INTAGLIO PRINTING

I N the biological sciences the massing of illustrations into plates is still the favourite method of illustration, although text-figures have recently become more numerous.

This is partly due to innate conservatism, for most of the earlier memoirs were so illustrated, doubtless because it saved time, since if wood engravings were used with a view to text-figures, the compositor had to wait for the blocks, whereas in the case of plates the compositor and the engraver worked independently. Also the possibilities of plates are enormous; they may be very beautiful indeed besides being biologically satisfactory, for much finer results can be obtained by engraving metal than by engraving wood. Then again there are many different processes available for the making of plates, so that if one proves unsuitable for a subject an excellent reproduction may be obtained by another.

Before passing on it is desirable to point out the essential differences in the three ways of printing.

Intaglio printing. If the finger-tips be examined, many ridges and furrows will be seen on their under surfaces; if now a thick ink be well rubbed into these so as to fill well the furrows, and the superfluous ink be wiped off from the general surface, an impression will be obtained of the furrows on pressing the fingers on to a piece of smooth white paper. Better still, if the copper plate of a visiting card be examined, the name will be found cut into the surface. If an intimate mixture of tallow and lamp-black be well rubbed into these depressions and the excess of ink wiped off the surface of the plate, an impression can be obtained by placing a piece of damp paper on the plate and passing both through the domestic mangle—the kind with rubber-covered rollers. In each case the principle is the same, the pressure forces the paper into the depressions of the plate so that it takes up the ink.

Plane surface printing. This is characteristic of lithography and allied processes. Writing or a design well chalked on a blackboard can be transferred on to a smooth piece of paper merely by a little vigorous rubbing on the back of the paper placed in position over the drawing. The transfers of childhood provide a further simple illustration, so also does the hectograph (jellygraph).

Relief printing. In this case, the design is raised above the general surface of the substance. A rubber stamp is an obvious example.

It will be noticed that intaglio and relief are the reverse one of the other, whilst plane-surface printing is intermediate between these extremes. In intaglio, the ink is taken from a depression; in relief from an elevation; and in flat printing from a plane surface.

INTAGLIO PLATES. There are several methods of making intaglio plates, but only a few are used in the illustration of scientific papers; attention however may be drawn to the others, not only for their own sake, but also on account of their influence on some modern photo-mechanical processes.

LINE-ENGRAVING. Line engraving, by which is meant cutting lines into copper, steel, or other suitable material with a burin or graver, is a very ancient art. Its employment for illustrative purposes is an outcome of the art of the metal workers—particularly the Florentine goldsmiths of the fifteenth century—who filled up the lines cut in the metal with a black enamel of silver and lead sulphides (niello) which was made by heating together a mixture of these metals with sulphur. This enamel when once in was very hard to remove, so that in order to see how their lines were progressing, the artists rubbed well into the metal, in order to fill up the lines, a sticky ink. The superfluous ink was then wiped off the general surface of the metal and a piece of paper was placed in position and pressed sufficiently hard to make it enter the depressions, which alone contained the pigment, and take up the ink. A print was thus obtained of the work and so its state was ascertained.

Metal engraving is carried out in the same fashion at the present time. A flat plate of copper or steel is well polished and is worked upon with a graver or burin, so that the picture is represented by lines cut into the metal. Any line, however fine, will give an impression on printing, hence it is hardly surprising that engraving has long been a popular means of expression by artists, since force, depth and delicacy are possible of attainment.

The printing is carried out in exactly the same way as by the early metal workers: the plate is covered with a thick ink which is forced well into the lines and then the superfluous ink is removed. The plate is now ready for printing; to do this, the plate is placed in the bed of a copper-plate press and over it is laid a sheet of damped paper which is covered with two or three layers of blanket. The whole is then passed under the roller which forces the paper into the incised lines, so that not only is the ink picked out, but a mould of them is taken on the paper, hence the very finest lines will give an impression. Having passed through the press the paper is carefully peeled off, and thus the print is obtained.

With regard to the metal employed, copper is commonly used, since it is soft and easy to work; its softness however is, in a sense, a disadvantage, since the plate will soon wear, the finest lines being the first to go, so that a limited edition of good impressions only is possible. To overcome this difficulty, the plate may be faced with steel, by which means it is rendered very durable.

Steel, although once popular, is not much used nowadays owing to its hardness and the rapidity with which it rusts. As compared with copper engravings, steel gives a somewhat harder line, whilst copper gives a soft line, but this, of course, does not mean that steel engravings are harsh; the finest work can be done on steel and of remarkable delicacy.

At the present day line engraving is seldom or never used as a means of illustrating scientific work. It is obvious that the average scientist has not the time and he certainly does not possess the skill to make his own plates; the engraver must translate the originals into lines, so that much consultation would be necessary. Further, a line engraving takes a long time to make, and most publishers would certainly look at the expense.

In the past, however, the line engraving was much used, and very beautiful work was often accomplished. The following works contain outstanding examples.

