Drawing and processing

Drawing and processing - Flash animation

Processing the drawing to prepare it for printing is one of the critical phases of lithography. Because mitsakes at this point can easily destroy the drawing on the stone, the greatest care must be taken to proceed correctly and to understand fully each step involved.
The aim of processing is to scparate chemically the image and nonimage areas of the drawing so that they will receive or reject ink consistently. When proicessing begins, the image areas consist of pasages drawn with greasy lithographic crayon or tusche. Through chemical processing the fatty-acid particles contained in the drawing are liberated, permitting them to combine with the stone itself. Once the grease has been transferred, there is no further need for the drawing materials that contained it. The greasy areas are now ink-attractive and form the printing image. Simultaneously, the undrawn or nonimage areas are so treated that they will receive water and repel grease. This reaction is brought about through a process called etching, in which a mixture of gum arabic and acid is applied to the stone, desensitizing its surface. The strength and formulation of the etch is determined through consideration of the character of both the lithographic drawing and the stone.
A sequence involving two separate etches is employed. The first etch partially desensitires the stone, so that the drawing materials may be removed with solvent, a proress called the washout. The cleaned image, now an integral part of the stone, is inked with a roller, a process called the roll-up. The inked image is then given a second etch to complete the desensitization of the stone and to provide a durable separation of ink-attracting and ink-rejecting areas during subsequent inking and printing.

The steps in processing

  1. The stone is given a first etch that (a) liberates grease from the drawing into the stone and (b) desensitizes the nondrawn areas of the stone so that they will no longer attract grease. The nondrawn areas become hygroscopic; that is, they now attract and retain water films. The drawn areas become hydrophobic, or resistant to water but attractive to grease. By magnification it may be seen that the process affects each minute dot and gaiin of the stone's surface.

  2. The drawn and etched work is washed out with solvent to remove surface materials; these materials are replaced during the roll-up with a layer of printing ink. While in this condition, the stone may be proofed at the press or minor corrections may be made. Additional desensitization is usually required before extended printing.

  3. The inked image is protected, cleaned., and given a second etch, completing desensitization of the stone. If the etches have been correct in strengt prolonged printing of the stone will now be possible.

1. Talc and rosin

Drawings to be processed must first be protected with a resistant material in order to withstand the corrosive properties of the acid contained in the etching solution. Rosin and talc (French chalk) are used in the process, each performing certain tasks better than other. Rosin is the major etch resist because it possesses the following properties: (1) it is avalaible in a finely ground fluorlike form; (2) it is insoluble in water; (3)it is soft and easily soluble in turpentine; (4) it is easily melted by heat; and (5) its particles bind together instead of remaining separated under the acid. Talc also is unaffected by nitric acid; however, it does not exhibit the uniting properties of rosin, and it tends to separate under waterly solutions. Consequently, talc is not recommended as an absolute resist. Its most valuable function is to overcome the tendency of the greasy image to repel the watery solutions of gum and etch. By destroying surface resistance, it permits these solutions to lie more evenly on the work during the course of etching.

Since the rosin is accepted as the major acid resist, it should be applied to the image before talc.
Both rosin and talc must be used also in the second etching. They should be applied separately. Mixing talc and rosin together for application in a single operation is not recomended - the rosin particles will be attached first to the ink; hence the protection of the image agains the etch could be uneven.
Both rosin and talc are gently applied to drawn and inked surfaces witch tufts of cotton or soft brushes; all excess powder is removed after each application.

2. First Etching

The etching solution is tested on the borders of the stone, to determine the strenght of reaction on that particular stone. The contact of the acid mixture with the carbonates of the stone will produce an effervescence of carbonic gas bubbles, which will indicate the strenght of the etch. If the reaction is violent and instaneously produces a rich white froth, the etch is probably too strong; it should be diluted with gum arabic. A satisfactory etch should react almost instantanelously, producing a gentle frothing. Weak etches will not efferversce until several second have elapsed; then they do so very mildly. Strong etches applied through pools of gum react like weak etches. If the etch appears too strong during application, it should immediately be diluted by adding gum arabic, or it should be brushed away from the drawing and toward the borders of the stone. Such etches should not be removed with a dampened sponge, because the water-soluble ingredients of the drawing may be endangered or smeared.

The basic application procedure is to flow the etch across the whole stone, rapidly and evenly subjecting all areas of the image to equal amounts of solution. The area of the stone to be etched may be reduced by first covering all the borders with the etch. Next a generous brushful of etch is carried in a single sweep across the top of the image from one side to the other. The brush is charged again and a second stripe is carried across the stone, just overlapping the first. The brush will need reaching for each stripe, because its solution will be partically exhausted as well as somewhat neutralized by reaction with the limestone. This pattern is repeated until the entire stone is covered. At the same time the strongest effervescence is brushed away from the weaker areas of the drawing and is deposited on the stronger. When the stone is completely covered, the remainder of the solution is applied more deliberately, at right angles to the first application. Further manipulation of fluid over the greasier areas of the drawing is advisable at this time.

