Heliogravure IV – Some Overall Thoughts

Cu_29_65_Gris_96Heliogravure has some particular characteristics that don’t facilitates the system calibration in order to arrive to a sort of WYSIWYG. Working from a film negative, it is a different experience if the user prints the negative on photographic paper before to perform the heliogravure or passes directly to the heliogravure. In the first option, what we want to see in the heliogravure print is conditioned by what we have seen in the photographic positive print. Therefore, the making of the transparent positive has a goal previously determined. If instead, the heliogravure is done using the photographic negative with no other intermediate than the positive transparency, then the judgement about tone and contrast will be preferentially performed at the final stage when the print come out of the press. Those aspects of the work method are not trivial, since we are always conditioned by the previous vision we have of a given image. Additionally, the photographic print paper may or may not have different reflection properties relating with the heliogravure final print paper. If both the photographic and the heliogravure prints are done over, for instance, satin neutral white papers, the perceptive experience looking at the two prints may be very close to one other. Conversely, using a premium glossy baryte paper for the photographic positive print and since most of heliogravure printing papers have matt surfaces, will probably conduct to a highly different perceptive experience when both are observed under the same lighting level.

In a different way, for those working from digital image files, it is sometimes painful to get on the print what is seen on the computer screen. It is necessary to note that in reality, this an unachievable goal. With a given printing black ink, it may be possible to achieve a dense and opaque black that allows for the same visual perceptive experience as when you look at a “0” black in the computer screen. Conversely, the whitest paper that you can use in heliogravure printing cannot reflect as much light as the white “255” transmits through the computer screen. Therefore, some preventions must be taken in the attempt to transport our experience of looking at the screen to the imprinted piece of paper. This is not an exclusive problem of heliogravure prints, but a common situation with any imprinted version on paper relating with the on screen viewing.

In my own approximation to the question, I have followed some steps that perhaps can help someone other. A summary is as follow:

  • Calibrate the screen brightness to approximately match the reflected brightness of a piece of printing paper evenly illuminated with a light level suitable for critical print detail analysis. Respective measures of brightness can be conducted with a camera exposure meter. The screen brightness can be adjusted until it matches the suggested exposure combination for the sheet of printing paper.
  • Prepare a digital step wedge divided in 10% increments, or the sequence do you prefer, and print it with the same printer settings as for the positive. A model with eleven steps and a continuous ramp is shown in the Fig., 1. You can download here a printable version of Grayscale Color Mode, 8bit color depth and 360ppi of spatial resolution. Stouffer step wedges are well indicated if the positive transparency is on photographic film, as Stouffer is. This ensures a comparable transparency and behaviour to the UV light exposure. Nevertheless, translucent films like Pictorico or PermaJet used to print the positive transparency from digital image files have a quite different opacity relating to the photographic film. Beside that, if there is not available a densitometer, the Stouffer test marks are unusable, excepting by visual comparison. Dealing with digital images, I prefer to prepare the step wedge referred to pixel Gray Value (0 – 255) and Ink Coverage (100% – 0%) with exactly the same printer settings and ink I use to print the positive image transparency. I also use this 10Step test to include in the imaging plates to control at a glance the etching progress.
  • Determine the minimum exposure time that produces an enough gelatin thick in the black. This can be done by incrementally exposing that test and then transferring the gelatin on a piece of glass. If a densitometer is not available, the result can be photographically reproduced and then the pixel grey value suitably measured as a consequence of the gelatin thickness presence in the different steps. A free software for any computer operative system that allows many kind of measurements and calculations over digital images is ImageJ.
  • Determine, with the digitally prepared screen, the maximum etching time that avoids the open bite. This can be done by exposing a piece of carbon tissue through the screen with increments in time. Finally, the tissue is transferred to the copper plate with the usual method and the plate is etched in a, for instance, 40ºBé ferric chloride solution. The etching is stopped sequentially in time covering strips of the plate with packaging tape. Analysing the combined results with a magnifier and on the resulting print, is not so difficult to arrive to some conclusion about the approximate limits for the total etching time. The goal is an even and dense black without any signal of open bite caused by the lateral etching.
  • Finally, with all those data, it is necessary repeat the trial with the complete exposure sequence, first to the screen and then to the positive transparency in order to fine tune the combined exposure times.

In my opinion, all that calibrating steps will provide a suitable exposure combination which will serve only as a starting point. The successive experience and most important, the characteristics of each image, will suggest some departures from this point of reference. Numbers, test targets and calibration graphs informs about the materials response but images are also a visual and perceptive experience. The fine tuning each image in a particular way, often can provide a much better visual and aesthetic result.

Reduced10StepHelio_CMitja

Figure 1. Step Wedge specially prepared to be imprinted with the same printer and ink used for the positive transparency. This file is a reduced version and not suitable to be used as an actual step wedge to print. Go to the link in the text to download it (click on the image to view an enlarged version).

The step wedge shown in the Fig., 1 can be used both calibrating the system or to include it in the plate, above or beside the image. This helps to visually control de etching progress. When printed with the native resolution of 360ppi, it measures 105x30mm, little enough to avoid excessive copper discard and sufficient in size to control at a glance the etching changes in each ferric chloride dilution. After downloading it, check for the actual grey values of the steps. If there is some departure from the noted figures, check the clipping or colour settings (colour space, automatic ICC profiles, etc.) in the digital image processing software. The five steps at lower right corner are of respectively 100%, 75%, 50%, 25% and 0%, from down to up.

Some examples of calibrating process. As David Morrish recommends in his book (1), transferring the exposed gelatin on glass can help to better decide about the appropriate exposure time that provides a minimum thick of gelatin resist in black areas. In spite of the practical difficulties I have encountered to maintain the gelatin on the glass during the hot water wash, it is still possible to extract some conclusions from the example shown in the Fig., 2.

