Read Ebook: The manufacture of earth colours by Bersch Josef Bersch Wilhelm Editor Salter Chas Charles Translator

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PAGE INTRODUCTORY 1

THE RAW MATERIALS FOR EARTH COLOURS 8

White Raw Materials and Pigmentary Earths 11

Limestone 11

Gypsum 18

Barytes, or Heavy Spar 19

Talc, Soapstone, Steatite 20

Clay 21

Yellow Earths 23

Brown Ironstone 23

Ochre 25

Yellow Earth 26

Terra di Siena 27

The Red Earths 27

Red Ironstone 28

Bole 31

Alum Sludge 32

Mine Sludge 32

Blue Earths 33

Azurite, or Ultramarine 33

Vivianite 33

Green Earth Pigments 34 Green Earth 34 Malachite 35

Brown Earth Pigments 36 Umber 36 Asphaltum 37

Black Earth 38 Black Schist 38 Graphite 38

THE PREPARATION OF THE COLOUR EARTHS 40 Crushing Machinery 43 Crushing and Sifting 77 Calcining 81 Mixing and Improving 81 Moulding 85

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

YELLOW EARTH COLOURS 127 The Ochres 128 Calcining Ochre 132 Ochres from Various Deposits 136 Artificial Ochres 138 Ochres as By-products 146

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 Burning in the Muffle 158 Caput Mortuum, Colcothar 160 Calcining Ferric Oxide 161 Ferric Oxide Pigments from Alum Sludge 164

BROWN EARTH COLOURS 168 Terra di Siena 168 True Umber 170 Cologne Earth 173 Asphaltum Brown 174

GREEN EARTH COLOURS 176 Green Earth, or Celadon Green 176 Artificial Green Earth 180 Malachite Green 181

BLUE EARTH COLOURS 183 Malachite Blue 183 Vivianite or Blue Ochre 184

BLACK EARTH COLOURS 185 Graphite 185 Black Chalk 194

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

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:--

Pigments occurring native in a finished condition, and only requiring mechanical preparation to fit them for use as painters' colours. 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. 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 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.

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 , or are artificial products . 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:--

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.

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 , by the old process, from green vitriol . 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.

THE RAW MATERIALS FOR EARTH COLOURS

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.

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