The etching should proceed with continous manipulation for two or three minutes; after this the total volume of the solution an dmost of the chemical reaction will have been spent. The etch should not be left on the stone too long, however, for it cannot then be dried into a thin coating.
The tinner and evener the coating, the better will be the desensitization of the stone. The wiping and polishing technique forces the film of etch or gum into the valleys of the stone grain and exposes the image for easy solvent removal during the washout. The adsorbed films of the etch or gum coating serve as a water-bearing and ink-rejecting mask, covering each grain and pore of the nonprinting surface. Coatings that are too thick or uneven often cover parts of the drawing, making it difficult for the washout solvent to penetrate and dissolve the image. Moreover, unevenly adsorbed coatings cannot retain water films equaly, hence they impair the inking of the image and can, in serious cases, produce gum streaks which are impossible to remove from the stone and which appear on the impressions printed from it.

Etch an gum coatings should be dried with two folded cheesecloth pads; the first mpos up and evens the bulk of the etch solution, and the second polishes and dries the remainder to a thin, even film.
When the first etch is thoroughtly dry, either the stone can be washed out, rolled up, and given a second etch or it can be stored for future completion the processing. It is preferable to complete the processing by inking the stone and giving it a second etch before storing for long periods.

3. Washout

After the first etch the fatty components of the drawing become an integral part of the stone, and the nonprinting image comprises a surface that is partially water-receptive and ink-rejective. The original ingredients of the drawing materials are no longer needed; these are removed in the washout process, which prepares the exposes image on the stone to receive ink.

  1. The work is washed out through dry coating of etch or gum, using turpentine and clean, dry rag. The solvent has the ability to penetrate the thin etch coating without dissolving it. The etch coating acts as a protective mask over the nonprinting areas, preventing the dissolved sludge from making contact with the stone.

  2. The dissolved sludge is wiped away and the stone is fanned dry, permiting the solvent to evaporate.

  3. fter few minutes – asphaltum solution should be applied on the stone with a clean, dry rug.

4. Asphaltum

The ink receptivity of a lithographic image is directly proportional to the extent of its fatty deposits. Heavy deposits of grease will tend to attract ink from the roller more rapidly than weak deposits. Hence, during the roll-up, the dark areas of an image will sometimes appear fully inked before the light areas have achieved their full tonality. If the stone has also been inadvertently overetched, even slightly, the light areas will accept ink all the more slowly. Inking further to assist the development of the light areas may result in overinking of the darks.

Liquid asphaltum may be applied to the image to fortify the fatty deposits and to equalize ink receptivity.br /> When liquid asphaltum applised on the image and dried, its greasy constituents fortify the fatty image deposits, leaving a tenacious and tacky film which is highly receptive to ink. Because of its greasy nature, asphaltum is also beneficial in reinforcing areas that have suffered minor grease loss from overly strong etching.

  1. The drawing is washed out througr the dry- gum mask, following the washout procedure.

  2. Asphaltum solution is applied with a clean cloth in an even film over the entire work an then is fanned dry.

  3. The dried coating is washed out with water. This removes the etch or gum mask from the nonimage areas. The image areas will retain a firm tacky coating which forms the ink base.

  4. The stone is kept damp and is immediatly rolled up with ink. It will be seen that the image will accept ink more quickly and evenly. Slightly overetched areas will regain their original value. If they do not, the overetching has been severe, and very little can be done to correct the error without couneretching the stone.

5. The Roll-up

The application of ink after the washout is one of the critical phases of stone processing. The objective is to deposit a layer of ink on the image, exactly duplicating the visual characteristics of the original drawing. The inking procedure must be executed with care and deliberation, inasmuch as the stone is only partially desensitized by the first etch.
The roller, lightly charged with ink, is passed over the work rapidly and lightly four or five times without stop- ping. Examination of the work will show that the image is beginning to receive ink in an increasing or decreasing degree, depending on its particular characteristics. The procedure of rapid rolling with minimum pressure limits the discharge of ink and enables the printer to assess the receptivity of the image to the ink. Within the period of the first few passes of the roller, the printer must determine the rolling techniques necessary for that particular stone. If ink acceptance is slow, the rolling should proceed more slowly and with graduall increased pressure. If ink acceptance is fast, rapid rolling and minimum pressure should continue. As the image develops, the roller is recharged with fresh ink from the slab, the stone is redampened, and the rolling is continued. These steps are repeated with periodic inspection of the work until the image is fully charged with ink and duplicates the tonalities of the original drawing. In this condition it is ready to receive minor corrections; these are followed by the second etch for the completion of desensitization.