How to create  tone  in etchings

Etchings and the etching process explained.
Ryepress Etching Studio
39 High Street Hastings
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Part 4
For most printing processes printing is done off one surface and so using one colour effectively means any mid-tones have to be achieved by an optical illusion. With the mechanical half tone screen the original is photographed and the amount of light is separated by a grid into black or white (sort of like the way in which the UK is divided into parliamentary constituencies each returning one seat in parliament)  This is the way most commercial lithography is produced - the Dots per Inch measurement is the measure of how convincingly tones will be rendered:. A half tone of 300 DPI fools  the human eye - 150 gives about newspaper photo quality and is less believable!
The engraver working on a plate by hand creates the illusion of tone by using a similar method but with cross-hatched lines or stippling. As the metal  has to be removed by physically cutting into the plate a certain consistent amount of force has to be applied - resulting in hatched lines of the same depth often running parallel and in many cases giving a mechanical or more controlled appearance. Accomplished engravers can be judged by the looser, fluid and more varied marks they use - reproduction bookplates and engravings that are copies of paintings are often noticeably more constrained and consequently lack the vibrancy of the original .
The obvious problem for the artist working in monochrome (most often Black and White) is how to avoid the overly mathematical patterns caused by half tone screening or the monotonous and systematic cross hatching used in reproduction prints. The Holy Grail of such artists is Continuous tone as best achieved in the traditional Black and white photograph.
Traditional photographs work on the principle of a flat surface covered by a fine grain that reacts to light. Each particle of grain is effectively analogue: it can be an infinite number of shades of grey (as opposed to halftone screening which is essentially Digital (Black or White). This photographic grain reacts according to the amount of light and the size of the grain - Fast film reacts quicker and is obviously grainier with less fine detail and higher contrast, slow film is more detailed. (Photographic papers work the same)
Which comes to Etching’s trump card.
Cross hatching used to give the illusion of tone
Cross hatching and Half tone. Example of tone being achieved from one colour by half tone screen, and hatching,
Roy lichtenstein Benday dots
Cross hatching tones
Half tone dots showing tones with black ink
Deriving its name from the etching process originally used to reproduce the washes of watercolours, aquatint works in the opposite way to standard etching. A fine resin powder is agitated in a sealable box using a paddle turned from the outside. After a period of time to allow the heavier particles to settle the box is opened (taking care not to breath in any of the powder). A degreased plate is placed face up on a mesh shelf and the box is closed. After allowing the fine resin particles to settle on the plate it is carefully removed and gently heated. This melts the coating of resin dust and bonds it to the plate's surface. The dust particles form a permeable resist to the acid: each particle preserving a tiny island of the original plate surface round which the acid can bite. Stop out varnish is painted on any areas required to remain unbitten, the plate is immersed in acid, removed, stopped out again for the next tone, and so on. Depending on the density of the particle distribution, biting times can be very short; the resin forms what will eventually print as white dots against the much wider expanse of exposed plate.
Etching is related to engraving in that it is an intaglio process; the ink is pulled out of the incised lines and deposited on the paper by the action of a heavy press as opposed to transferred from the surface, as in relief printing. The work of removing metal from the lines is done by the action of acid and is infinitely more controllable than the purely physical action of the engravers burin. For this reason the depth of lines can be precisely controlled so that they consequently hold varying amounts of ink. Whilst deeply bitten lines will be almost exactly the same width as more shallow ones (especially with Dutch Mordant on copper) they will print a correspondingly darker and richer tone.
Using cross hatching in conjunction with a series of biting stages means closely hatched areas, lightly bitten can be "interlaced" with widely hatched areas heavily bitten to create rich textural tones unachievable in any other printmaking media.
The effect is to create an elaborate network of intermeshed "trenches" in the metal each dug to a specific level. These are far more delicate than standard hatched lines and can  be subtly modified by burnishing to create new tones even within the existing tones. The action of the burnisher rounds off the edges of the "trenches" lightening  and softening the steps between tones  to create smoky graduations ideal for clouds and for modelling contours.
Etchings using tones created by a combination of soft ground and hard ground - not Aquatint
An example of etching using only aquatint
Norman Ackroyd RA,
Blasket, Co. Kerry,
Etching - Aquatint
1 2 3 4