Helio04_GelatineOnGlass

Figure 2. Gelatin transferred on glass. The exposure have been done without any previous screen. Find the explanations in the text (click on the image to view an enlarged version).

  • Looking at the black extreme of the step wedge, at left with thinner gelatin resist, only the 8min exposure provides some separation between the respective 100% and 90% steps (take into account that this is an exposure without screen).
  • The whites are better separated with 4min and barely separated in the 8min exposure step.

This probably suggest that the system admissible dynamic range is shorter than the dynamic range of the step wedge. Those data relates only to a given printer, ink and positive transparency printing media. If we don’t want or cannot change that, a possible solution is to digitally apply a dynamic range reduction to the step wedge image file. Reducing the equivalent to 1step (10%) would allow to expose for 5 or 6min with a fine separation both in highlights and shadows.

10Stepx4_ReproPrint

Figure 3. Exposure time sequence of the 10 Step Wedge with 10% reduced tonal range (click on the image to view an enlarged version).

Fig., 3 shows a print obtained from a sequentially exposed step wedge. The step wedge image file was processed in order to reduce its dynamic range the equivalent to 1step (10%) as discussed above. The exposure sequence is, from down to up, 4min, 6min, 8min and 10min. The digital screen was previously exposed for 6min, the correct time found to allow for a dense black. The Fig., 4 shows a graph with two plots corresponding respectively to the 4min and 6min step wedges in the Fig., 3. Over each plot and with the same colour, simple linear regressions have been also calculated in order to help in the interpretation. While the 4min exposure allows for a better separation in the highlights, the 6min plot shows a slightly better separation in the shadows area and a greater slope that indicates a higher image contrast. In this case, the separation is better equaly spaced between the overall steps. Although this is not shown by the plots, it is important to realize that the 6min exposure will provide also the thickest gelatin resist which in turn, will need changes in the etching sequence to penetrate a given (or desired) depth in the copper plate. Up to the extend of my knowledge and from my reduced experience, we cannot qualify anyone of worse or better result. We can advantageously consider that for images that need a higher contrast and/or a better separation in the shadows, 6min would be the chose. Conversely, those images with no large shadow areas, with critical highlights and/or limiting contrast, would be better processed with 4min of exposure time. In between, we have still several possible exposure times around the 5min, that will provide intermediate results. Then, the control of the image tone and contrast is the result of suitably combine digital image processing, transparency exposure and etching sequence.

Plots_Helio08_4min6min

Figure 4. Chart with two plots corresponding to the step wedges shown in the Fig., 3. In blue, 4min exposure time; in red, 6min exposure time (click on the image to view an enlarged version).

Screen Scheme and Etching. In order to better look at the kernel of the method, that is the sequential etching, I have taken the following photomacrographs from small areas of a plate and its corresponding print (Table 1). The image on the plate is the same step wedge with 6min of exposure previously studied. There are shown only five equally spaced steps corresponding, from left to right, at 0, 77, 128, 204 and 255 grey values. Clicking on the pictures, will access to a 50X magnification view on average computer screens (screen pixel size around 20-25µm). As a reference value, the actual size of the figure 255 over the plate and the print is of 2.7mm length.

PlateGV000_MagnifiedX50 PlateGV077_MagnifiedX50 PlateGV128_MagnifiedX50 PlateGV204_MagnifiedX50 PlateGV255_MagnifiedX50
PrintGV000_MagnifiedX50 PrintGV077_MagnifiedX50 PrintGV128_MagnifiedX50 PrintGV204_MagnifiedX50 PrintGV255_MagnifiedX50

Table 1. Click on the images to access to a 50X magnification for an average of computer screen. Depending on its pixel size, the magnification will vary slightly. The figure of 50X is calculated for an screen with a pixel size of 22µm.

As can be seen magnifying the previous images, although in the plate pictures it is clearly visible the screen structure, in the print magnifications it is almost disappeared after the ink is merged with the paper fibers. This means that the screen scheme aspect is not so important because it disappears under the pressure of the press roller. Some amount of ink is forced to enter in the paper fiber while other is spread over the surface, all that depending on the amount of ink present in the corresponding step. This ink amount is related with the etching depth. Finally, looking at the black (0) step, it is obvious that although there are present recessed areas caused by the lateral etching, they aren’t deep enough to provoke open bite. Perhaps this example shows an endpoint before the open bite appears.

AverageGreyValue_PrintHelio08Steps

Figure 5. Plot of average grey values from sections of the respective prints shown in the Fig., 3 and Table 1 (click on the image to view an enlarged version).

As a final test, some average samples of pixel grey value have been taken from the print pictures of the previous series. This average results have been plotted in a graph that is shown in the Fig., 5. As can be observed in this case, the positive transparency exposure time, the etching sequence, the inking, the plate wiping and the etching press action have resulted in a quasi linear sequence, as is expected. Exaggerated departures of this linear behaviour, will show the effects of causes related with all those method steps.

References

1. MORRISH, D. and MacCALLUM, Marlene (2003) Copper Plate Photogravure, Demystifying the Process. Ed. Focal Press, Burlington MA.

