Photo-mechanical Processes

Photo-mechanical, with the exception of the Woodburytype, refers to a print made with ink where a photographic process has been used in producing the printing block, plate or stone. Woodburytype is the exception as the image is formed by a relief image of gelatine containing carbon, it was, though, a commercial process worked along side other photo-mechanical processes. Most of these processes were worked as an industrial reprographic process, hand-pulled photogravures were also produced by photographers as original art work.

Printing can be categorised into three types:

• Relief - where the ink lies on raised portions of the plate, e.g. woodcut, letterpress.
• Intaglio - where the ink lies below the surface of the plate, e.g. engraving.
• Planographic - where the ink lies on the surface of the plate, e.g. lithography.

Each of these has been used in a photo-mechanical context.

A photograph is continuous in tone, in the photogravure process light to dark shades of the original are represented by areas of varying depth on the plate that transfer more or less ink to the paper. In other printing processes the mid-tones are simulated by lines, dots or areas that are either inked or not inked. In intaglio processes after inking, the plate has to be wiped clean of surplus ink, for this it is necessary for the ink to be contained in lines or cells surrounded by raised lines, without which the ink would be pulled from the plate.

Collotype

The origin of the collotype process lies in Alphonse Poitevin patent of 1855 though it was a few years before the practical steps were developed sufficiently for it to be worked commercially.

The process uses a sensitive layer of bichromated gelatine that is treated with heat and water, this causes the gelatine to swell and form random microscopic reticulations across its surface, it is these that hold ink and allow mid-tones to be reproduced. When exposed under a negative the parts of the reticulated surface affected by light (shadow areas) harden and become water repellent, the unexposed regions remain receptive to water. Prior to inking the surface is treated with a water/glycerine mixture, the water is repelled by the exposed areas which will then take-up ink, the unexposed areas will hold water and reject ink.

The process is screenless and uses a continuous tone original. The fine and irregular ink deposits produced high-quality prints that are often difficult to distinguish from photographs. The surface is often left matt but could be glazed, different colour ink was used. The process was used extensively for high-quality book reproductions, postcards and prints.

Collotype, like other photo-mechanical process, developed into several different variants and sub-processes and was worked in different ways by commercial companies, it was also combined with other processes such as photo-lithography.

In its basic form the process consisted of:

• Coating a glass (later metal) plate with a sub or bonding layer of albumen, gelatine or similar.
• The plate was coated with a light-sensitive gelatine bichromate solution.
• The coated plate was heated and allowed to cool.
• In this condition the reticulations have formed and the plate could be exposed under a negative.
• The exposed plate was then washed in a cold water to remove the excess dichromate.
• After drying the plate was soaked in a water/glycerin mixture.
• The plate was then ready to be inked and printed from.

Collotype was known by several names including Phototype, Albertype. A three-colour version of the process was developed. The process was never that easy to use and during the twentieth-century it was replaced by lithography and high-resolution half-tone work.

The image on the right shows the surface of the print taken with a microscope magnification of 48x.

Heliotype

Heliotype is a variation of the Collotype process developed by Ernest Edwards around 1870. Chrome alum is included in the gelatine/bichromate layer which hardens the gelatine making it less prone to swelling. After exposure a slight dampening of the surface is enough to repel greasy ink in the unexposed areas. The gelatine sheet is transferred to a pewter plate prior to printing. The gelatine surface is sufficiently hard to be printed from in a flat bed press. The gelatine was usually rolled with two inks, a stiff ink for the fully exposed areas and a thin for the mid-tones. The process is screenless and uses a continuous tone original.

The process allowed 1000 to 1500 prints to be made from a single gelatine sheet (matrix). Advertising stressed that the print was ready for binding as it came from the press, it did need to be trimmed and mounted.

Half-Tone (Relief)

Half-tone is not an end-to-end printing process but, as the term is used here, a technique for translating the continuous tones of an original into a set of dots that will be represented by ink when printed (similar to the modern process of digitising). The most common method was to translate the original into dots of differing size by the use of a cross-line screen consisting of a fine grid pattern ruled on a glass plate. The resultant half-tone image would be used in either a letterpress or photo-lithographic printing process. When viewing an image produced by half-tone the dots fuse together to simulate the continuous tones of the original. An important factor in developing a half-tone relief process was the ability to print images alongside type in the same press.

Simply breaking up the image into lines or dots, by the use of screen, would not produce a half-tone image, it is necessary for the dots to vary in size in proportion to the tone of the original.¹ The screen is placed a short distance in front of the photographic plate, light passing through the grid will spread out and produce dots on the plate proportional in size to the light falling on that part of the screen. A high-contrast plate was used that reduced the image to simply black and white without mid-tones. The stop in the lens was not always circular, its size and shape had a direct bearing on the dots produced. Screens came in different degrees in fineness, measured in lines per inch, a finer screen will produce an image with more detail and be more faithful to the original. A course screen would be used when printing newspapers or posters that are viewed at a greater distance.

Colour printing by the half-tone process involved producing three negatives for each of the three subtractive primary colours from which printing plates could be made. A fourth plate, printed in black, showing shade variations was often included.

The image on the right shows the surface of a half-tone print taken with a microscope magnification of 48x.

