The initial gray value of the image decides the final coverage or density of the screen.The file resolution ( ppi) determines the size of the smallest island or the narrower channel present in the screen.Then, preparing a digital file to make a random or stochastic screen, some questions must be taken into account:
All that concludes in bigger lakes that will difficult the ink retention during the plate wiping, producing the so called open bite in the shadow areas of the image.
If the etching is prolonged beyond some limiting time, the lateral etching will connect some channels eliminating the little islands. Beside this etching in depth, there is also a more or less important lateral etching that progresses beyond the perpendicular of the hardened gelatin, in such a manner that at the end of the process the etched area will be a bit more large that expected. The etching will be proportionally deeper relating with the time the copper is immersed in the acid.
After the gelatin is transferred to the copper plate and the non hardened gelatin is washed away, the ferric chloride will attack the part of the copper represented by the black pixels in the digital screen. During the first exposure, the transparent spaces in the screen allows the UV light to pass and to harden the gelatin. The general method includes to generate a bitmap image in which the proportion of white (transparent in the film) and black pixels matches the desired density, that is usually around a covering of 50%. This allows for a reproducibility in the method less depending on variety in praxis. An alternative consist in the use of a screen digitally generated and filmed over high contrast photographic film. Between them can be cited the control over the particle size, the aquatint box volume, the mechanical system to shake the particles, the waiting time before put the plate into the box, the time of particle deposition over the plate, the melting method and the final cooling procedure. The islands represent the copper areas protected from the ferric chloride attack because the gelatin will be tanned or hardened, while the channels and lakes are the areas that will be etched by the acid.Īlthough the results obtained with this method can achieve exceptional quality depending on the methodology employed, some key parameters for the success are not so easy to standardize. Observing under magnification and in words of Luis Hernández (3), there can be observed that can be called islands, channels and lakes. The lack of determinism in the probability of spatial distribution and size pattern applies for that is called stochastic screen. In this way, an infinity of little particles fall over the plate in a sort of network with a shape and distribution completely unpredictable. The variety in size of particles and the random probability of landing onto the copper allows for a random spatial distribution. The resin or asphalt powder is applied by means of the called resin or aquatint boxes, where the product, finely grounded, is mechanically shaken and let fall down over the copper plate surface. As described in the technique of heliogravure, it is necessary to have some screen to illuminate the gelatinized paper or carbon tissue in order to bypass the use of a resin or asphalt aquatint.