 

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First Heliogravure Print

After some troubles and a quantity of trials with Grayscale step wedges, here is my first Heliogravure image. The plate measures 145x165mm and the image 120x120mm. I have used an stochastic screen digitally designed and printed on high contrast photographic film in a Pre-press service bureau. The digital image was edited in Adobe Photoshop and printed on Pictorico OHP film with an EPSON R3000 printer. The etching sequence was 45ºBé, 43ºBé, 42ºBé, 41ºBé and 40ºBé. The etching begun after 5min in the 45ºBé solution and most of tones have been etched between 43ºBé, 42ºBé and 41ºBé. The plate was in the 40ºBé solution only for 1min. The total etching time is of 25min at 21ºC. This a proof with the step wedge still on the upper plate area. Clicking on the image you will see a life size version (only approximate and depending on the size of your screen pixels).

ReproHelio10_LifeOnStone

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Heliogravure III – Ultraviolet Light

Updated 26/01/2016

Cu_29_65_Gris_96Hardening the gelatin. In heliogravure, it is necessary an illumination source rich in ultraviolet (UV) light. This UV light must provoke the cross link reaction into the gelatin previously sensitized with potassium bichromate (K2Cr2O7). This reaction is explained by several and sometimes slightly different theories. Luis Hernández (2), explains that the UV light acts on the gelatin generating a tanning effect. The reaction consists in the reduction from the hexavalent Chrome (Cr6+) to trivalent Chrome (Cr3+). The necessary water is taken from the gelatin, that becomes dehydrated. The result is a hardened gelatin with a higher melting point and a reduced solubility in water. Other important effect is a lower higroscopicity or capability to absorb water.

According to Cartwright (1), it is well known the insolubility effect that provokes the treatment of gelatin with metallic salts. This effect is reversible in most of cases except with potassium and ammonium bichromate. Although the complex composition of proteins molecules like the gelatin difficult a complete and reasoned explanation of the reaction, empirical data based on changes in the color of the solutions suggest that, in presence of organic substances, the bichromate is reduced by the light action (the same effect is achieved in the dark but it needs more time) giving a chrome compound that is in turn absorbed by the gelatin. The gelatin becomes then more insoluble. The mentioned changes in color conducts to deduce that the compound is the chrome hydroxide (Cr(OH)3).

Even though is specified that the exact reaction is not completely explained by the chemists, the Hunter-Penrose documentation (3) proposes that the gelatin tanning is produced as a result of a redox reaction that is accelerated by the short wavelength radiations. According to this documentation, the equation for the bichromate is as follows:

Redox_Dicromat

Under the action of the UV light, the colloid is oxidized by the bichromate forming trivalent Chrome. As the metallic trivalent ions form colloids, the areas exposed to the UV light don’t be washed out by the presence of water. Note that the explanation is very close to that of Cartwright.

Spectral sensitivity. In Cartwright text (1), the spectral sensitivity of the bichromated gelatin extends to the band between 350nm and 420nm, falling down rapidly beyond 455nm. Even though the glass of the contact presses absorbs the radiations below 325nm, the useful band passes through the glass. Hunter-Penrose (3) locates the actinic band between 320nm and 380nm. Finally, Luis Hernández, citing the technical documentation of Autotype, defines the activity between 330nm and 430nm, with a maximum peak at 370nm. Also from Autotype, the sensitivity continues beyond the 430nm to fall down at 470nm and practically disappear at 500nm. The little differences between those data can be attributed to the different pigments merged with the gelatin by the manufacturers of carbon tissue.

From those data can be deduced that is necessary to have a light source rich in UV-A band (315nm400nm). By now and for a printing studio, there are available several possibilities:

  • In first place, specific devices coming from the Graphic Arts market as the NuArc or Amergraph. Both provide collimated light beams from a tubular metal-halogen bulb and a parabolic mirror. They incorporate electronic stabilization with instantaneous ignition and easy control of the exposure time. Both instruments also incorporate a vacuum contact press. There are other devices that use punctual direct light bulbs of metal-halogen. In order to reduce the difference between the angle of illumination of the center and the edges of the contact press board, the light source is located far away of the assembly and then, those devices are taller that the collimated source ones.
  • A second option is the same metal-halogen lamp mounted independently of the vacuum press and at a distance large enough to be considered a punctual light source.
  • The third option is a mixed tungsten/metal-halogen bulb as the Ultra Vitalux from Osram at a sufficient distance that ensures a uniform illumination over the vacuum press frame.
  • Finally, a battery of fluorescent tubes emitting the specific band of UV wavelength.

The specific instruments solve the illumination and the vacuum contact press in an unique device. The financial cost is relatively high for a non commercial installation. The mixed lamps as the Osram Ultra Vitalux are a good choice both related with economic cost and the emission spectrum, very close to that of the metal-halogen lamps. Nevertheless, its opal glass and the bulb physical size provide a relatively diffused light. This is not the best option for digital screens (Heliogravure II – Stochastic Screen). Locating the bulb far away enough, provokes a loss in actinic power. Finally, in terms of cost, energy savings and comfort of use, the fluorescent batteries are a very fine election. On the other side and caused by its inherent diffused light, they prevent the correct exposure of digital screens (Heliogravure II – Stochastic Screen).

Lamp Installation. As a corollary, the system chosen in my case is a lamp of metal-halogen used as a reasonably punctual light source at a sufficient distance from an independent vacuum press. The bulb used is the OSRAM Supratec HTC400-241-R7s High Pressure Lamp. Its emission spectrum is shown at the Fig., 1. From the total 460W of nominal power, 12W correspond to the UV-B band (280nm315nm) that is absorbed by the vacuum press glass, while 82W are emitting the band of UV-A (315nm400nm) useful for the gelatine hardening. Comparing with the data of the Osram Ultra Vitalux, this has a nominal power of 300W but emitting only 13,6W in the UV-A band, about six times less.

Espectre_Supratec

Figure 1. Spectral Radiation Distribution of the OSRAM Supratec HTC400 lamp in the band between 250nm y 450nm. There can be seen high emitting peaks in the specifically useful band for the gelatin hardening.

