ThermaNin is a ninhydrin derivative for developing fingerprints on thermal paper. Regular ninhydrin and DFO solutions are either based on polar solvents (like acetone or ether) or contain them. These turn the top (active) layer of thermal paper gray or black. Japanese researchers have published that hemiketals of ninhydrin are soluble in petroleum ether (or other polar solvents like pentane, heptane) without the need for addition of polar solvents. These solutions develop fingerprints on thermal paper without darkening of the surface.