PIGMENTS, PAINT
AND
PAINTING

PIGMENTS, PAINT
AND
PAINTING

A PRACTICAL BOOK FOR PRACTICAL MEN


BY

GEORGE TERRY



London
E. & F. N. SPON, 125 STRAND
New York
SPON & CHAMBERLAIN, 12 CORTLANDT STREET
1893

INTRODUCTION.

Hence the need for a handbook such as this volume aims at being. Granted that our technical schools and colleges are affording a liberal and invaluable education to the workman who will avail himself of the opportunities given him, still a man does not remain for ever at school, and he needs a guide-book, handy of reference and accessible in price, to refresh his memory and supplement the information gained in the class-room and workshop.

To fulfil this useful purpose is the aim and object of this unpretending volume.

CONTENTS.

ILLUSTRATIONS.

CHAPTER I.

PRELIMINARY.

If all the rays are cut off from an electric light except those which are in and beyond the violet, and a flask containing a solution of sulphate of quinine is held in that portion of the spectrum, it will become luminous. The same thing will occur even more strikingly on placing a piece of uranium glass in the ultra-violet rays. The explanation of this phenomenon is that beyond those rays which give light there are others which do not give light, i. e. which do not cause us to experience the sensation of light; the reason being that their vibrations are too rapid. But when certain other substances, such as sulphate of quinine, or a thin slip of uranium glass, are placed in the path of the rays, this rapid motion is arrested and modified, and these rays, which in themselves are not luminous, are reflected back to our eyes as luminous rays. The rapidity of the vibrations being moderated, our retinas become sensible to them as rays of blue light.

Colour does not depend only upon chemical composition nor solely upon the aggregation of the particles, but upon these and other things besides not yet explained. All matter is in a state of motion. If you heat a substance you communicate an increased activity of motion to the particles of which it consists. When certain coloured rays of light are falling upon a substance, these coloured rays of light have a motion peculiar to themselves. It may be that the degree of motion in that substance, either existing in it naturally without heating, or communicated to it by artificial heating, is such that these rays of light are precisely those which that substance is not capable of sending back to our eyes. They are then absorbed or destroyed in some way, by the particular state of that substance upon which they fall; and those rays which the substance is capable of reflecting back are mainly sent back to our eyes. Certain colours, such as blue, yellow, and green, absorb certain rays more or less perfectly, and reflect back in the main blue, yellow, and green to our eyes. Hence it is incumbent on those who are studying colour, and who are interested in the purity and permanency of colour, to comprehend at least the principles of that science of light which tells of the action of light upon various bodies that are used as pigments in painting.

If we put together two substances one of which destroys or modifies the chemical condition or state of the other, then certainly one of those substances, and very probably both, will lose the colour which it had before it came into contact with the other. It is therefore most important that all engaged in the preparation and use of colours should make a study of this science of light. Of almost equal value is a study of the science of heat. We have seen what heat can do in changing the conditions of a substance. To give another instance. The black sulphide of mercury, after sublimation by heat, exhibits properties, imparted to it by the heat, which it did not possess before, i. e. it can, by trituration, be brought to display a red colour.

On showing the spectrum on a screen, if some solution of soda or other sodium salt be held in the course of the light, almost all the coloured rays but one will be cut off, and a little band is seen in the yellow part of the spectrum. This is because the sodium flame is almost “monochromatic,” or single-lined: it cuts off all the colours but the yellow. Again, if metallic thallium is held in the flame, the only band remaining in the spectrum will be the green; and if a lithium salt, the only surviving colour will be red.

Pigments.—The term “pigments” is applied to those colouring matters which are mixed in a powdery form with oil or other vehicle for the purpose of painting. They differ in this respect from the dyestuffs, which are always employed in solution. A very large proportion of the pigments in common use are derived from the mineral kingdom, the most notable exceptions being found in the blacks and lakes. All pigments are required to possess “body,” or density and opacity; to be insoluble in water and most other solvents, except the stronger mineral acids; and to be inert, or incapable of exercising chemical or other influence on each other or on the vehicle or drier with which they are mixed prior to use. They may be conveniently classified according to their colours in the first place, reserving the consideration of their preparation for use for a later chapter. The chief classes are Blacks, Blues, Browns, Greens, Reds, Whites, and Yellows.

