Abstract: The document or substrate containing the information to be protected is covered with a liquid crystal or other such material capable of being made transparent using a process such as heating it to a higher temperature than ambient room temperature. In this manner the information on the substrate is normally invisible and thus not accessible to individuals without special equipment. For copying, the coated substrate is then placed on a copier machine in which the transparent glass plate of the copier is heated, for instance by electrical resistance heating, to a temperature matching the temperature-transparency window of the coating on the document. The information on the substrate may also be made visible for viewing by covering the coating with a transparent plate heated to the proper temperature. A second coating of different color but substantially the same temperature transparency may be selectively applied to the first coating to provide normally visible information.