Bojanus: Anatome Testudinis Europaeæ, Vilnae, 1819-1821. The plates are beautiful engravings by Lehmann after the drawings by the author.

Chatin: Anatomie Comparée des Végétaux. Good steel engravings illustrating the structure of various plants.

Curtis: Flora Londinensis, London, 1777. The illustrations are hand-coloured copper engravings by Sowerby and others, many of which, particularly the earlier ones, are of outstanding excellence. The engraving is often nothing more than the mere outline of the plant, whilst in cases where the structures are more massive, a certain amount of shading is used. The colouring is very good indeed, and it is obvious that much care was taken not only in the actual painting but also in the choice of pigments which, as far as can be judged, are as fresh now as when first used.

Curtis's Botanical Magazine and Edwards's Botanical Register contain some excellent examples of hand-coloured copper engravings.

Levaillant: Histoire Naturelle des Oiseaux d'Afrique. Paris, 1805-8. This work contains beautifully coloured engravings by Feesard. The original drawings were by Reinold.

Lyonet: Traite Anatomique de la Chenille. La Haye, 1762. The plates are amongst the best illustrative of zoological science.

Martius: Flora Brasiliensis. The earlier volumes, e.g., Vol. 13, Part I, contain excellent engravings.

Passæus: Hortus Floridus. Arnheim, 1614-17.

Sowerby and Smith: English Botany. London, 1790-1866. The illustrations are hand-coloured copper engravings.

Thuret et Bornet: Etudes Phycologiques. Paris, 1878. This work contains the finest plates ever published in a botanical work. Riocreux drew from the preparations, and his drawings were engraved on steel by Picart, Thomas and others.

ETCHING. Etching is a term very loosely used; strictly speaking it consists in corroding a metal plate or a flat stone with acid, or other substance possessed of a kindred action, so that depressions are formed. A pen and ink drawing, although usually so termed, is not an etching. Briefly the method is this: a well polished copper, steel or zinc plate is covered with a substance, known as the etching ground, consisting commonly of a mixture of asphaltum, white wax and pitch, which resists the action of the acid. The ground may be laid in more than one way; the simplest, perhaps, is to dissolve the etching ground in some solvent such as chloroform, which readily volatilises, and to pour the solution on to the plate, which is tilted this way and that until the liquid is evenly distributed; the excess is poured off and what remains is allowed to dry, the plate being kept level during the process.

The plate is then warmed until the ground is softened, when it is held over a smoking candle and is rapidly moved here and there so that if properly done the fine soot is evenly incorporated in the ground. When the plate is cold, the drawing may be made by cutting through the etching ground, so as to expose the underlying copper, with needles of various sizes. The work is then etched by means of dilute nitric acid.

When this is satisfactorily accomplished, the ground is cleaned off, the plate well inked with copperplate ink, and the surface ink removed by coarse muslin. The plate is then gone over with fine muslin, but the ink must not be removed from the depressions; finally the damped paper is placed in position and impressions obtained by the use of the copper-plate press.

Etching, although suitable, especially when natural-printed,* for the illustration of many scientific subjects, is but seldom employed at the present time for this purpose; the preliminary announcement of Warburg's Die Pflanzenwelt, however, states that some of the illustrations are etchings.

*A plate is said to be natural-printed when all the ink is removed except from the depressions; in artificial printing some ink is allowed to remain on the flat parts. Artists frequently, after removing the superfluous ink, lightly dab the plate in order to make the pigment spread slightly beyond the actual limits of the depressions; this is known as retroussage.

SOFT-GROUND ETCHING. This is a somewhat rare method of reproduction nowadays; it may, however, be described briefly, for it would appear to be suitable for scientific purposes, since it should not prove a matter of great difficulty for an author who is a sufficiently skilled draughtsman to make his own plates. The polished copper plate is laid with ordinary ground to which is added lard in a quantity according to the warmth of the weather.

Over the plate is then placed a sheet, larger than the plate by an inch or two, of damp, thin, grained paper, the edges of which are folded over and pasted to the back of the plate. When the paper is dry it will be well stretched and in close contact with the plate. With the hand resting on a bridge, in order to avoid inadvertent touching of the plate, the drawing is made on the paper with a pencil of a hardness suited to the softness or otherwise of the etching ground. When the drawing is finished the paper is carefully removed; wherever the pencil has been used, the etching ground will adhere to the paper, so that in such places the metal will be exposed. The plate is then etched and printed as in the normal process.

No reproductions of drawings of scientific subjects apparently have been reproduced by this method. Examples can conveniently be examined in The Seven Lamps of Architecture by Ruskin.

MEZZOTINT. The characteristic feature of mezzotint is that the subject is translated into tones rather than lines as in the preceding intaglio methods.

The surface of a smooth metal plate—usually copper—is raised into innumerable and minute projections by going over it in all directions with a curved steel tool, known as a rocker, the edge of which is finely toothed. An impression taken of the plate in this condition will give a deep rich tone. The high lights are obtained by scraping and burnishing away the elevations so that there are no pits left to hold the ink, and, similarly, intermediate tones are produced by partly removing the pile so that the pits are made of varying degrees of shallowness and consequently will print in tones according to their depth.

