Part 4

The positive is now rinsed in water, and is sometimes stained with an aniline dye in order to render the film more visible. Next it is developed in a stream of water until the surface of the metal is visible between the dots, the last traces of the soluble gelatine being removed with warm water. After drying, the plate is evenly heated over a Bunsen burner until the dots of gelatine mixture turn chocolate colour, when the plate is allowed to cool gradually. This is known as burning in. The plate, if necessary, is now touched up and the back, sides and margins varnished in order to protect them from the acid: when the varnish is dry, the plate is etched in a weak solution (about 2-1/2 per cent.) of nitric acid if the metal be zinc; if the plate be copper, it is usually etched with a solution of iron perchloride.

On taking a proof, there is almost certain to be a lack of contrast, the plate is then fine etched, by which means a considerable improvement can be made; and, by covering certain parts with an acid-resisting substance ("stopping out"), it is possible to fine etch locally.

Incidentally it may be mentioned that machine etching, by which a fine spray of the etching fluid is distributed over the plate, has recently come into vogue, for it is claimed that the results print better and are in other ways an improvement upon the older method.

The plate may now pass through the hands of an engraver, who removes any blemishes, as far as is possible, improves the high lights, and so on; in fact, a skilful engraver can improve the plate considerably.

After the plate is trimmed, and the superfluous metal cut out by means of a routing machine, it is firmly tacked to a wooden mount, usually of oak, but sometimes of mahogany, especially if the plate is large. In order to obtain the best results, the printing, in a typographical machine, should be done on highly calendered paper--so-called "art" paper; in fact, this is absolutely essential if a fine screen has been used; it is only the blocks made with the coarser screens that will give fair prints on ordinary paper. For this reason reproductions made by the half-tone process are very generally treated as plates unless the glazed paper is used throughout the book. The process is used principally for the reproduction of photographs, and for pencil or wash drawings.

With regard to photographs, it has already been mentioned that authors should send the negative or two or three prints differently toned, in order that the operator can choose the one most likely to give the best result.

It is sometimes difficult in a photograph of a landscape to obtain a negative in which the particular feature it is desired to represent--_e.g._, in photographs of vegetation--stands out with the requisite contrast. This is due to the position of the sun at the time of exposure, or to the use of ordinary plates. The remedy for the first is to take the photograph when the proper light obtains; with regard to the second, the use of colour correct plates, together with a colour screen in front of the lens, will obviate the defect. Since for scientific purposes the correct interpretation of the various tones of the vegetation, for example, may not be essential, variously coloured screens may be used in order to emphasise a particular feature. For instance, it will be noticed how well the bushes in Plate 7 stand out. This effect was obtained by the use of a panchromatic plate in conjunction with a red colour screen.

With regard to drawings in wash, charcoal or pencil, in which there are half-tones; these are better drawn on an enlarged scale, especially if the author is not a skilled draughtsman, for improper gradations in shading and other imperfections will not appear so noticeable in the reduced reproduction. Originals should all be made in one colour; in the case of wash drawings, diluted Indian ink (really Chinese ink) will give excellent results.

In making pencil drawings, a fairly stout hand-made paper with not too much grain should be used. If the drawing is to be of some size, the paper may be damped and pasted by its edges on to the drawing board, it will then be stretched quite flat and will not cockle when dry.

The outline of the object may first be sketched in lightly with a soft pencil and then the shading may be proceeded with. To do this, broad-pointed soft pencils, 2 B, 3 B, or 4 B, should be used, and it is better generally to work from the high lights to the shadows. To avoid rubbing finished parts, the work should proceed from the top of the board downwards, especially in the case of large drawings.

In order to obtain a nice gradation and a more smooth appearance--more especially when a very coarse paper has been used--the work may be gone over with paper stumps of appropriate size and softness, and, of course, India rubber may be employed where it is desired to reduce the density of the shading.

When finished, the edges of the various parts may appear woolly owing to the rubbing of the lead; this may be cured by cleaning up the edges with a trimmed piece of India rubber, but in so doing there is always a risk of rubbing out part of the shading, especially if the outline be at all intricate. If preferred, all the shaded parts may be fixed by painting them over with a suitable solution, gelatine for instance, paying particular attention in following the correct outline. When dry, the application of soft india-rubber will soon clean up the blurred edges.

