The specific nature of the tissue to be preserved is not critical, and I am not aware at this time of any soft vegetal or animal tissue that could not be converted by this invention to a solid body retaining many properties of the fresh tissue, particularly optical properties, yet virtually immune to decay and capable of withstanding substantial mechanical stress. The specific nature of the fluid precursor composition employed as an impregnant is also unimportant within obvious limits. The precursor must not chemically attack the tissue while fluid, and many monomers and prepolymers now commercially available and yet to be discovered meet and will meet this condition. Entirely satisfactory results have been obtained so far with acrylic resins, epoxy resins, polyester resins, polyurethanes, and silicone resins varying widely in their chemical properties and their processing characteristics, particularly in the conditions under which they are formed by polymerization of monomers or intermediates.