicle to make it fit for immediate use as a painters’ colour.
Moreover, the same oxide is obtained, in large quantities, as a by-product of the treatment of other minerals. From the point of view of chemical composition, this by-product is of very low value, by reason of the large supplies of native oxide available. By means of a very simple chemical treatment, however, this by-product oxide can be considerably improved in commercial value, being, in many cases, convertible, by merely heating it to certain temperatures, into a variety of colours which sell at remunerative prices.
Consequently, in view of the present condition of the chemical industry, the term “earth colours” can be enlarged to include a number of waste products which fetch good prices as colours, though otherwise practically valueless in themselves.
The number of earth pigments is very large, and comprises representatives of all the principal colours. For painting purposes, few pigments beyond the earth colours were known to the ancients; and most of the colours in the paintings which have come down to us from antiquity are pure earth pigments, thus affording proof of their great durability, having retained their freshness unimpaired for hundreds—and some for thousands—of years.
The earth colours might be divided into such as occur ready-formed in Nature, and require only mechanical preparation, and which either require special treatment (e. g. calcining), or are artificial products (like the iron oxide mentioned above). Since, however, such a classification would not advantage our knowledge of the nature of this class of colours, it appears useless and superfluous, and we will therefore simply confine ourselves to arranging the earth pigments according to their colour—white, yellow, red, etc.
Adopting this classification, the following minerals and chemical products may be considered as earth colours:—
White.—These include the varieties of calcium carbonate, such as chalk, marble, precipitated chalk, calcium phosphate, calcium sulphate (in the form of gypsum, alabaster, muriacite and the precipitated gypsum produced as a by-product in many chemical works), heavy spar, the different varieties of clay, and magnesia.
Yellow.—This group comprises ferric hydroxide (hydrated oxide of iron) in the form of the various minerals known as ochre; all the preparations chiefly composed of this hydroxide, and all those prepared by artificial means. A very important member of this group is orpiment; the other arsenical compounds frequently met with native, being however, on account of their poisonous properties, no longer used as pigments.
Red.—Chief among the red earth colours are those consisting of ferric oxide (iron oxide), under various names. The only other member of the group is the far rarer vermilion.
Blue.—The blue earth colours are few in number and of no particular beauty; but they are of importance on account of their cheapness and because all the artificial blue pigments are rather expensive. Two products in particular merit attention in this connection, namely, ultramarine, and the mineral known as blue ochre or blue ironstone. The latter, as a matter of fact, cannot be used for anything else than a painters’ colour, and can be obtained at a low price; whereas ultramarine also forms a valuable raw material for the recovery of copper, and is therefore dearer.
Green.—This group, again, contains only two members, viz. malachite green (chrysocolla), and the green earths (seladonite), known as Verona, etc., green. These occur fairly often in Nature, and the green earths in particular find a wide industrial application by reason of their low price. Malachite green is very similar, in chemical constitution, to ultramarine; and both form sources of copper and are consequently expensive.
It should be mentioned that both ultramarine and malachite green can only be profitably made into pigments where the minerals can be obtained cheaply, since both of them can be manufactured where artificial pigments are produced, and are put on the market under the same names as the native articles. The very low price of the green earths makes them highly popular as colouring matters in certain branches of industry, and they are very largely used by wall-paper manufacturers.
Brown.—This is a large group, and the pigments composing it are specially distinguished for their beauty and depth of colour, on which account they are used in the finest paintings. Here, again, it is ferric oxide, in combination with water—and therefore ferric hydroxide—that furnishes a large number of the members of the group. Like the renowned Siena earth, the artists’ colours known as Vandyck brown, bole, Lemnos earth, umber, etc., mainly consist of more or less pure ferric hydroxide. These minerals are, moreover, specially important to the colour manufacturer, inasmuch as most of them enable a large number of different shades to be obtained by a simple method of treatment consisting merely of the application of heat in a suitable manner; and these colours are among the most excellent we possess, by reason of their beauty and permanence. Amongst this series must also be classed native manganese brown, which chiefly consists of a mixture of manganese oxide and the hydrated peroxide of the same metal.
Black.—There is really only one member of this class, which, however, is frequently used, viz. that form of carbon occurring as hexagonal crystals and known as graphite. Another natural black natural product, occasionally used as a painters’ colour is the so-called black chalk. However, since black pigments can be produced very cheaply by artificial means, the natural colours find only a limited application; and only in one instance is graphite used alone, viz. for making blacklead pencils.
As already mentioned, certain chemical industries furnish by-products which are of very little value in themselves, and many of them, indeed, may be classed as worthless, since chemical manufacturers naturally endeavour to get everything possible out of their materials in the course of manufacture.