This lamp, of high pressure discharge, needs a specific starting and power maintenance system. The power source is composed by three basic elements:

  • Ballast HPS CCG 400W
  • Ignitor of 4-5kV
  • Compensating condenser of 50µF

The lamp has been installed in a specifically designed support and fixed to a former LPL photographic enlarger column with the enlarger head removed. In order to compensate the loss in weight and to avoid the risk of unwanted sudden displacements, 2.5Kg of weightlifting discs have been mounted on top of the set (Fig., 2).

Figure 2. OSRAM Supratec HTC400 mounted on a former photographic enlarger column, allowing for vertical displacement (click on the image to view an enlarged version)

The starting and power supply system is shown in the Fig., 3. The upper cover of the ballast incorporates schemes about the items connexion. In any case, it is not difficult to find indications about the correct connections for those kind of lamps in Internet. In order to facilitate the changes in distance from the vacuum press, all the power supply system is connected to a socket controlled by a switch.

Supratec_Ignitor_1000pix

Figure 3. Set of items and connexions of the OSRAM Supratec HTC400 power supply (click on the image to view an enlarged version).

Supratec_Ignitor_Carcasa_800pix

Figure 4. Previously described power supply protected with a cover and ready to use (click on the image to view an enlarged version).

Thus the lamp cord is in turn connected to this socket avoiding the need to place the power supply closer to the lamp in the vertical support. This is specially indicated because only the ballast weighs more than a kilogram. All the power supply set is protected by an aluminium cover and mounted on an isolated holder (Fig., 4). Following the manufacturer instructions (Fig., 5), there is always a minimum waiting time of 2min before the lamp emission is fully stabilized. Finally, it is necessary a cooling time of 5min before re-connect after turning it off.

StartUp_Supratech

Figure 5. Chart of the emission flux as a function of time for the lamps OSRAM Supratec HTC 400W and 1000W. 1) Cold 1000W lamp. 2) Cold 400W lamp. 3) Both lamps from medium charge.

Safety Recommendations. UV light it is not healthy. Specially UV-B can cause severe and irreversible lesions in the eyes and skin. While UV-A is not so dangerous, their effect can be accumulative and continued exposure to it is also dangerous. Using the lamps described in the text it is necessary to wear industrial safety goggles that ensure protection against the specified radiations. It is too highly recommended to wear gloves if there is necessary to touch the object during the exposure. While UV-B is absorbed by the vacuum press glass, it is hitting the hands of the operator. A simple and safely solution is to provide the lighting installation with a thick fabric black curtain that is closed around the lighting area before switch on the lamp.

References

1. CARTWRIGHT, H. M. (1961) Ilford Graphic Arts Manual, Volume 1-Photoengraving. Ed. Ilford Lted, Ilford, Essex.
2. HERNÁNDEZ, Luis (2010) El Heliograbado por el Procedimiento Talbot-Klíč – Antecedentes, uso y principios para el control del tono. Tesis Doctoral, Dir. José M. Guillén. Universidad Politécnica de Valencia-Facultad de Bellas Artes-Dept. de Dibujo.
3. HUNTER-PENROSE (2006) Photoengraving Glue Datasheet. Ed. Hunter-Penrose in Technical Notes, London.

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Heliogravure II – Stochastic Screen

The Aquatint Screen
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 resin or asphalt aquatint. 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. The variety in size of particles and the random probability of landing onto the copper allows for a random spatial distribution. 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 lack of determinism in the probability of spatial distribution and size pattern applies for that is called stochastic screen. Observing under magnification and in words of Luis Hernández (3), there can be seen that can be called islands, channels and lakes. 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.

Although 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. 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. An alternative consist in the use of a screen digitally generated and filmed over high contrast photographic film. That allows for a reproducibility in the method less depending on variety in praxis.

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%. During the first exposure, the transparent spaces in the screen allows the UV light to pass and the hardening of the gelatin. 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. The etching will be proportionally deeper relating with the time the copper is immersed in the acid. Beside this etching in depth, there is also a 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. If the etching is prolonged beyond some limiting time, the lateral etching will connect some channels eliminating the little islands. All that concludes in bigger lakes that will difficult the ink retention during the plate whipping, producing the so called open bite in the shadow areas of the image. Then, some questions must be taken into account:

  • The file resolution (ppi) determines the size of the smallest island or the narrower channel present in the binary image.
  • The initial gray value of the image decides on the final coverage or density of the screen, that is, the proportion between the white and black pixels in the binary image.
  • Remember that the black pixels constitute the area that will be finally etched and the lateral etching will be added to some extent to this initial black area.

Screen Resolution
As has been explained, the file spatial resolution will determine the smallest island present in the image. In a first approximation, smaller elementary units in the screen difficult the detection by the observer and preserve little image details. The resolution of a given system breaks down from its weakest step and in our case, this is the screen. The little detail visible in the final image depends on the screen resolution regardless the positive image resolution. Nevertheless, a high resolution screen can be accompanied by some risks that is important to evaluate. In first place, the illumination system employed will translate the screen to the gelatin in a different way depending on its properties: collimated, punctual or diffused. The collimated systems generate a parallel beam of rays in a such way that they arrive completely perpendicular to the vacuum press glass. This kind of illumination is incorporated in professional devices as the NuArc or Amergraph. Actual punctual system are not possible in the real world but a little bulb far away enough from the vacuum press can behave as reasonably punctual. Finally, diffuse system are constituted by the batteries of fluorescent tubes.