Development of Half-Tone

Charles Petit (1878) proposed printing a photograph on to bichromated gelatine, after development this formed a mould from which a plaster cast was taken. The cast was coloured black and then ruled with a V-shaped tool to produce a series of lines at right angles to each other. Where the cast was deepest only a small cut would be made, where the cast was thickest the tool would make a wider line, the result was a series of dots that varied in size. This was re-photographed to produce a new original. ²

F.E. Ives (1881) proposed a similar method, he produced a relief cast from a photograph, a rubber sheet which was uniformly covered with small cones was coated in ink and pressed against the cast. The rubber cones were more or less spread depending on the relief of the cast leaving large or small dots of ink on the cast. The cast was then re-photographed.³

Modern half-tone was introduced by Ives in 1886 when he used a cross-line screen placed a short distance from the plate. Cross-line screens consisting of two ruled and etched glass plates cemented together at right angles were perfected by Max Levy around 1890.

Photochrom

This was a hand-coloured half-tone process printed from lithographic plates.

The working details of the process remained secret but it seems that a black & white print was hand-coloured, this was then photographed through coloured filters and masks to produce colour separation negatives, one for each colour used at the colouring stage (possibly several partly coloured prints were photographed). These were then printed onto lithographic plates, after being rolled-up with the appropriate colour ink they were printed successively to give the final print. A half-tone screen was introduced somewhere in the process.

The process was developed at the firm of Füssli in Switzerland but licensed to other printers, it was most popular during the 1890s.

Photogravure

The origins of photogravure can be traced to W.H.F. Talbot's work in the 1850s, his patents are general in nature covering photo-engraving but include the use of bichromated gelatine which forms part of the photogravure process.¹ Bichromated gelatine was a light-sensitive substance used to transfer the original image to the printing plate and provide a resist to the etching fluid. This was the first use of bichromated gelatine, Talbot also introduced the use of screens to breakup the image.² Photogravure was developed into a commercially workable process by Klíč in 1879 and was known as the Talbot-Klíč process. At first the process was worked with flat-bed presses, later Klíč developed a process suitable for rotary presses (known as rotogravure). Photogravure was used as a general term encompassing rotogravure prints.

Photogravure is an intaglio process, that is, the ink lies below the surface of the etched plate. Its important characteristic is that ink is retained in small pockets of varying depth that deposit varying quantities of ink on the print. To achieve this a screen or irregular aquatint ground is used to divide the image on the printing plate into either a fine grid pattern or, when an aquatint ground is used, minute irregular areas.³

Prints have great tonal range with a soft velvety look to the shadow areas. They are often printed in brown or sepia ink and can be difficult to distinguish from a real photograph.

Photogravure

was mainly used for the reproduction of photographs and fine-art printing. The process starts with a continuous tone positive which is printed onto a carbon tissue.⁴ The gelatine of the tissue becomes less soluble in proportion to the amount of exposure through the positive transparency. This gives a relief image of the original which acts as a resist at the etching stage. Several successive etching baths of different strength are used, local control can be introduced by stopping out the etch with varnish. As later practised a photographic image of an aquatint ground or screen could be printed onto the carbon tissue at the start of the process rather than using a ground on the printing plate.

The individual steps are:

• An aquatint ground is given to a polished copper plate by dusting with fine bitumen, resin or similar particles, these are fused to the plate by heat.
• A carbon tissue is exposed to light under a positive transparency.
• The exposed carbon tissue is attached to the copper plate paper side uppermost, the paper is removed with warm water.
• The less exposed and more soluble gelatine is dissolved in hot water. The more exposed, less soluble, gelatine remains on the surface of the plate to act as a resist of varying thickness.
• The plate is etched in ferric chloride (perchloride of iron) solutions of different strengths. The plate is then cleaned of any remaining residue and is ready for printing in a flat-bed intaglio press.

Rotogravure

was used for high-volume printing such as magazines and post-cards using a cylinder press. In principle the process is the same as photogravure except that a regular screen is used rather than an aquatint ground. The screen provides a datum surface for the doctor blade to wipe the cylinder.

The image on the right shows the surface of a rotogravure print taken with a microscope magnification of 48x.

Woodburytype

The Woodburytype process was developed by Walter Bentley Woodbury (b. 1834, d. 1885) based on a series of patents from 1864. The process sits between being photographic and photo-mechanical; it can be thought of as producing carbon prints mechanically, on an industrial scale. It is a continuous-tone process, no screen or ground is used.

A relief image, from a photographic negative, was produced in bichromated gelatine, this was placed in contact with a lead block and subjected to considerable pressure in an hydraulic press. The result was a lead mould varying in depth in proportion to the tonal areas of the original subject. Pigmented gelatin was placed in the mould and transferred to paper using a press. The Woodburytype produced a continuous tone image consisting of varying depths of gelatine, as no screen or ground was required the result was indistinguishable from a carbon print. The gelatine relief could be re-used to produce further lead moulds.

Woodburytypes were normally printed on thick paper or card, they could be produced in any colour but most often they were in a sepia tone that resembled a photograph, an extra coating of gelatine or similar was often applied to the finished print. Due to the pressure needed to create the mould the images tended to be small. Woodburytypes were most suitable for high-quality reproductions and were typically used for book illustrations, cartes de visite and also lantern slides. By the 1890s its use in book illustration was declining as processes such as photogravure proved more economic.

Stannotype

Woodbury introduced a related process called Stannotype. In this process the gelatine relief is not pressed into a lead block to produce the mould, thus the need for an hydraulic press was removed. The process starts with a positive transparency, from that a gelatine relief negative was produced, that was coated with a thin layer of india-rubber varnish. Tinfoil was laid over the gelatine relief and passed through a pair of rollers that pressed the tinfoil into the gelatine relief to produce the printing mould. Printing from the mould was the same as in the Woodburytype.

Woodbury continued to work on photo-mechanical printing including printing directly from the gelatine relief combined with a screen to reproduced half-tones. Disderi also patented a photo-mechanical process in 1867.