If charcoal be used the same procedure may be followed. Charcoal and pencil drawings should be fixed, in order to prevent rubbing, before sending to the block makers. A suitable fixative may be purchased or one may be made by dissolving white resin in alcohol and applying it to the paper by means of a scent spray or an atomizer. A very good fixative may be made by dissolving a little gelatine in hot water and applying it whilst hot by means of a broad, flat camel hair brush, or ordinary milk may be used in a similar way. After the fixative has been put on, the drawing should be pinned up by one corner--unless, of course, it was pinned up before the fixative was employed, which is the best way when the fixative is an alcoholic solution--and allowed to dry; it may then be placed under pressure in order to flatten it, for fixed drawings generally shew a tendency to curl, especially when the preparation used for fixing has only been applied to one surface of the paper.

In making drawings for reproduction by means of the half-tone process, there are a few general points to which attention should be paid.

It should be remembered that there is not infrequently a tendency towards flatness in the reproduction; it is therefore important that the originals should be "plucky," and, on the whole, it is better to exaggerate with regard to high light and shade, especially if there is much modelling or perspective.

Finally, with regard to lighting, it is better for the majority in drawing their objects--solid objects in relief are referred to--to use a more or less lateral illumination and to represent only the high lights, shades and shadows referable to this main direction of illumination. A high relief will thus be obtained, and the effect will prove more satisfactory than if minor sources of illumination are unsuccessfully dealt with. This is especially important in drawing complicated structures such as models of vascular tissues, embryos, etc.

In cases where many such figures are to occur on one page, it is highly desirable that the lighting of each should be from the same direction.

The use of the half-tone block is now almost universal, so that it is hardly necessary to mention examples, more especially as they are hard to judge without seeing the original picture. Those in the present book are all of a high quality. Excellent examples will also be found in Tansley's _Types of British Vegetation_ (Cambridge, 1911) and in the _Journal of the Royal Horticultural Society_.

Proofs should be carefully compared with the originals, particular attention being paid to the rendering of the tones; as already remarked, fine etching will clear up a block and will often prove a remedy to flatness.

An author will naturally consider whether a photograph is to be reproduced by means of photogravure, collotype or half-tone. It is impossible to lay down any laws on the subject, but the following points should be considered.

If it is essential to have the reproduction in the text, a half-tone block must be used; it must, however, be remembered that the paper used for the letterpress may be very unsuited for the printing of half-tones. On the other hand, if it be immaterial where the picture is placed, then the relative merits of photogravure, collotype and kindred processes and half-tone must be weighed.

Provided that expense need not be considered, photogravure will, in the majority of cases, give the best results; on the other hand, if this process is too costly, then the choice lies between collotype and half-tone. The latter method will often give a result with more contrast as compared with collotype, whilst collotype will give a truer interpretation of the tones.

As has already been remarked, the best results with half-tone blocks only are to be obtained by the employment of a paper which seemingly has no lasting qualities; it therefore follows that if the reproduction forms an important record, the use of collotype is indicated, since many varieties of good paper are available. As a general rule photo-micrographs are best reproduced by collotype.

In order that the respective qualities of these three processes--photogravure, collotype and half-tone--may be compared, Plates 8, 9, and 10 have been made from the same photograph, a view taken by Dr. F. F. Blackman of the Bouche d'Erquy, a salt marsh in Brittany, which was selected chiefly on account of the large number of tones it contains.

These three plates are not entirely comparable, since the heavy shadows in the right hand bottom portion of the photogravure have been touched up by the engraver. This was not intended by the author, but the plate was retained as it shews that directions regarding this point should not be omitted when sending the drawing or photograph to be reproduced. It also indicates that for critical work, when an exact a facsimile as possible is required, collotype should be used, for the plate cannot be touched up.

With regard to the reproduction of drawings shaded by means of wash or pencil, the same remarks apply, with the addition that if it be possible to express what is desired by other means, suitable for reproduction by line block, these latter should be employed. To illustrate this point, Figures 5, 6, and Plate 11 have been inserted; all illustrate the vascular skeleton of a fern (_Marattia fraxinea_), the first one is in outline and the second is shaded by lines of varying thickness; both of these are reproduced by means of the line block, whilst the third is a reproduction by half-tone of a pencil-shaded drawing. In order to obtain a fair comparison, the half-tone is reproduced as a plate, owing to the fact that it would not print satisfactorily on the paper used for the letterpress.

THE HALF-TONE THREE COLOUR PROCESS.--This process is much used for colour reproductions of various subjects; and, in view of the fact that the best results can only be obtained by the best photography, the object should, if possible, be sent to specialists for reproduction. In many cases, however, this is impossible, _e.g._, landscapes and animal and plant portraits amidst their natural surroundings, so that the scientist, if unable to make a water colour drawing, which will give by far the best result, must make his own negatives.

