A transfer sheet for spirit duplication to produce concealed images containing an oxidizing agent capable of oxidizing a subsequently applied iodide to produce a color forming iodine reactant, in which the transfer sheet comprises a flexible paper base sheet having a transfer coating containing a colorless, soluble copper salt and an amine selected from the group consisting of piperidine, piperazine, phenylenediamine, tallowamine, trichloromelamine, triisopropenolamine, and melamine to improve the hygroscopic character of the copper salt.