
Paper-based feedback systems containing hidden entries comprising an image forming reactant selected from the group consisting of dithiooxamide, the N,N'-disubstituted derivatives of dithiooxamide and an iron-complexing compound having a benzene ring and at least two hydroxyl compounds substituted in adjacent positions on said ring. The hidden entries are developed by contacting the entries with an image-forming coreactant contained in a wax matrix in the form of a crayon or sheet.











