Embedding 



PARAFFIN SECTIONS 



99 



bottom and 2j^ of the clear molten mate- 

 rial above. Thus, when the object is placed 

 in one of these vials it will droj) until it 

 reaches the solidified layer, where it will 

 remain in contact with molten wax at 

 exactly the melting point of the wax. It is 

 obvious that the room in which this opera- 

 tion is to be conducted must be at a fairly 

 constant temperature and be free of drafts, 

 but only a very large volume of embedding 

 work justifies the purchase of an expensive 

 thermostatically controlled oven. If such 

 an oven is to be purchased it is liighly de- 

 sirable to avoid one in which the heat is 

 distributed bv convection. Such an old- 



Vacuum ovens are occasionally required 

 for the impregnation of the most dilficult 

 material but should he avoided whenever 

 possible. If a vacuum oven is to be em- 

 ployed, moreover, it is necessary that all 

 volatile solvents be removed from the 

 material before it is placed in the vacuum 

 so that it is always desirable to precede 

 exposure in a vacuum oven by a consider- 

 able period of embedding in an ordinary 

 oven. 



Assuming that the material has been 

 passed through dehydrating and clearing 

 agents, and is now awaiting embedding, 

 there are two main methods by which this 



Am INIAKC 



Fig. 38. Circulating air embedding oven. 



fashioned convection oven is seen in Fig. 

 52 and is to be found all too frequently in 

 laboratories. Unfortunately these ovens, 

 as any cook could tell any microtomist, 

 vary enormously in temperature from top 

 to bottom. The thermostat is usually 

 placed at the top and, in a fairlj^ large 

 oven, there may be as much as a ten- 

 degree differential between the lowest 

 shelf and the top one. The oven shown in 

 Fig. 38 in which a circulating fan continu- 

 ously moves the air and thus maintains 

 a uniform temperature throughout the 

 whole oven, is infinitely to be preferred. 

 It is the high cost of such circulating-air 

 ovens which leads the writer to believe 

 that much more use should be made of the 

 very simple rachant-heat embedding de- 

 vice discussed previouslj'. 



may be done. luther be the oliject may 

 transferred directly to a bath of molten 

 wax, or it may be passed through a graded 

 series of wax-solvent mixtures. The writer 

 is strongh' in favor of the latter course. 

 Let us suppose benzene has been selected 

 as the clearing agent and that the object 

 is in a vial containing a few milliliters of 

 this solvent. Chips are then shaved from 

 the block of embedding agent and add('(l 

 to the vial. These usually dissolve very 

 slowly and form a thickened layer at the 

 bottom of the tube through which the 

 object to be embedded sinks. The average 

 object will be satisfactory if left overnight. 

 The tube is then placed in the embedding 

 oven, maintained at a temperature slightly 

 abt)ve the melting point of the wax, and 

 as many further shavings as possible are 



