• Webster's Revised Unabridged Dictionary
    • n Heliotypy A method of transferring pictures from photographic negatives to hardened gelatin plates from which impressions are produced on paper as by lithography.
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Century Dictionary and Cyclopedia
    • n heliotypy A photographic process in which from an ordinary negative is made a positive of such character that from it a direct impression in ink can be obtained by means of a printing-press. In the Edwards process, as practised in the United States, a film of gelatin sensitized with bichromate of potash, and having chrome alum incorporated with it, is formed on glass, stripped off when dry, and exposed to light during a certain time under the negative. The film is then washed to remove the sensitive principle, and is attached to a plate of metal or other solid back. Those parts of the film which have been affected by the light during exposure under the negative are left in such condition that they can be made to take printing-ink, while the parts not affected, owing to the opacity of the corresponding parts of the negative, resist the ink. This process depends upon the fact that a gelatin film sensitized with bichromate of potash becomes by the action of light insoluble in water, while the parts which have been shielded from the light, and from which the potash has been eliminated after the exposure, swell when moistened. The films are technically called skins. In other processes a mold of gutta-percha or other material is prepared from the film, and copper is deposited on this by electrotypy. The resulting plate can be printed on an ordinary printing-press. See photogravure and photo-engraving.
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Chambers's Twentieth Century Dictionary
    • n Heliotypy hē′li-ō-tī-pi a photo-mechanical process in which the gelatine relief is itself used to print from in some form of printing-press, instead of being covered with tinfoil as in the stannotype process
    • v.t Heliotypy to produce a heliotype picture of
    • v.i Heliotypy to practise heliotypy
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