Collotype is a transliteration of English collotype. Colrotype printing belongs to the field of lithography. It is one of the earliest photolithography. Because it uses thick glass as the base, it is also called "glass printing".

The Loro plate is coated on a thick frosted glass plate with chromium gel photosensitive solution, and exposed to light with the negative film and the photosensitive film layer, and then a photochemical reaction occurs to cause the gel layer to harden to form an image. The density of the image density is not composed of dots of different sizes, but is caused by the difference in the amount of light received by the adhesive film, which makes the adhesive film harder. The degree of hardening determines the difference in the expansion of the film. Different degrees of expansion form fine wrinkles with different density and density. The greater the amount of light received, the greater the degree of hardening, the smaller the swelling, the more wrinkles, less moisture absorption, more ink adhesion, and the darker the color tone after printing. Luoban printing is direct printing, so the pictures on the printing plate should be the reverse image.

The process flow of Coro plate making and printing is as follows: grinding glass → coating photosensitive liquid → contact exposure → development, wetting treatment → printing.

1. Grinding glass plate

Flat glass with a thickness of about 10 mm, with an appropriate amount of emery, and then with the same thickness of glass plates, grind each other to make the smooth glass plate into a rough surface, after washing, drying, coating a layer of silicon Sodium mixed solution (a mixture of sodium silicate and protein solution) is used as the bottom layer. The bottom layer is coated to prevent the glass surface from staining and to make the photosensitive film adhere to the glass plate more firmly.

Second, coating photosensitive liquid

Apply a certain amount of heated dichromate gelatin photosensitive solution to the surface of the glass plate coated with the bottom layer in a dark room, and dry it flat in an oven at 40 ～ 60 ℃. Bubbles will be generated at high temperature. Once the photosensitive film is dried, fine wrinkles will be formed on the surface. The wrinkles will be thicker when the temperature is higher, and the wrinkles will be finer when the temperature is lower.

3. Exposure

The continuous negative image negative film is exposed in close contact with the photosensitive plate. Due to the difference in the amount of light received by each part of the plate surface, the photosensitive film produces different degrees of hardening reaction. The hardened part due to light irradiation becomes difficult to absorb moisture and easily absorbs ink. After exposure, a full exposure is performed from the back of the photosensitive plate, so that the adhesive film is closely attached to the plate base.

4. Development and wetting treatment

Develop the exposed photosensitive plate with water. The unexposed part of the adhesive layer swells and then dissolves away. The swollen part of the light-receiving part is less swollen. According to the density of the negative film, it absorbs water. Let it dry naturally after shadowing.

Using glycerin-based wetting agent, wetting the plate surface, the purpose is to keep the water sufficiently and prevent the water from losing.

5. Printing

The printing is carried out on a special Collo flat-bed printing machine. The structure is a round flat type. It is manually wiped, rolled and rolled. It requires less fluidity and viscosity of the ink. The water resistance of the binder is good, and the pigment Easy to disperse and soft structure, so as not to damage the printing plate. When inking, the reaction roller is used for black ink, and the rubber roller is used for coloring ink. Because the swollen portion of the plate surface absorbs much water, it is difficult to absorb a large amount of ink. The concave portion of the plate surface is filled with more ink, which can be printed with greater density. The environment temperature and humidity are stricter when printing.

The gradation of the Colo prints can faithfully reproduce continuous-tone images, and its effect can exceed that of lithographic and letterpress prints. It is suitable for printing sophisticated high-end prints. It is precisely because of this feature that the oldest photo-engraving method can The continuation of this process is maintained. In addition to being printed on paper, the Corot version can also be printed on silk and silk. Because the base is glass, the printed part and blank part are both gelatin, so the printing power is not high. Generally, a printing plate can only print about 500-3000 copies, and the maximum is not more than 5000 copies. Therefore, it is not suitable for printing urgent items and high-volume products.

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