CHAPTER II.

BLACKS.

The principal impurity to be watchful of in the vegetable and lamp blacks is a small quantity of oily matter which may seriously interfere with their drying qualities. They should leave very little ash after being burned in a crucible. Bone and ivory blacks are sometimes valued as much for their mineral matter as for their colouring matter. The proportion of this mineral matter is ascertained by heating a certain weight of the black to red heat in a crucible till every trace of black has disappeared, and then weighing the residue. The residue should next be boiled in strong hydrochloric acid till it is dissolved; if there is any which will not dissolve it is most probably barytes, which has been added as an adulterant and to make the black weigh heavy. When the solution is complete, the addition of ammonia will throw down a precipitate of phosphate of lime, which should equal 60 to 70 per cent. of the original weight of mineral matter. If much less than this, it is likely that whiting or gypsum has been mixed with the pigment. As carbon is not acted upon by acids or alkalies, it follows that all pure carbon blacks are in themselves perfectly stable and permanent pigments, and that they exert no influence on other pigments with which they may be mixed.

Animal-black.—This substance is almost identical with bone-black, but is generally in a more finely divided state. Any animal refuse matter may be used in its preparation, such as albumen, gelatine, horn shavings, &c. These are subjected to dry distillation in an earthenware retort. An inflammable gas is given off, together with much oily matter, ammonia, and water, while a black carbonaceous mass is left behind. This is washed with water and powdered in a mill, the product being animal-black. It is largely used in the manufacture of paint, printing ink, and blacking.

Bone-black.—When bones are heated in a retort or crucible, the organic constituents are decomposed and carbonised. A mixture of combustible gases is given off; some of these do not condense on cooling, others condense in the form of a heavy oil, called bone-oil. Also much water containing tarry water and ammoniacal salts in solution passes over. The residue in the retort or crucible consists of finely divided carbon, in intimate mixture with the inorganic constituents of the bones: this mixture constitutes ordinary bone-black, or animal charcoal, as it is sometimes called. The inorganic portion may, if required, be removed by washing the residue in dilute hydrochloric acid.

The process, as worked on the large scale, is carried on in different ways, according as it is desired to collect the volatile condensable portion of the distillate, or to allow it to escape. In the latter case, when it is required to obtain only bone-black, the apparatus employed is of a very simple nature, and the amount of fuel needed is comparatively small. The carbonisation is effected in fire-clay crucibles, 16 in. high and 12 in. diameter. These are to be preferred to crucibles made of iron, which were much used at one time, since they do not lose their round form when subjected to a high temperature; in consequence of this, they fit more closely together in the furnace, less air can penetrate, and therefore less of the charcoal is consumed by oxidation. The furnace is an ordinary flat hearth, having a superficial area of about 40 square yards, and is covered in with a flat arch, all of brickwork. The fireplace is situate in the middle of the hearth; the crucibles are introduced through doors in the front, which are bricked up when the furnace is filled; each furnace holds eighteen crucibles. The crucibles, filled with the coarsely broken bones, are covered with a lid luted on with clay. To economise fuel, the furnaces should be in a row, and placed back to back.

The arrangement of the furnace and pots is shown in Figs. 1 and 2. A is the fireplace; B, the crucibles, eighteen in number, spread over the floor of the furnace in a single layer; c, d, e, and f are the flues for conducting away the heated gases arising from the calcination of the bones, as well as the waste heat itself; the last portion of the flue is fitted with a damper g. The furnaces are intended to be built in fours, back to back, the waste heat serving in a great measure to conduct the operation of the revivifying apparatus placed in the centre of the group, and marked C.