The first thing to do is to purchase a set of colour-filters, adapted to the colour-correct plates to be used, from firms who specialize in these matters, Messrs. Paget or Messrs. Wratten for instance, and from them the inexperienced should obtain full information regarding exposure, etc., for it is essential that the exposure of the negatives should be correlated in order that all may have the same tone-value.

The colour-screens, blue, green and orange, are made by dyeing gelatine with suitable stains; the films are stuck on to perfectly plane glass and are mounted in frames. In practice these screens are usually placed behind the lens, in which case a special camera is necessary, or they may be adapted to fit on to the front of the lens. In either case the procedure is the same; three negatives are taken one after the other through each colour filter, the exposure being modified in order that the tones in each case may be of equal value.

There are thus obtained three negatives which, of course, yield positives which look very different one from the other. These prints may be sent to the block makers, but it is better, on the whole, to send the negatives with clear indications as to the colour of each.

From each negative there is made by contact a transparency, and from these positives there are prepared a set of half-tone negatives from which are made the half-tone blocks.

The reproductions are made by superposed printing of the three blocks, yellow being printed first, then red, and finally blue (Plate 12).

As indicated above, it is hardly worth while to make negatives for this process unless the operator is a really skilful or at least an efficient photographer, and even then the final product may prove unsatisfactory.

Better results are generally to be obtained by sending to the block maker a Lumiere colour photograph with full instructions regarding any corrections in the colours which may be necessary.

Examples:--

Bateson: _Mendel's Principles of Heredity_, Cambridge, 1909. Church: _Types of Floral Mechanism_, Oxford, 1908. Seward: _Darwin and Modern Science_, Cambridge, 1909.

PHOTO-MECHANICAL LINE BLOCKS.--The photo-mechanical line block, commonly known as a zinco, is in a sense the lineal descendant of the wood block. As a means of reproduction the possibilities of line blocks are very great, for not only is it possible to reproduce by their means all kinds of line drawings, but also drawings in charcoal and crayon, provided they be suitably executed on a proper grained surface. In fact, an artist or draughtsman who has a thorough knowledge of the process and its capabilities can obtain extraordinary results. The process has the further advantage of being both quick and inexpensive, a few hours only being required to make the finished product.

Their mode of manufacture is the same in principle as for half-tone blocks; in the case of the latter, the method known as the enamel process was described; in the present instance a different procedure may be dealt with.

A photograph of the drawing is taken on a negative, the wet collodion process being generally followed, although dry process plates may be used.

A highly polished zinc plate is sensitised with bichromate of potash and gelatine, or by other means, and, when dry, is exposed under the negative.

The exposed metal plate is then taken into the dark room and evenly, but thinly, coated with etching ink. When the ink is dry, the plate is developed in water; the unexposed gelatine, and with it, the ink, will come away, its removal being helped by the judicious application of a dabber of wet cotton wool.

The plate may next be "rolled over" with an ink which will more stoutly resist the action of the acid than that used in the first inking, but prior to this it is usual to soak the plate in a mixture of gallic acid, phosphoric acid and gum. This second rolling up must be carried out as if the plate were for lithographic reproduction; and, when dry, powdered resin may be applied, in order to make a better acid-resist, as in the preparation of a lithographic stone.

The plate is now etched slightly in a weak solution of nitric acid; it is then rinsed, dabbed dry and placed upon a hot plate until the resin has stuck well to the ink. When cool, the margins, sides and back are protected from the action of the acid by means of a varnish and the plate is given its first real etch, which is a very slight one. After rinsing and drying, the plate is again heated until the ink and resin have melted and flowed down the exposed sides of the ridges of metal produced in the first etching. This application of ink and resin must be repeated in order that the exposed sides of the ridges may be well covered with the acid-resist and so will not be undercut. The plate is then given its second etch, and this is done with a stronger acid, after which the sides of the lines are again protected with resinous material in the same way as before. The third etch follows, after which the metal is thoroughly cleansed from all the ink, etc.

In order to smooth the shoulders of the lines, the plate is given a finishing etch: the cleaned plate is warmed and rolled up with hard etching ink; the metal is then heated until the ink becomes glazed, and, when cool, is placed in the acid bath for the requisite amount of time. If necessary the plate, after cleaning, is touched up with a graver, and the superfluous metal is cut away. Finally it is mounted on a block of wood, and after the corners and sides have been trimmed square, the block is ready for the press.

