5. Infiltration and Embedding in Paraffin Wax 



The paraffin matrix in which tissues are embedded serves to 

 support the tissues against the impact of the knife and to hold the 

 parts in proper relation to each other after the sections have been 

 cut. These functions are best performed if all cavities within the 

 tissues are filled with the matrix and if the matrix adheres firmly to 

 the external and internal surfaces of the material. Infiltration consists 

 of dissolving the paraffin in the solvent containing the tissues, gradu- 

 ally increasing the concentration of paraffin, and decreasing the con- 

 centration of solvent. The solvent is eliminated by decantation or 

 evaporation, or both, depending upon the character of the solvent 

 and the process used. 



Properties and Preparation of Paraffin 



The properties of the embedding paraffin are important factors 

 in the success or failure of sectioning. Desirable properties are as 

 follows: 



1. Constant and known melting point and appropriate hardness; the 

 waxes used for most botanical work have melting points between 50 to 55°C., 

 with a tolerance of 2° for a given grade. 



2. Smooth, even texture, with a minimum of crystalline or grainy 

 structvne. 



3. Absence of particles of dirt, included water, and volatile or oily com- 

 ponents. 



Commercial paraffins from different sources differ widely in 

 properties and suitability for embedding. Purchases made from a 

 given source may vary from time to time — some lots giving satis- 

 factory results, whereas other lots, treated by indentical methods, are 

 unsatisfactory. For these reasons paraffins from the available sources 

 should be tested as to melting point, texture, behavior under the 

 casting methods used, and cutting properties with familiar subjects. 



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