The Figures 1, 2 and 3 show comparative idealized schemes of the above described light sources behaviour relating with two different screen resolutions. In the Fig., 1 can be observed as for a collimated sources the translation of the screen dimensions into hardened gelatin is perfect regardless the area of the plate and the size of the screen elements. The only one constriction is the total area that the source is able to illuminate as a function of its design. Then, this system do not show any difficult to faithfully translate the screen scheme.

InsolacioTrama_Collimated_eng

Figure 1. Idealized schemes of the gelatin hardening through two screens of different spatial resolution by means of a collimated light source. Up, gelatin hardening; down, gelatin transferred over the copper plate (click on the image to view an enlarged version).

In the Fig., 2 there is shown the effect caused by a punctual source. It can be observed as the more far is the position from the center of illumination, the more the screen foot changes in position and size. There is a reduction in size for the hardened gelatin parts and then is changed the relationship between those parts and the preserved by the opaque areas in the screen. Nevertheless, even in the exaggeration of the example, the effect is not so important if the bulb is small in size and sufficiently away from the vacuum press.

InsolacioTrama_Punctual_eng

Figure 2. Idealized schemes of the gelatin hardening through two screens of different spatial resolution by means of a punctual light source. Up, gelatin hardening; down, gelatin transferred over the copper plate (click on the image to view an enlarged version).

Finally, the Fig., 3 shows as a diffused source can harden the gelatin even under the opaque element of the screen, mainly in the case of high spatial resolution. This system allows to illuminate large spaces with a relatively reduced price and a low range of energy consumption. Nevertheless and attending to its constrains, the diffused systems are really only useful for the illumination of positive images.

InsolacioTrama_Diffusse_eng

Figure 3. Idealized schemes of the gelatin hardening through two screens of different spatial resolution by means of a diffused light source. Up, gelatin hardening; down, gelatin transferred over the copper plate (click on the image to view an enlarged version).

The second question is related with the lateral etching. Depending on the method employed to binarize the original image that will be used as screen, the channels will be closer between them and that implies that the lateral etching will easily connect it. This provokes the apparition of larger lakes that, if are large enough, will conduct to the open bite in the shadow areas. Another related question is that if the screen resolution is very high to improve the visibility of detail in the image, the screen and image exposure time must be carefully controlled to avoid the risk of a prolonged etching that will in turn facilitate the open bite.

Finally, in third place and although I haven’t found any reference about that, it is possible that the non-hardened areas have a minimum size beyond which any ferric chloride dilution will be able to penetrate. The physical implications of the ferric chloride penetration in the gelatin resist is very complex and out of the knowledge of who write this text. Nevertheless, the capability of liquid penetration in narrow spaces is closely related with viscosity. A high resolution screen, with narrow spaces between hardened resist, may need a more diluted concentration of ferric chloride to allow its penetration in the shadow areas. This can alter the correct progression of the etching. The preservation of clean highlights, will conduct in this case to an insufficient etching of the shadow areas. Then, it is necessary a balanced compromise between the screen resolution, the exposure times and the total etching duration.

All that considerations implies that the screen generated by digital means would be as little as possible but avoiding the upper explained constriction. A little screen is not only useful to avoid its detection by the naked aye of the observer, but improves the visibility of the image detail. Attending to the historic evolution, it could be enough creating a screen imitating the rosin aquatint aspect and size.

To the extend of many texts consulted by the author, there are no numerical data published about the average size of the rosin aquatint particles. David Morrish (4), in his book Copper Plate Photogravure, Demystifying the Process, recommends to use screen with a spatial resolution between 250lpi (lines per inch) and 300lpi. Those resolutions correspond to a particle sizes of 102µm and 85µm respectively. It is specially significant that the first figure is coincidental with the average resolution of the naked eye at the distance of distinct vision. It seems that those figures come from the spatial resolutions used in the printing industry for the regularly spaced half-tone screens, but are not actually related with the rosin particle size. If the image positive transparency is generated by a digital inkjet printer and attending that its maximum actual resolution is of 1440dpi, we can say that the screen to be employed could have the same or lower spatial resolution. This is because a higher screen resolution cannot improve the positive quality coming from the inkjet printer. Taking the values recommended by David Morris, we can consider multiple values as 600ppi, 900ppi or even 1200ppi, always below the limiting resolution of the inkjet printer.

Screen Scheme
If the goal is to emulate the structure obtained by the rosin grain, that’s needed is to prepare a bitmap image with a random distribution of the white and black pixels. There are two possibilities. The first one is to begin with a smooth Greyscale image which pixels have the grey value corresponding to the density or ink coverage desired. Applying some quantity of noise to that image will conduct to a random grey value distribution. If now a threshold of grey value 128 is applied, all pixels are converted to white (255) or black (0) depending on its original value being respectively over or below the medium grey value of 128. The key in this method is how to adjust the noise tool and it depends on the software used. No matter the software and tool employed to do that, the final black ink coverage must be measured. For instance, if we want a final coverage of 50%, the original image would be a smooth one with all its pixels of a 128 grey value. After the application of noise and threshold, the average value of the resulting image must be 128 or a very close value. In most of digital image processing software, this can be corroborated by the histogram data.

The second method available is to begin in the same manner as previously described but applying a binarization by mean of a stochastic screening in frequency modulation (FM). In Adobe Photoshop, the option is the menu Image > Mode > Bitmap and to choose the option Diffusion Dither (Fig., 4). This system applies a binarization of the original gray values to 0 or 255. This binary conversion is applied to the first pixel in the upper/left corner of the image and then the error committed depending on its original grey value is diffused over the subsequent pixels an so on, until the last pixel in the bottom right corner of the image.

Selección de la opción Diffusion Dither en el cuadro de diálogo de Adobe Photoshop para aplicar tramado estocástico en el Modo de Imagen Bitmap (click en la imagen para observar una versión ampliada).

