InfiUration with Parafjin B5 



Infiltration with Paraffin 



Paraffin is considered to be either soft or liard. The melting point 

 of the former lies in either the 50-52°C or 53-55°C ranges; of 

 the latter in the 56-58°C or 60-68°C ranges. The choice of melt- 

 ing point depends upon the thickness at which the tissue is to be 

 sectioned, or upon the type of tissue — hard paraffin for hard tissues and 

 soft paraffin for soft ones. If relatively thick sections are to be cut, use a 

 soft paraffin; otherwise the sections will not adhere to each other in a 

 ribbon. If thinner sections are desired, down to a thickness of 5-7 

 microns, use a paraffin in the 56-58° grade. For extremely thin sections 

 of less than 5 microns, sometimes the best results can be obtained with 

 a hard paraffin of 60-68° melting point. The sections will retain their 

 shape and size without excessive compression and will ribbon better 

 than if the paraffin is too soft. In addition, room and temperature con- 

 ditions can influence the choice of paraffin. Often hot weather will force 

 the use of a harder paraffin than might be chosen during cool weather. 

 If it is impractical to stock more than one kind of melting point paraffin, 

 usually a 56-58°C is the safest choice. 



Except in the case of friable tissues, the following step may be 

 omitted. After the tissue is well cleared by benzene or other clearer, 

 begin to saturate the soltuion with fine shavings of paraffin until some 

 of the paraffin remains undissolved. Leave the tissue in the sattirated 

 clearer for 4-6 hours, or overnight for large pieces. Then with a warm 

 spatula remove the tissue to melted paraffin already prepared in an 

 oven. 



In normal cases which eliminate the preceding step, tissues are trans- 

 ferred directly from clearer to paraffin. Keep the oven temperature only 

 high enough to maintain the paraffin in a melted state, no higher. This 

 lessens the danger of overheated tisstie, which can initiate hardness and 

 shrinkage. Paraffin standing in a warm oven in a melted condition for 

 several days or weeks is better for infiltrating and embedding ptirposes 

 than freshly melted paraffin. After ^-1 hour in the first bath the tissue 

 is removed to a fresh dish of paraffin for a similar length of time. Two 

 changes of paraffin are stifficient for most normal requirements, but for 

 some cases of difficult infiltration, such as horny skin or bone, a third 

 change may be necessary, and the time of infiltration may need to_b( 

 extended to as much as six hours o\er-all, e\en o\ernight. Su( Iv 

 fortunately are rare. 



The use of a vaciumi oven lor infiltrating ^vill remo\e air Irom some 



