Collotype
Light pressure ( also Phototypie, Collotypie, Albertype ) is a now rarely -applied Darkroom procedures.
In a broader sense so that all photomechanical ( photolithographic ) planographic printing method for reproducing halftones without grid called, but developed only in 1856 by Louis- Alphonse Poitevin under the name Collotypie in the narrow sense and in 1870 by Joseph Albert improved methods with which longer runs could be produced. Karl Klietsch developed in 1879 from the photogravure.
Use
The light pressure was very popular around 1900, but was ousted by the dissemination and improvement of offset printing. Following the closure of Dresdner light pressure workshop today exist worldwide, three light printing: the Leipzig Museum of the Printing Arts, the world's oldest photo workshop of the brothers Alinari in Florence and in Benrido printing in Kyoto, Japan.
In addition to the colored lithograph he was mainly used for the illustration of books or printing in small quantities, such as viewing, Memorial postcards, there were no other known method, drawings, paintings or photographs, reproductions of comparable quality.
Artists, too, have used the light pressure for independent statement for graphics. An example is Willi Baumeister with his twenty original light printing, which was published in a published folder with 200 copies Sports and machine in 1929 by the gallery Flechtheim.
After the distribution of the process in the middle of the 20th century gradually declined, it is still used today outside the immediate area for artistic facsimile of works of art such as paintings, medieval manuscripts and documents. To this end, the playback quality is unmatched.
The light pressure is set with its means and possibilities - like photography - an art form dar. In Leipzig developed techniques and processes of artistic creation on the pressure plate in collaboration of light printers and artists such as Olaf Wegewitz. Thus, the light pressure original graphic was its own form of graphic art. Due to the proven possibility of exposure with different semi-permeable and permeable materials on the printing plate, a new means of expression in the photographic work emerged. These two types are recognized as a distinct art forms.
Platemaking
Support of the printing form is a 10 mm thick matt etched very flat glass plate (plate glass without mirror layer ) or a metal plate. On the plate ( prelayer & printing layer ) is applied a light sensitive emulsion of Chromatgelatine and dried in semi-darkness in two layers. The emulsion consists of gelatin, which was made sensitive to light with ammonium dichromate or potassium dichromate.
Then a photographic halftone negative using light is exposed onto a high UV content on the disk. The negative is - unlike most printing processes - not rasterized. When exposing the chromate salts contained in the gelatin alter the solubility of gelatin in water and set the negative image into a gelatin relief. This process is called tanning of the gelatin. The amount of incident light determines the amount of " tanned " gelatin. The special feature of this emulsion in its relation to the amount of light to the degree of cure:
- Little light = low curing
- Much light = strong hardening
After the copying operation, the board is immersed in cold water 5 ° C to 10 ° C, so that the chromate be washed, an additional exposure is prevented. In this case, the characteristic " reticulation " forms are to be identified at the ready light unmistakable prints at high magnification.
Is a multi-color result desired, each corresponding printing plates are made for each ink color. Sometimes up to 20 plates for certain colors are overprinted For facsimiles of works of art. Thus, it is possible to obtain prints of high fidelity.
Pressure
Before printing, the printing plates are usually moistened with a glycerin - water mixture. The swelling of the gelatin depending on the degree of cure - hence depending on exposure level - the reproduced tone is determined. There is a relationship between the exposure and the recording ink at the corresponding location. The following applies:
- Low curing = severe swelling, a little ink
- Strong hardening = slight swelling, a lot of ink
The water in strongly swelling areas contained abundant plays a role: it repels the greasy ink from, whereas the more hardened areas to reject this. In light of the pressure - low profile cylinder press, the printing takes place in this printing process. This light pressure quick presses are similar in construction to offset monochrome Andruckflachpressen. Often converted stone presses were used. The daily number of pieces is between 600 and 1000 arc and requires a high level of professional experience and concentration of the printer. The fine "Source relief" to be controlled by another subsequent wetting of the printing form in contrast. Stronger humidification reduced, weaker humidification increases the contrast.
The appearance of the pressure can be influenced by chemicals which are applied to the printed layer of gelatin. Formalin gelatin, alum makes individual print darker parts because it affects curing. Cyanide is hygroscopic, it absorbs water, making the soft gelatin and so are individual pressure parts brighter.
The relative humidity in pressure near must be constantly kept the same. In this respect, the Fort pressure must be constantly monitored. One person at the recording sheet is inserted in the printing machine and the printer it is removed after the printing operation of the machine, and evaluated. The pad can reach 1000-2000 prints, then a new screen has to be created. The gelatine layer is occupied by the printing processes and damaged.
The achieved editions are low and the amount of work in the light pressure is very high. Light pressure is expensive, but the quality of reproduction justifies the high price. In early times, the light pressure was comparable in workload with other printing processes and put in pressure result in other ways not achievable Printed Media ago.
A special feature of the light pressure is the oil-based, but especially solid and tough light ink. The processing is only possible with special strong color grater blades. Before each use, it is under immediate addition of oil varnish again with this color meter (printer Patel ) durchzuwalken so that the paint is ready for use.
Light prints, when they are stored completed relatively airtight in book form, retire a stronger odor of the chemicals used, for example, by ox bile, which is used during printing to avoid disturbing mitdruckende edge tones.