Figure 4. Diffusion Dither option from the Adobe Photoshop menu Image > Mode > Bitmap.

In a pictorial image, the applied binarization proportions a new image version where the tone is represented by a more or less dense groups of black pixels over a white background. The black pixels represent the ink in the final printing and the white ones are the color of the paper. From a smooth image of constant gray value the final version is composed by a random distribution of white (255) and black (0) pixels that emulates the original image grey value or density. Although Adobe Photoshop don’t explain which is the algorithm employed, it is surely a version of the so called of Floyd Steinberg (2). Even though the several software implementations available incorporate modifications of the original algorithm to avoid the artefact, this binarization system tends to show a checker board result when is applied to a smooth image of medium grey value of 128. Then, if we want to obtain a final coverage of 50%, it is necessary to begin with an image which grey value approximates but don’t equals the value of 128. Conversely, 117 or 138 can be taken, respectively corresponding with black coverings of 55% and 45%. The Fig., 5 shows the resulting binarizations by the two described methods and beginning with a grey of 45% in both cases. At left, by means of applying noise; at right, with diffusion dither. It is necessary to realize that the parts of the plate that will be etched corresponds with the black pixels in the screen if it is filmed as positive. This prevention can only be avoided if the density is of an exact value of 50%.

Trama_Bk45_NoiseUnif100_Dither

Figure 5. Enlarged samples of stochastic screens. At left, coming from applying noise to a grey image. At right, after diffusion dither over the same grey image (click on the image to view an enlarged version).

Observing the screens shown in the Fig., 5, the obtained by mean of applying noise (left) is more similar to the traditional schemes published from the classic rosin aquatint, with islands, channels and lakes of several sizes and random distribution. Conversely, the created by diffusion dither (right) shows also a random distribution but there is a constant value of the separation between islands, channels and/or lakes. This constant separation is always of one pixel and equals the file resolution value. In both schemes there can be seen isolated individual pixels. Those individual pixels are the smallest ink point or paper space in the final stamp and are too the most prone to be affected by the lateral etching.
This lateral etching, if occurs, will not affect in the same manner to the two screens. With the screen shown at left, the lateral etching will enlarge the size of the original lakes and then will increase the risk of open bite in the shadow areas. With the screen shown at right the risk of lateral etching is also present but with a more predictable behaviour and with the same effect all over the image. Then, the system by means of diffusion dither (right) would present less difficulties and risk of open bite. This constrain with the method shown at left, imitating the rosin aquatint, is explicitly explained by H. M. Cartwright (1) about the screen of rosin aquatint, “… and give difficulties in etching because the elements vary so much in size with random distribution and the finer ones tend to be etched away.”

Attending to this different structure, the screens generated by application of noise would be used with files of higher resolution in order to control the final size of the original lakes, that are always several times bigger than the image resolution. This allows to avoid the risk derivative from the lateral etching. In both cases, with noise or diffusion dither, the image resolution must be adjusted attending to some before mentioned issues:

  • Risk of detection by the naked eye of the observer.
  • Behaviour relating with the illumination system.
  • Reasonable ferric chloride penetration window. The need for a diluted solution to penetrate the shadow areas, can result in a premature penetration of the image highlights.

Physical Support of the Screen
After the file is prepared with one of the two described methods, there are three ways to take the screen over a physical support, laser printer, inkjet printer of photographic quality or filming over photographic high contrast film. Laser printer is the cheapest option but presents some difficulties derivative from its usual low spatial resolution. Schemes with spatial resolution higher than 450ppi could be poorly reproduced.

The inkjet printer of photographic quality can achieve work resolutions of 1440dpi and even 2880dpi in some units. This allows for a fine reproduction of screens with spatial resolutions of 600ppi and 900ppi, but the system tends to create diffused edges constituted by ink drops of different opacity. This compromises the reproduction of the screen scheme and alters the initial covering or balance between white and black regions of the screen. There are available in the market some RIP (Raster Image Processor) for those printers that corrects this problem common in Graphic Arts, but the price is quite elevated for a non commercial use. A cheaper RIP as the QuandToneRIP provided by Roy Harrington is very interesting for the calibration of the positive image printing but cannot completely solve this kind of problem with screens.

The most reliable method is to send the file to a service bureau that develops the file onto photographic film using a high resolution image-setter. For a reasonable price, sizes up to DIN-A2 and bigger can be obtained. The resulting black parts are constituted by metallic Silver depositions of a high density (4.0OD5.0OD) and the edge resolution is nearly perfect. The black metallic silver is additionally more resistant to the UV radiations than the inkjet ink. The only one problem is that those services are progressively substituted by the so called Computer To Plate (CTP) technology and there is no future for it at medium term. May be in the next years the systems to print the screen with inkjet printer will improve enough. In the meanwhile the service bureau is available, the option is to provide a number of screens to cover the work over a reasonable period of time. Taking care in handling, a screen over a photographic film can be a long lasting item.

Screen Exposure
There are several authors that recommend an exposure time for the screen, that in most cases is expressed as a percentage of the positive exposure time, previously determined. In most cases, the recommended exposure time exceeds that of the positive attending to provide a thicker resist than the obtained in the image highlights. Nevertheless, it is difficult to advise a concrete time or percentage provided that the respective physical support are different and consequently is different too its transparency to the UV light. If both the screen and the positive transparency have been obtained over photographic film, the respective base+fog opacity may be relatively equal and then a slightly longer exposure time for the screen is correct. In the other extreme, if the screen is done over photographic film and the positive, for instance, inkjet printed on Pictorico OHP transparency, the exposure time for the screen, even being shorter than the applied to the positive, can provide a thicker resist. It is therefore necessary to check the respective densities by a suitable densitometer or indirectly by experimentation, always for a given light source.

