Residual Films on Lithographic Plates and Methods for their Removal.

It has been proved definitely that albumin plates, as usually made, have a thin film of residual albumin on their non-image areas. Desensitizing gums do not adhere well to such a film, and the plates are not desensitized as well as they would be if the etches were laid down on truly bare metal. The measurement of contact angles of water on plate surfaces has been utilized in studying methods for the removal of residual albumin. The lower the contact angle, the less residual albumin and the better the plates are desensitized. Contact angle measurement and press tests showed that the use of a gum pre-etch reduces the amount of residual albumin. Washing the plates with an ammonia solution following development removes part of the residual albumin. The combination of a pre-etch and an ammonia wash is better than either one alone.