The final step of processing is infiltration with a cell-supporting medium. Paraffin wax is the most common infiltration and embedding medium. Paraffin wax is miscible with xylene and isopropanol but not with other alcohols or aqueous fixatives. Various infiltration media are available to suit every tissue type and application.
Factors such as wax hardness, stickiness, and brittleness during microtomy should be considered with the technique and tissue in mind. As a general rule, the higher the melting point of paraffin, the harder the paraffin and the more complex the ribboning. This also indicates a high amount of polymer within the paraffin. The lower the melting point, the softer/stickier the paraffin (lower polymer content), and the easier it is to ribbon during microtomy. Most common paraffin have a melting point between 55–58°C, and the paraffin is typically kept at 2–4°C above the melting point on the processor.
During this last processing step, heat and vacuum are utilized for optimal paraffin infiltration. However, too much heat and time in liquid paraffin may harden tissue and create microtomy problems. As with the previous processing steps, the optimal infiltration time for each tissue type/size must be determined for optimal results.