References
1. CARTWRIGHT, H. M. (1961) Ilford Graphic Arts Manual, Volume 1-Photoengraving. Ed. Ilford Lted, Ilford, Essex.
2. Diffusion Dithering Algorithms
3. HERNÁNDEZ, Luis (2010) El Heliograbado por el Procedimiento Talbot-Klíč – Antecedentes, uso y principios para el control del tono. Tesis Doctoral, Dir. José M. Guillén. Universidad Politécnica de Valencia-Facultad de Bellas Artes-Dept. de Dibujo.
4. MORRISH, D. and MacCALLUM, Marlene (2003) Copper Plate Photogravure, Demystifying the Process. Ed. Focal Press, Burlington MA.

Posted in Early Photography, Heliogravure (english) | Tagged , , , , , , , , | 1 Comment

Heliograbado III – Luz Ultravioleta

Cu_29_65_Gris_96Endurecimiento de la gelatina
En el caso del heliograbado, se necesita una fuente de iluminación rica en rayos ultravioleta (UV) que provoque la polimerización (cross link) de la gelatina sensibilizada con dicromato de potasio (K2Cr2O7). Según Luis Hernández (2) la luz UV actúa sobre la gelatina dicromatada generando un efecto de curtido. La reacción consiste en la reducción de parte del Cromo hexavalente (Cr6+) a Cromo trivalente (Cr3+). Para ello se necesita agua que se toma de la gelatina, quedando ésta deshidratada. El resultado es un endurecimiento de la gelatina y la elevación del punto de fusión, al tiempo que se merma su solubilidad en agua. Otro efecto importante es un descenso de su higroscopicidad.

En palabras de Cartwright (1), es bien conocido el efecto de insolubilidad que provoca el tratamiento de la gelatina con soluciones de sales metálicas. Este efecto es reversible en la mayoría de los casos excepto en el de los dicromatos de potasio y amónico. Aunque explica que la complejidad de las moléculas de gelatina en tanto que proteínas dificultan una explicación completa y razonada de la reacción, datos empíricos deducidos de los cambios de color de las soluciones conducen a pensar que el dicromato, en presencia de sustancias orgánicas como la gelatina, es reducido por la acción de la luz (o de forma más lenta en la oscuridad) para dar un compuesto de cromo que es absorbido por la gelatina que se vuelve así más insoluble. Los cambios de color mencionados sugieren la presencia de iones de cromo trivalente (Cr3+) y por lo tanto, el producto ligado a la gelatina es el coloide Hidróxido de Cromo (Cr(OH)3).

Aunque también se especifica que la reacción exacta no está todavía del todo explicada por los especialistas químicos, la documentación de Hunter-Penrose (3) propone que el endurecimiento o curtido de la gelatina se produce como resultado de una reacción de oxidación-reducción que es acelerada por la radiación de onda corta o UV. Según esta documentación, la ecuación de la reacción para el dicromato es la que sigue:

Redox_Dicromat

Bajo la acción de la luz, el coloide es oxidado por el dicromato y se forma Cromo trivalente (Cr3+). Como los iones metálicos trivalentes forman un coloide, las áreas expuestas a la luz no se disolverán al lavar con agua. Nótese que esta explicación coincide con la de Cartwright.

Sensibilidad espectral
Según Cartwright (1), la sensibilidad espectral de la gelatina dicromatada se extiende en la banda entre 350nm y 420nm, para decaer rápidamente a partir de 455nm. Aunque el cristal de las prensas de contacto absorbe las radiaciones más cortas por debajo de 325nm, el rango útil queda en la banda que consigue atravesar dicho cristal. Hunter-Penrose (3) sitúa la banda actínica útil entre 320nm y 380nm. Finalmente, Luis Hernández (2), citando la documentación técnica de Autotype, define como banda útil la situada entre 330nm y 430nm con un máximo en los 370nm. También según Autotype, la sensibilidad continúa más allá de los 430nm para decaer al llegar a los 470nm y prácticamente desaparecer a los 500nm. Las pequeñas discrepancias entre los diversos datos citados pueden ser debidas al tipo de pigmento, generalmente rojo, que se incorpora en la gelatina para facilitar la detección de su presencia sobre el cobre. De todos estos datos se deduce que es necesario disponer de una fuente de iluminación rica en radiaciones de onda corta o ultravioleta. En la actualidad se dispone de diversas alternativas:

  • Instrumentos específicos del ámbito de las Artes Gráficas como las insoladoras NuArc o Amergraph. Ambas proveen un haz de rayos colimados por un reflector parabólico a partir de una lámpara de descarga de metal-halógeno. Estos aparatos disponen de control electrónico del tiempo de estabilización de la lámpara y del tiempo de exposición. También equipan la prensa de contacto con bomba de vacío.
  • Otros instrumentos de Artes Gráficas con el mismo tipo de iluminación pero en modalidad puntual. Al no disponer del reflector parabólico, la lámpara se sitúa a mayor distancia de la prensa de contacto y por lo tanto, el instrumento tiene una altura mucho mayor que los dos anteriores.
  • La misma lámpara metal-halógena citada dispuesta a una distancia suficiente de la prensa de contacto independiente como para considerarla una fuente razonablemente puntual.
  • Lámpara combinada de metal-halógeno y filamento incandescente como la Ultra Vitalux de Osram dispuesta del mismo modo que la anterior.
  • Baterías de tubos fluorescentes con emisión específica de las longitudes de onda citadas.

