AbstractHardness of the blocks of polymerized methylmethacrylate (MMA, a plastic embedding medium widely used in calcified tissue histology) was measured at the top, the bottom, and the sides of the blocks. In addition, the most superficial layers of resins were removed with a grinding machine to explore the hardness in the inner parts of the blocks. A kinetic study of hardness changes was also done in all these areas during a period of 60 days. We observed an altered polymerization at the top of the blocks that we attribute to incompletely excluded atmospheric oxygen. However, a defect in the polymerization process was also observed on the lateral sides and the bottom of the blocks, suggesting complex changes induced by surface contact: MMA continued to harden quickly after blocks were been removed from the molds, and polymerization was not completed even if the blocks looked hard and suitable for sectioning. Heightening the tissue specimen with polyester foam or sponge (to avoid contact with the bottom of the mold) and waiting one or two days after removal from the mold were easy ways to avoid embedding, sectioning and staining artifacts.(The J Histotechnol16:65, 1993)