This article describes a method for patterning certain solids—insoluble salts and reduced metals—in sheets of paper. Chemical reactions that produce these solids as precipitates occur when a stamp of patterned paper inked with an aqueous solution of reagent comes into contact with a paper substrate containing a second reagent. The pattern is determined by a hydrophobic barrier on the stamp that restricts delivery of the ink in two dimensions. The technique is one of the few methods available to generate two-dimensional patterns of solids within the bulk of sheets of paper. It can reliably produce features with lateral dimensions only down to 1 mm, but is convenient, inexpensive, and amenable to use in large-area patterning. The method can be used to introduce function to paper-based systems: (i) paramagnetic salts precipitated within paper allow pieces of it to be manipulated or separated from mixtures with a bar magnet, (ii) the precipitation of transition metals or enzymes can be used to position and store catalysts on paper for subsequent use, (iii) catalysts patterned in two dimensions in paper can be used to generate dynamic 3-D structures when exposed to appropriate substrates: for example, a ring of Pd⁰ deposited on a sheet of paper will generate a cylindrical “cage” of rising oxygen bubbles when exposed to an aqueous solution of hydrogen peroxide, and (iv) patterns of catalysts or colored precipitates can serve as deterrents to counterfeiting.