Some of these by-products, however, can advantageously be used as pigments, a good example of this being afforded by the iron oxide formed as a by-product in the manufacture of fuming sulphuric acid (Nordhausen oil of vitriol), by the old process, from green vitriol (ferrous sulphate). In itself, this oxide is practically valueless, but, by very simple treatment, it can be converted into very valuable pigments which have a market value far in excess of the original material. Although it has hitherto been the custom to confine the term earth colours to such as occur ready-formed in Nature and only require simple mechanical treatment to make them ready for immediate use as pigments, the author is nevertheless of opinion that a book dealing exhaustively with earth colours should also make some mention of all the mineral colouring matters which can be easily made into pigments by simple processes, such as calcination or bringing into association with other substances. In accordance with this view, the present work will describe all the pigments that are obtainable in this manner. Most of the earth colours consist of decomposition products of certain minerals; and this applies particularly to such of them as contain iron oxide. According as the decomposition of the original mineral has been more or less extensive, the natural product exhibits different properties; and the manufacturer must consequently endeavour to treat them in such a manner as to ensure that the pigment obtained will be as uniform as possible in shade and permanence. In order to accomplish this it is essential to have an accurate knowledge of the origin of the raw material under treatment, and of its chemical and physical properties. In view of this, the author considers it necessary to deal more fully with the pigmentary earths forming the raw materials of the earth colours, before passing on to the preparation of the colours themselves.
CHAPTER II
THE RAW MATERIALS FOR EARTH COLOURS
The minerals constituting the raw materials for the preparation of the earth colours occur under very divergent conditions in Nature. Some of them, such as chalk, form immense deposits, even whole mountains, whilst in other cases, e. g. the blue ferruginous earths, the occurrence is connected with certain local conditions, and many are found only in isolated deposits, as pockets or beds. This last is the case, for instance, with the handsome brown iron pigments; and indeed the names by which they are known indicate that they are only found in well-defined localities, or that they are met with of special quality there. The brown earth colour known to all painters as Terra di Siena, is found at many other places as well as near Siena, but the product from that city acquired aforetime a special reputation for beauty, and therefore all similar earths, provided they are equal to that from Siena, also bear the same name in commerce.
A number of raw materials for the preparation of earth colours are found, it is true, in many deposits, but their utilisation depends, in turn, on local conditions. For example, many copper mines contain, in addition to the other cupriferous minerals, those used, in the powdered state, as ultramarine or ultramarine green, and not infrequently lumps of mineral are found containing both blue and green together. However, it is only when these minerals occur in sufficient quantity to make the necessary sorting profitable that their manufacture into pigments can be regarded as practicable.
Before commencing to work a deposit it is essential to make sure whether the raw material, or pigmentary earth, is actually suitable for the manufacture of earth colour. Even the general character of the material is important, those of soft, earthy consistency being much easier to treat, and the cost of preparation smaller, than if the raw material be hard, tough and crystalline.
The extent and thickness of the deposit, and the ease with which it can be worked, also play an important, and even decisive part, since, other conditions being equal, it will not pay to erect a colour works unless the raw material is available in sufficient quantity and is cheap. Generally, the deposit is not homogeneous throughout, the mineral being purer in some places and more contaminated with gangue in others. The percentage of moisture also varies, and in short, a number of circumstances must be taken into consideration in forming a conclusion as to whether a deposit is workable or not.
In order to arrive at a reliable opinion on all these conditions, sampling is indispensable. If the samples are of uniform character, they can be mixed together to make an average sample. But if they differ considerably in appearance, general character, proportion of gangue, etc., it is preferable to examine them separately, more especially when the area which each represents is large.
The examination should extend, on the one hand, to the natural percentage of moisture, and, on the other, to the purity of the material. The water content is determined by thoroughly drying a weighed sample, bearing, however, in mind the fact that pigmentary earths of a clayey nature vary in water content according to the time of year, besides changing in accordance with the weather when the won material is stored in the open.
The purity can only be ascertained by an examination in which a sample of the soft, clayey material is crushed and passed through a narrow-mesh gauze sieve, the amount of the coarse particles—sand, small stones, etc.—remaining on the sieve being determined. A more accurate method, of course, is to separate the true pigmentary earth from the gangue by levigation. For this purpose, a weighed quantity of the crushed, air-dry sample is placed in a relatively narrow glass vessel and thoroughly mixed with water, the turbid supernatant liquid being poured off after a short interval. The residue is repeatedly treated in the same way, until no more fine particles remain in suspension, the residue then consisting of impurities, or gangue. Of course, the washings can be collected, the suspended matter allowed to settle, and finally weighed in an air-dry condition. By this means an approximate idea of the yield of earth colour can be obtained at the same time.
Raw materials which are not amorphous, soft and clayey must first be crushed, an operation facilitated by heating to redness and quenching in cold water. Oftentimes the heating causes a change of colour and improves the covering power—a point to which reference will be made later on.
In the following description of the various raw materials, the chemical composition of the pure minerals will be given, together with an enumeration of the most common impurities.