The Artotype process is very similar to the Collotype, a slight improvement being made by Obemetter. A mixture of soluble glass and albumen was used for the foundation film, thus avoiding the necessity of first exposing to the light. In all other respects the process is similar to the Collotype.
Chromacollotypes are produced with a number of Collotype plates, using differently colored inks. The negative to be reproduced is first blocked with opaque varnish in all parts except those required to be reproduced in a certain color. From this a collotype plate is made to be used with that colored ink. The varnish is then removed and the negative blocked for another color, and so on. In printing, the paper is passed through with the different plates until all the colors have been printed on it, forming a complete many-colored image. Instead of blocking out the negatives they are sometimes made by photographing the object through different colored screens, which have the effect of shutting off or absorbing some colors and allowing others to pass through undisturbed to the sensitive photographic film.
The Indotint process was invented by Roche, of New York. A copper plate is used, roughened by exposure to the sand-blast, causing the sensitive film to firmly adhere. The film is also roughened by the addition of alcohol to the dichromatized gelatine. After exposure the unaltered dichromate is removed by washing, and the plate dried.
process was invented by Husnik, of Prague. He takes a thick plate of chromatized gelatine and exposes it under a negative. This is then attached to gutta-percha, zinc, or wood as a foundation. The image is next developed with a solvent such as a saturated solution of alkaline dichromate. This has the double effect of dissolving the unexposed parts, and deepening and strengthening the relief. The high lights are protected, and the relief again exposed to the light, after which it is again developed to deepen the whites and further strengthen the image. Plates thus produced can be used in the ordinary printing machine with the letterpress. Electrotypes can also be made from these plates when a very large number of copies is required.
TheHeliotypie process is closely allied to the Collotype. The most important difference is that
the gelatine film containing the image is hardened with chromic alum, and detached from its support. It thus forms a thin sheet or " skin," as it is termed, which can be laid on to a zinc plate, or attached to the cylinder of a printing machine. Further, it can be easily preserved for printing from on future occasions when desired.
In our next class, i.e., those processes in which the image is printed from stone, we have to mention the following :—Photo-lithography, ink-photo process, and the photo-caustic process.
In an image is either made upon the stone direct, or first upon specially prepared paper and afterwards transferred to the stone. The transfer process is the one most general. A full description of the process will be fonnd under Photo-lithography.
The Ink is a method of photo-lithography in half-tone. The process was invented by Sprague, of London, the details being kept a secret.
is the name given to photo-lithographic images in half-tones by means of a Meisenbach veiled negative.
With regard to those processes of typographic or block printing where the image is printed from a metallic surface in relief, they may be conveniently divided into two divisions, z e., the swelled gelatine processes and photo-etchings. Under the first we have the photo-electrotype, photo-engraving, and Mosstype processes. There are a very large number of photo-electrotype processes, differing in minor details. The process may be briefly described as follows :—A sheet of chromatized gelatine is exposed under a negative. Parts are thus rendered insoluble and incapable of absorbing water, the remaining portions, protected by the image of the nega tive, being soluble and capable of absorbing cold water. After exposure the gelatine is immersed in cold water, when the absorbing parts swell up, or else the soluble parts are removed with warm water, acetic acid, or other solvent. The next process is the making of moulds in wax, or plates from the gelatine image ; and from these copper blocks can easily be made for typo graphic printing by the electrotype process. If half-tones are to be represented the negative must be broken up into lines, dots, or stipple.