Photo-Trichromatic Printing
C. G. Zander
Published by Raithby, Lawrence & Co., Ld., Leicester
1896Many of the readers of this booklet, who have followed the brief outline of the principles of chromatics and particularly the explanation of the Young-Helmholtz theory of colour vision, will probably ask themselves if the scientists who call
RED, GREEN AND VIOLET
primary colours, have wiped out of existence the notion of the three primary colours of the artists
RED, YELLOW AND BLUE.
This, however, on closer examination we shall find is not the case. Let us first remember that the scientist deals with coloured light, and in making mixtures adds one light to another, whilst the artist, by the superposition of pigment colours (if transparent), takes away more and more of the light reflected from the white surface on which he works.
The scientist defines as primary colours those which cannot be produced by the mixture of any two colours of light, and which on the other hand cannot be further disintegrated. We must also recollect that if the three primary colours—red, green and violet—be combined in suitable proportions, they produce white light.
It will be remembered that any colour sensation other than the primaries may be produced by mixture of two of the primary colour sensations in suitable proportions, with or without the addition of a certain proportion of white.
The artist calls primary colours those which cannot be produced by superposition of any two transparent pigment colours. If the three primary pigment colours, red, yellow, and blue, be mixed in suitable proportions, they will produce grey to black, according to their density. Any colour may be matched by mixtures of two or three of the primary pigment colours, or by mixing two primary colours with a proportion of grey (i.e., diluted black) or white.
The artist calls secondary colours mixtures of two primary pigment colours. These secondary colours are supposed to be complementary colours of that primary colour which has not entered into the combination that makes up the respective secondary colour. This statement is, however, not strictly correct, as can be proved by experiment. The secondary colours of the artist ought rather to be called contrast colours of their primaries. For an explanation of the term complementary colours, refer to the first part of this book. The secondary colours of the artist are the primary colours of the scientist, red being obtained by superposing the transparent pigment yellow upon the transparent pigment red (a bright crimson), green by superposing the transparent pigment blue (cyan blue) upon the yellow, and blue-violet by superposing the bright crimson upon cyan-blue.
It is, therefore, evident that although red seems to be common to both scientists and artists as a primary colour, the primary red of the scientists, i.e., the fundamental red of the spectrum, is quite different from the primary red of the artist. The fundamental red of the spectrum is similar in hue to vermilion, or, perhaps, a bright scarlet lake as made up by some artists' colourmen. This red produces good orange when mixed with yellow, but mixed with blue it produces only a dirty looking violet. It will also be impossible to produce a crimson by mixing small proportions of blue with vermilion, but scarlet can successfully be mixed from yellow and crimson lake (if the latter is not too purple). Let us take crimson lake in place of vermilion for the purpose of mixing violet, and we shall see that it produces a satisfactory violet, with cyan blue. We see now that crimson makes with yellow equally good orange as it produces good violets when mixed with blue, and that we cannot produce crimson by any mixture of two pigment colours. We, therefore, are justified in terming crimson a primary pigment colour. The correct shade of this crimson will scientifically be determined by a mixture of the red and blue-violet rays of spectrum, i.e., all the spectrum colours minus the green sensation. The crimson lake of some of the leading artist's colourmen is a very fair representation of this primary red of the artists.
Using familiar terms, we may call the primary red of the scientist, "scarlet," and the primary red of the artist, "crimson."
The primary yellow of the artist may be defined as a combination of the fundamental red and the fundamental green of the spectrum, i.e., all the spectrum rays minus the blue-violet. The representative amongst pigments is what artists usually term "lemon yellow," a mixtureof chromate of zinc and chromate of barium, and which printers usually call "primrose yellow." Pale cadmium yellow also is of the same hue. This colour will produce good orange in combination with crimson, i.e., the primary red of the artist, and good greens with- cyan-blue (not violet-blue, such as ultramarine).
