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obtained. The more modern theory, first broached by Young and more fully developed by Helmholtz, considers that there are three primary colours, red, green, and blue, although some authorities add a fourth.
However, it must be confessed that while the modern theory accurately explains all the phenomena of colour producible by the use of the spectrum colours, yet the older theory of Brewster more easily explains the phenomena of colour as produced by the admixture of the various colouring-matters, pigments, and dyestuffs in common use; this arises not from any fault in the newer theory, but from the compound nature of the light which is reflected or transmitted from the colouring-matters in question. Of the newer theory it is not intended to deal, although it is advisable for colourists to make themselves acquainted with it; as to the old theory, it will be sufficient to say that when any two of the primary colours are mixed together a so-called secondary colour is produced ; thus red and yellow produce orange, red and blue produce violet, while yellow and blue make green. When the secondaries are mixed together they produce what are called tertiary colours, of which there are six, known as buff, citrine, sage, slate, plum, and russet. The nomenclature of these tertiary colours is very indefinite, and different authorities give them different names.
The common theory of red, blue, and yellow is not wholly satisfactory, as it does not account for all the shades which may be produced by the admixture of pigments; thus a mixture of ultramarine, a blue, with yellow ochre, a yellow, does not produce a green, as the theory would expect, but a kind of greenish-grey; this effect can, however, be explained by the blue-red-green theory when we know the kind of
reflected by the two pigments in question. Reference must be made to text-books on colour for a further development of the subject.
Colours.—It has been explained above that the term “colours" is used in two senses—first, to express the sensation which light of various kinds evolved from bodies excites on the retina of the eye, and which sensation is purely functional ; second, to denote those bodies which, having the property of selective absorption of coloured rays from the light which falls upon them, appear to be coloured and which have the property of imparting this colour to other bodies ; such bodies are known as colouring matters and may be divided into two groups, dyestuffs and pigments; the former are mostly soluble in water and are used solely to dye cotton, wool, or other textile fibres, while the latter are insoluble, and are used in the preparation of paints.
Besides these two classes of coloured bodies there is another group which are distinguished by the fact that while possessing colour yet they cannot impart this colour to other bodies ; such are bluestone (sulphate of copper), nitrate of cobalt, chrome alum, &c.
Pigments. These are a fairly numerous class of colouring matters which are used to give colour to paint. They are nuostly derived from the mineral kingdom, although a few are obtained from organic sources. As a class they are distinguished by being insoluble in water, turpentine, and most other solvents with the exception of the strong acids ; they are opaque or nearly so; and they should be perfectly inert bodies exercising no action of any kind on any other substance with which they may be mixed. As typical examples of pigments may be taken barytes, oxide of iron, yellow ochre, chrome green, and umber.
In dealing with pigments in detail they will, as a rule, be considered under the divisions of white pigments, red pigments, yellow pigments, and so on; but here and there, deviations from this rule will be made, as in the case of Derby red (which will be dealt with under the head of yellow pigments) and in the case of lakes, where it is thought that the composition and properties of the particular pigments can be more conveniently pointed out, if dealt with in one group.
Paint.-Paint is the name given to a liquid composition which is used very extensively for two purposes—first, to act as a protective substance to preserve the body on which it has been applied from the destructive action of the weather ; second, as a decorative agent.
