56 MATERIALS AND PROCESSES OF SLIDE-MAKING 



removed from the specimen by molten wax, so that if they must be used they 

 should always be washed out with a hydrocarbon before the wax bath. Rela- 

 tively small traces of any essential oil will destroy the excellent cutting proper- 

 ties of any wax mixture and, as the oils are nonvolatile, there is no chance of 

 getting rid of them in the embedding oven. 



Choice of an Embedding Medium. It is to be presumed at the present 

 time that no one will endeavor to use a plain paraffin but will use a mixture. 

 If a plain paraffin is preferred, then it is necessary to buy (in the United States 

 by importation) a carefully fractionated and very expensive wax. Ordinary 

 cheap paraffin is a mixture of a great variety of compounds of slightly different 

 melting points; it is essential in the use of pure wax that a wax of a very 

 sharp melting point should be obtained. 



The choice of an embedding medium should be dictated less by the nature 

 of the specimen than by the conditions under which it should be cut. If pure 

 paraffin is to be employed, it should be of such a melting point that will give 

 the hardened wax a crisp section at the required room temperature. Since the 

 introduction of any foreign substance automatically lowers the melting point 

 of the wax, it is obviously desirable to use mixtures rather than the pure 

 material. For ordinary routine preparations, the author's preference is for any 

 of the paraffin-rubber-bayberry wax mixtures. The introduction of rubber un- 

 doubtedly increases the stickiness of the wax and makes it easier to secure 

 continuous ribbons. Bayberry wax not only prevents the crystallization of the 

 paraffin but also lowers its melting point. Two of the best mixtures are: 



Hance's Rubber Paraffin: 



Stock rubber solution 



Cut 20 Gm. of crude rubber into small pieces and dissolve, with con- 

 stant stirring, in 100 Gm. paraffin heated to smoking. 



Embedding wax 



Paraffin 100 Gm. 



Stock rubber solution 4 Gm. 



Beeswax 1 Gm. 



Maxwell's Embedding Wax: 



Paraffin 100 Gm. 



Hance's stock rubber solution 4 Gm. 



Bayberry wax 7 Gm. 



Beeswax 1 Gm. 



It is not recommended that these embedding waxes be prepared in the lab- 

 oratory. There are many proprietary embedding waxes, sold under a variety of 

 trade names, which have essentially, the composition of the two mixtures 

 just described. 