Impressions are taken in the same way as in the case of etchings.

Mezzotint apparently has never been used for the reproduction of scientific subjects. Indeed, in a sense, this process is much too artistic for the purpose. At their best, illustrations reproduced by this method have mystery and depth and give the imagination much employ; in a word, they are subjective rather than objective, qualities unsuited for our purpose.

PHOTOGRAVURE. Photogravure may next be considered, for although it is a photo-mechanical process, it corresponds to mezzotint. Excellent results may be obtained by its use provided the drawings—usually executed in monochrome such as sepia—be really good, otherwise they are hardly worth reproducing by this relatively expensive method.*

*This account refers only to plates made and printed entirely by hand, not to photogravure for printing on a rotary machine.

Photogravure is particularly suitable for the reproduction of drawings showing a large amount of detail and made up of a variety of tones rather than lines or stipple.

The photographic part of the process is essentially the same as making a carbon print from a photographic negative. This consists in exposing under the negative the carbon tissue, which is a mixture of gelatine, in which is dissolved bichromate of potash, and a suitable pigment. Such a film of bichromate gelatine is, when dry, sensitive to light. If no light gains access to it, the gelatine is readily soluble in warm water; if light acts upon it, the gelatine becomes insoluble in proportion to the degree of its exposure. Obviously, the pigment will be retained in varying degrees according to the relative insolubility of the different parts of the gelatine.

The carbon tissue having been exposed, is rolled down on a wet sheet of paper covered with some adhesive and is dried under pressure. The paper is then soaked in warm water when the basis of the carbon tissue easily peels off; the picture is developed by laving in warm water, which will dissolve the gelatine in proportion to its exposure to the light. The print when dry is remarkably permanent and, from the picturesque point of view, is infinitely superior to the ordinary silver print.

The method of making the photogravure plate is, in outline, as follows:

The original drawing is photographed, and it is very important that the negative should be as perfect as possible.

From the negative, a positive is made upon transparency carbon tissue which is mounted upon a sheet of plate glass. The procedure is, in essentials, exactly the same as described above for the making of a carbon print. This positive when dry may be touched up; after which a negative, which also may be touched up, is made from it upon an ordinary sheet of carbon tissue.

The negative so obtained is transferred to a prepared plate of copper, developed with warm water and dried.

The copper plate is prepared as follows: after being well polished until quite free from all scratches, the surface is dusted over with finely powdered resin or, more usually, bitumen. The plate is then heated until the dust adheres.

After the carbon negative has been stuck on to the plate, developed and dried, the margins and back of the copper are protected with an acid-resisting varnish. When dry, the plate is placed in the etching bath of nitric acid or, more generally, of ferric chloride. The etching fluid will pass through the thinnest parts of the negative first, so that the surface of the copper will be etched to a degree corresponding to the thickness of the gelatine. The high lights on the negative obviously will be represented by thick coatings of gelatine, consequently such parts will be but slightly etched and vice versâ.

If the plate had not been laid with resin, the surface after etching would show more or less extensive depressions and elevations; but the grains of resin protect the copper immediately beneath them from the action of the acid, which consequently can only dissolve the exposed parts of the metal between the resinous particles. The result is, therefore, that the plate is covered over with numberless fine pits of varying depths. The deepest ones will, on printing, give the darkest tones, since they will hold more ink, the shallower ones will give the lighter tones, whilst the shallowest and those parts unetched will give the high lights.

The plate is usually etched three or four times successively in varying strengths of fluid, after which the etching ground and gelatine is cleaned off.

A strong copper-plate ink is then well rubbed in by means of a dabber, after which the ink from the surface is removed, first with a coarse piece of muslin and finally, with fine muslin. The ink must not be removed from the pits.

The first pull is then taken as in a line engraving with a copper-plate press, and its appearance shows what corrections are necessary. The plate nearly always requires a certain amount of engraving; the high lights may be improved by means of a burnisher, the shadows by means of a rocker or a roulette—a small steel wheel the rim of which is beset with fine teeth—and so on. Finally, if a large edition is required, the plate is steel faced.

Although much used for the reproduction of pictures, photogravure is too rarely employed for scientific purposes; this is to be regretted, for the process is admirably suited to the reproduction of photographs and drawings with delicate tones. As compared with the usual half-tone, the cost is high, and this no doubt militates against its use.

Examples of outstanding excellence will be found in the New Phytologist, Vol. xi, 1912, plates 5 and 6. These are absolute facsimiles of the original drawings by Mr. McLean, both as regards colour and reproduction of tones. Plate 8 may also be examined and compared with plates 9 and 10 which are reproductions of the same subject in collotype and half-tone respectively.

PLANE SURFACE PRINTING

Plate 1.—An original lithograph by Mr. Harry Becker.