The biggest challenge faced by any printing technology is the problem of representing tonal variations using only one colour or a minimal combination of colours. In colour printing the range of colours is usually the familiar CMYK - Cyan, Magenta, Yellow and Black. K is used for Black in the abbreviation as it is the KEY colour - usually printed last to mask any slight variations in registration (the process of lining up each colour). It is the “key” colour to which all the other are aligned.. As each colour cannot be printed lighter or darker it is necessary to give the illusion of lighter tones by breaking the image up into variably spaced and sized dots. This is called half tone screening and the results can easily be seen either through a magnifying glass or by standing close up to a billboard. The CMYK 4 colours are a refined and optimised  version of the  basic primary colours: Blue, Red and yellow - which, with  Black gives the painter  an infinite variety of colours. The 19th Century art movements   Impressionism and pointillism show how artists were already looking at different ways of “mixing” colours optically by using adjacent primary colours at about the same time colour printing was evolving. The full circle was effectively reached with Roy Lichtenstein’s use of cheaply printed comic book iconography with its crude and obvious “Benday” dot patterns.
A traditional etching press
Aquatint box showing paddle mechanism
Preliminary work on the plate - sketching out areas requiring tones - can be lightly etched into the plate using soft ground in much the same way as a watercolour can be lightly sketched out in pencil.
The final result of aquatint is fine white unbitten dots against a grey background and it should be noted each unbitten "pinnacle" of metal will be extremely susceptible to wear and the plate is unlikely to yield a large edition.
A similar but physically opposite way of creating steps of flat tone akin to aquatint is to run a plate prepared with hard ground through the press with a sheet of fine sandpaper face down on the surface. This will create the negative effect to aquatint with the surface covered in a in a galaxy of pinpricks of exposed metal. The plate is stopped out in stages as with aquatint but care must be taken to cover the plate evenly and densely with dots, and that the pressure is not enough to scratch the plate (if the pressure is too great the small pinpricks in the hard ground will still be evident on the surface of the metal even in  the stopped out (white) areas)
Aquatint box with handle for turning paddle to agitate Resin/Rosin
Although the first imitations of wash drawings in print making can be traced back to the middle of the 17th century, it was in the course of the 18th century that this method was perfected. The engraver Frantçois-Philippe Charpentier announced in the "Avant-Coureur" of the 10th of July 1762 that he had invented a machine "to engrave in a way that imitates wash processes". In 1780, Jean-Baptiste Le Prince, painter, drawer and engraver, whose sepia drawings had had great success, decided to reproduce them by means of engraving and consequently presented a paper to the Académie Royale de Peinture (Royal Painter's Academy) entitled Plan du traité de la gravure au lavis (Outline of a treatise on wash engraving). However, and despite Le Prince's personal success, his method had very little success in France and it was only quite some years later, after having undergone various modifications abroad, that aquatint returned to France at the end of the 18th century to occupy a place of honor. The term aquatint was coined in London during this period of exile.
In the second half of the 19th century, when great innovations were being made in modern printing, aquatint suddenly had an unexpected development. Attempts were made to adapt this technique to the newly discovered photographic processes. Niepce had already tried to transfer photographs unto metal plates so as to make engravings. His first "heliographies" date back to 1827. In his experiments the inventor of photography used a photographic negative and syrian asphalt*, which, when exposed to light protected the plate on which it had been spread. The non-exposed parts could then be dissolved without any problems thus exposing the plate in the same places. All that was left to do was to bite the plate. After biting the protected parts of the plate became the black areas of the printed picture.
Further developments in photographic techniques made aquatint one of the finest of all reproductive processes. As of 1878 Karl Klietsch of Vienna used these new techniques to invent grain photogravure. In 1882, when the first half-tone screens were invented (described below), the graining process of photogravure was abandoned. It was finally in 1910 that photogravure printing on a rotary press was developed thanks to the possibility of making engraved cylinders. Needless to say the appearance of such cylinders was fundamental in increasing the speed of printing
Goya aqautint
Francisco Goya
With etching having been around for such a ling time  it is not surprising that a good many books have been written on the subject - the best by practicising printmakers.

The Art of Etching by E S Lumsden is the book I used when first learning etching and I still have that original battered and acid stained copy! This is the best technical guide to etching I have found and contains sate by state examples of the author’s own etching as well as a good selection of the best known Etchers
Also worth  buying:
I have in the past used acetone based car spray to create an aquatint but recently have discovered that small inexpensive atomiser spray bottles filled with a mixture of acrylic plate backing resist diluted with water will create a fine enough mist to give a more convenient alternative to a traditional aquatint. Place the degreased plate on a sheet of white paper and curve a funnel of corrugated card around it. Place a sheet of card on to and spray 18 inches above the plate in strong bursts. Any larger droplets should hit the back wall of card and the finer spray should fall evenly on the plate.(the white paper should give an indication of the density). The resist is removable with meths allowing stop out varnish to be painted on and removed with white spirit with no harm to the “aquatint”
Ryepress resources is an Amazon “Astore” in which I have gathered together a selection some of the Art Materials and equipment that I have used personally and can therefore recommend.