Los instrumentos de Artes Gráficas resuelven la iluminación UV y la prensa de contacto con bomba de vacío a cambio de un coste relativamente alto para una instalación no comercial. Las lámparas combinadas como la Ultra Vitalux son una buena elección tanto por coste económico como por su espectro de emisión, muy parecido al de las lámparas de metal-halógeno. Aún así, el volumen y el cristal opalino de su bulbo obliga a situarlas muy lejos de la prensa si se quieren evitar los efectos nocivos de la luz difusa, perdiéndose de este modo parte de su potencial actínico. En términos de coste, ahorro energético y comodidad de utilización, las baterías de tubos fluorescentes son también una muy buena elección, aunque comprometen la correcta exposición de las tramas estocásticas digitales. Respecto estos inconvenientes, Ver Heliograbado II – Trama Estocástica.

Por todo ello, el sistema escogido en este caso es el de una lámpara de metal-halógeno utilizada como fuente razonablemente puntual a una cierta distancia de la prensa de contacto independiente, dado que su bulbo es tubular y mide sólo 33mm. La lámpara es la OSRAM Supratec HTC400-241-R7s UV High Pressure Lamp. Su espectro de emisión se muestra en la Fig., 1. De los 460W nominales, 12W corresponden a la banda UV-B absorbida por el cristal de la prensa de contacto y 82W a la banda UV-A que es la realmente útil en nuestro caso. Si se comparan estos datos con los de la OSRAM Ultra Vitalux, ésta tiene una potencia nominal de 300W de los cuales sólo 13,6W emiten en el rango del UV-A de nuestro interés, es decir, seis veces menos.

Espectre_Supratec

Figura 1. Espectro de emisión de la lámpara OSRAM Supratec HTC400 en la banda entre 250nm y 450nm. Nótense los picos de emisión en la banda de longitudes de mayor sensibilidad espectral de la gelatina bicromatada.

A causa de su pequeño tamaño, la lámpara utilizada, de descarga de alta presión, precisa de un sistema de arrancado y mantenimiento de la ignición independiente que consta de:

  • Balasto HPS CCG 400W
  • Iniciador de 4-5kV
  • Condensador compensador de 50µF

La lámpara se ha montado en un soporte con reflector expresamente diseñado y atornillado a una columna de ampliadora LPL desprovista de su cabezal. Al retirar el peso del cabezal de la ampliadora, la fuerza del resorte de ayuda a la traslación vertical del mismo resulta excesiva. Para compensar este exceso, se han incorporado 2,5Kg en discos de pesas para halterofilia (Decathlon), recuperándose así la comodidad de trabajo sin esfuerzo ni riesgo de desplazamiento súbito al aflojar el sistema de frenado (Fig., 2).

Insoladora_MetalHalide

Figura 2. Lámpara OSRAM Supratec HTC400 fijada a un soporte de ampliadora para disponer de desplazamiento vertical (hacer click en la imagen para ver una versión ampliada)

La disposición de los elementos de arranque y alimentación se muestran en la Fig., 3. Sobre el balasto se pueden ver dos opciones de cableado y el documento Curso de Iluminación – Lámparas de Halogenuros Metálicos de la Universitat Politècnica de Catalunya (UPC) proporciona información completa acerca de las propiedades y formas de alimentación de estas lámparas. Para mayor comodidad operativa, el sistema de arranque y alimentación se ha conectado a una base de enchufe con toma de tierra y provista de interruptor.

De este modo, el cable de alimentación de la lámpara se enchufa en esta base sin necesidad que el conjunto de arranque esté junto a la lámpara en el dispositivo de desplazamiento vertical. Ello es especialmente indicado teniendo en cuenta que sólo el balasto pesa cerca de 1Kg. El conjunto descrito se ha protegido con una carcasa de plancha de aluminio y dispuesto en un soporte aislado que se coloca en las inmediaciones de la prensa de contacto y el soporte vertical de la lámpara (Fig., 4).

Supratec_Ignitor_1000pix

Figura 3. Conjunto de arranque y alimentación de la lámpara OSRAM Supratec HTC400 (hacer click en la imagen para ver una versión ampliada).

Supratec_Ignitor_Carcasa_800pix

Figura 4. Sistema de arranque y alimentación de la lámpara OSRAM Supratec HTC400 con la cubierta protectora. A la izquierda se muestra el cable de conexión a la red. El cable de alimentación de la lámpara se enchufa en la base de la derecha y el interruptor controla el encendido y apagado (hacer click en la imagen para ver una versión ampliada).

Siguiendo las instrucciones del fabricante (Fig., 5), se respetan 2min para la estabilización de la emisión una vez encendida la lámpara y 5min de enfriamiento antes de volver a encenderla después de un apagado.

StartUp_Supratech

Figura 5. Gráfico del flujo de emisión frente al tiempo desde el encendido de las lámparas OSRAM Supratec HTC de 400W y 1000W. 1) Lámpara de 1000W en frio. 2) Lámpara de 400W en frio. 3) Ambas lámparas a partir de media carga.

Referencias

1. CARTWRIGHT, H. M. (1961) Ilford Graphic Arts Manual, Volume 1-Photoengraving. Ed. Ilford Lted, Ilford, Essex.
2. HERNÁNDEZ, Luis (2010) El Heliograbado por el Procedimiento Talbot-Klíč – Antecedentes, uso y principios para el control del tono. Tesis Doctoral, Dir. José M. Guillén. Universidad Politécnica de Valencia-Facultad de Bellas Artes-Dept. de Dibujo.
3. HUNTER-PENROSE (2006) Photoengraving Glue Datasheet. Ed. Hunter-Penrose in Technical Notes, London.

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