The primary blue of the artist may be described as the combination of the fundamental green and the fundamental violet sensation of the spectrum {i.e., all the spectrum rays minus those of the red sensation). This is represented by some kinds of Prussian blue (ferric ferro-cyanide of potassium). Captain Abney, to dispel this confusion of primary colours of light and primary transparent pigment colours, has proposed that they shall be indicated by plus and minus terms. Thus, spectrum red is + R, spectrum green is + G, and spectrum violet is + V; whilst the primary pigment red (crimson) is — G, the primary pigment yellow (primrose) is — V, and the primary pigment blue (cyan-blue) is — R.
We have seen in the first part of this booklet that blue and yellow light produce white light, whilst the artist when mixing blue and yellow pigments produces green. How can this difference be explained? The fact is that the yellow pigment transmits both green and red, whilst the cyan-blue pigment transmits both violet and green. The green is, therefore, the only colour which both pigments transmit, and is the residual colour when they are superposed.
The colours nearest the caloric end of the spectrum, the red and orange, will give us a sense of warmth, as will, likewise, their pigmentary representatives. Yellow, which forms the most luminous part of the spectrum, gives us a sense of light. A slight wash of gamboge over some parts of a water-colour landscape will produce a sunlight effect.
The colours nearest the "actinic" part of the spectrum, the green blue, and violet, produce a sense of cold.
Hence, when we hear artists speak of warm and cold lights and shades, warm and cold greys, we may infer that in the warm lights, shades, and greys, the red or orange preponderates, whilst in cold lights, shades, and greys, the green, blue, or violet preponderates.
It has before been stated that crimson, yellow, and cyan-blue transparent pigments, mixed in suitable proportions, produce black, or if diluted with white a grey will result. Grey may also be defined as white deprived of part of its luminosity. All the constituents of white light are partly and equally absorbed by the body we term grey. A black pigment diluted with white will give a similar effect.
If we mix crimson, yellow, and cyan-blue to produce black, it will be found almost impossible to mix the pigments so that a perfect dead black should result, that is, a black that will absorb all the white light (except, perhaps, the slight percentage which all blacks reflect). It will be found that our mixture, and also most of the carbon blacks, reflect a little orange or yellow besides this small percentage of white, and this orange or yellow will appear brown to the eye. The artists' colourmen and the printing ink makers are well aware of this fact, and try to counteract it by mixing a small proportion of blue with the black.
Instead of diluting his black with white to make a grey, the watercolour artist utilises the transparency of his pigments, which allow the paper to reflect white light throught the paint.
There are colours which are widely spread in nature, and are well represented by pigments, which are not seen in the spectrum but can only be imitated by partly abstracting luminosity. These colours are maroon, russet (terra-cotta), brown, citrin, olive, sage, myrtle, navy-blue, slate, and plum colour. These are broken hues and many people like to call them "art shades." They may be obtained by mixing pure colours with varying proportions of neutral grey. They are often called "tertiary" colours from the erroneous notion that prevails amongst artists that they can only be made up by mixing two of the "secondary" colours. A little reflection and experimenting will show that by mixing two "secondary" colours we really bring together the three primary (pigment) colours in more or less equal proportions, making up a black or grey which saddens the mixture of the two remainmg predominating colours. Thus we have accomplished, by a roundabout way, what can be done more directly by mixing grey with pure primary or secondary colours as the case may be. Another way of stating the matter is that saddened colours are hues deprived of their luminosity.
Saddened red is called maroon.
" red-orange " brown.
" orange-yellow " russet or terra-cotta.
" yellow " citrin.
" yellow-green " olive.
" green " sage.
" blue-green " myrtle.
" blue " navy blue.
" violet " slate.
" purple " plum.
If we dilute pure hues with white we get tints which are also called by familiar names as follows: —
Diluted red is called pink.
" orange-red " salmon.
" orange " buff.
" orange-yellow " cream.
" yellow " straw.
" green " pea green.
" blue-green " sea green.
" blue " azure.
" violet " lavender.
" purple " heliotrope.
" purple-red " magenta.
We may also mention two more familiar colours: —Drab, which is a grey in which orange slightly predominates, and French grey, in which blue slightly predominates.
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