To illustrate the enormous improvement which may result from the block passing through the hands of a skilful engraver, two line blocks of a wood engraving by Riocreux (see p. 36) have been prepared. Fig. 7 is the impression given by the block as ordinarily turned out, whilst Fig. 8 is a precisely similar block which has been worked up by an engraver.

There are several other methods of making the blocks, but the principles are the same as in the foregoing process.

In examining the proofs it must be remembered that deletions are not the only alterations that can be made in the finished block; not only can lines be cut away, or their character altered by removing part of the metal from them, but additions can be made in reason. For instance, lines can be added across open spaces, and if part of the printing surface has been accidentally removed in cutting away the superfluous metal, the damage can be made good by building up with solder and working on this with the graver. If, however, the additions required are at all extensive or complicated, it is better to have a new block made.

Inasmuch as scientific illustrations are to describe and explain definite facts, the drawings must needs be materialistic rather than suggestive; in other words, a more or less conventional system must be employed.

In making their drawings for reproduction by line blocks, authors have at their disposal the black line, the white line, the black space, the white space, the black dot and the white dot, all of which may be combined in various ways. No tones, other than black and white, are available; if it be desired to represent half-tones, they must be rendered by the above-mentioned means.

In the majority of cases the originals should be made with black ink on white Bristol board or smooth white paper; ordinary lead pencil drawings on smooth paper are useless, and lead pencil, black crayon or charcoal in combination with grained paper or board should not be employed unless the draughtsman has the requisite skill and knows exactly the limitations of the line block. For all ordinary folk black ink and Bristol board cannot be improved upon.

The drawing may be made first with a soft lead pencil, using the camera lucida or other optical aids to correct delineation. The pencil lines are then gone over with ink; for this purpose a good black ink is necessary. Wolff's Indian ink, Higgins' waterproof ink and Steuber's waterproof drawing ink are highly satisfactory, and there are many others. With regard to pens, a suitable implement is all-important; Gillott's lithographic pens and Brandauer's No. 515 are recommended. For straight lines of an even thickness a ruling pen is very useful, and these may be obtained fitted with an adjustment which enables the worker to rule a line of a definite thickness, _e.g._, .5 mm. and so on.

All drawings should be made larger than it is intended the reproduction to be, for slight inaccuracies, ragged lines, and other blemishes will thus appear less obvious. This drawing on a large scale is often a stumbling block, because the work appears too open and the draughtsman is tempted to put in too much; this must be avoided, else the crowded lines may join together in the reduced reproduction. Also it must be remembered not to draw too finely, else the work in parts may disappear entirely in the reproduction. In drawing on an enlarged scale a certain amount of exaggeration may be employed, in order that when reduced the drawing may not be quite spiritless.

When representing a solid object, such as a plant or an animal, to shew the external morphology, it is to be borne in mind that form is the main thing to represent, and this can be expressed by outline drawing alone. In fact, more or less primitive methods must be employed, and better models cannot be followed than the best wood cuts.

An examination of figures 10 and 12 will shew that Fuchs[A] attained his object by simple outline drawings; he never employed local colour, and shading he used very sparsely indeed, and then only to give expression to the form of some thick part. Fuchs's celandine (Fig. 10) should be compared with the drawing of the same plant (Fig. 9) by R. G. Hatton.

[Footnote A: The methods followed by the illustrators of the Herbals may be conveniently studied in Hatton's "The Craftsman's Plant Book," London, 1909, and Arber's "Herbals," Cambridge, 1912.]

The methods of Matthiolus (Figs. 11 and 13) were somewhat more advanced, for he used shading not only to express form but also to give a certain amount of relief. It will be noticed that he shaded by lines which followed the moulding of the parts.

The work of Riocreux (Fig. 8) should also be studied; it will be observed that he managed to get a very high relief in his drawings by the simple means of straight or curved lines, according to the shape of the part, of varying thicknesses.

There is no necessity for keeping all the lines of even thickness. For instance, provided the character of the form is not altered, the outline on the shaded side may be made thicker than on the illuminated side; also distance can be indicated by the use of thinner lines, for these, although really black, will give the impression of greyness. Then again, a line may be drawn with local increases and decreases in thickness, as in ordinary writing, and such lines drawn by a skilled hand can be made to express a marvellous amount of modelling.

The draughtsman, however, is not restricted to lines; any marks which can be made with a pen and black ink may be employed, provided they be sufficiently firm and large.