The first object is brought about by making the paint with materials which are not acted upon by the various agents present in the atmosphere, such as water, acid vapours, light, oxygen, that exert a more or less destructive action on bodies which may be exposed to their action. The bodies which have been found to resist this destructive action of the atmospheric influences are the various so-called drying oils, resinous matters, and the pigments. A paint is a liquid composition which will remain liquid until it is applied to the body to be painted, and yet when so applied and afterwards exposed to the atmosphere will dry and leave behind it a firm, hard (yet elastic), and opaque coating, which may be more or less lustrous and be capable of resisting the weather. The opacity of the coating is obtained by using pigments of various kinds, which also tend to increase the resisting power of the paint, and these pigments are mixed with liquid bodies, such as oils and spirits, which are
used partly to obtain a composition that is easy of application, and partly to secure volatility, so that when alone or when mixed with resinous matters, they will evaporate away and leave behind a hard mass firmly binding the pigments to the body over which they have been painted. The liquid bodies which have been found to answer this purpose best are the drying oils, such as linseed oil, which when spread over a surface and exposed to the air absorb oxygen and dry into a hard mass; but as these oils, for various reasons (which will be more fully dealt with later on), cannot be used alone with satisfactory results it becomes necessary to mix them with some solvent, such as turpentine or shale naphtha, which is volatile. In some kinds of paints a little resinous matter is used, which dissolves in the solvent; on exposure the latter evaporates off, leaving the resin behind in the form of a dry coat on the surface to which it has been applied. Paint is always more or less coloured to add to the decorative effect. Its primary purpose, however, is to hide the character of the surface to which it is applied and, as has been pointed out, to protect this surface.
Varnishes.—These bodies are very similar to paints in their properties and uses. They differ in giving a transparent lustrous coat of a very resistant character to the destructive action of the weather. They may be coloured ; but, if so, transparent colours are used and not, as in the case of paints, opaque pigments. They are composed of a resinous matter dissolved in various oils and solvents, the latter forming the vehicle by means of which the resin is transferred to the surface to be varnished. The special properties of varnishes will be dealt with later on.
The white pigments are a very important group of painters' “colours," probably the most important, as while the red, blue, green, &c., pigments are used simply or almost entirely as colouring pigments, the white pigments are used in two ways1st, as "body colours," i.e., to give body or covering power to paint; 2nd, as “colouring pigments.” Thus, in making a red paint, white lead or barytes is added to give the necessary body and vermilionette is used to colour the paint. On account of this dual feature of the white pigments they merit a more detailed account of each individual member of the group than is necessary for other pigments.
The white pigments are a fairly numerous group of bodies derived entirely from inorganic sources. Many white bodies are known which could be used as pigments, but are not so used on account of expense, &c. The following list comprises all that are used either on a large or small scale :
White Lead, basic carbonate of lead, 2 Pb CO, Pb 1,02; this pigment is also sold under a variety of other names.
Lead Sulphate, Pb S 04; many pigments sold under various fancy names consist essentially of this body combined with other white pigments.
Lead Oxychloride, Pb, 001,, Pattinson's white lead.
Zinc Sulphide, Zn S; this body combined with barytes, &c., is largely used as a white pigment.
Barium Sulphate, BaSO4, barytes.
Of these, the most important are white lead, lead sulphate, zinc white, zinc sulphide, barytes, gypsum, calcium carbonate, and china clay.
White lead has been known and used as a pigment for centuries; the Romans and Greeks used the native carbonate of lead or
cerusse, as it was then called, from which the mineralogical name cerussite has arisen. This natural pigment is found only in comparatively small quantities, and it is no wonder that a process for the artificial production of white lead was soon found out and adopted, with the result that natural cerussite is not now used as a pigment.
It is not known when white lead was first made, who made it, or to what country it owes its birth. The oldest known method is that commonly called the “Dutch method,” from the supposition that it was invented in Holland ; it is described as the Dutch process in an English patent granted in 1787, and there is no doubt but that it is the process referred to in three earlier patents granted in 1622, 1635, and 1745, in which it is spoken of as an old process. Evidently white lead has been made for several centuries. During all this period there has been but little change made in the Dutch process. But in the interval inventors have not been idle, for there is no other pigment which has attracted so much attention at their hands as white lead;
and the number of processes and modifications of processes which have been devised, is almost innumerable. With all this invention, the ancient Dutch process still retains its pre-eminence as the best process for the manufacture of white lead.
MANUFACTURE OF WHITE LEAD.
White lead, the basic carbonate of lead, is manufactured by a variety of methods. It is not easy to classify these processes into groups, as they not unfrequently pass one into the other imperceptibly. The author suggests the following classification, which is based on the principles which appear to underlie the various methods adopted, or which have been proposed and used on a limited scale ::
1st Group.—Stack method.
3rd Group. Precipitation processes based on the action of carbonic acid gas on various lead salts.