Hardening 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:
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 (315nm – 400nm). 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 (280nm – 315nm) that is absorbed by the vacuum press glass, while 82W are emitting the band of UV-A (315nm – 400nm) 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.
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).
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.
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 one 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.
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.
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.