32 Botanical Microtechnique 



In most parts of the United States, the wax obtained from pe- 

 troleum is known by the name paraffin, whereas in some areas tlie 

 term wax is used. The tw^o terms are used indiscriminately in this 

 manual. 



Most of the paraffins sold for domestic canning have excellent 

 properties but are too soft for sectioning under ordinary room tem- 

 peratures or for cutting very thin sections. 1 his inexpensive paraffin 

 is satisfactory for sectioning soft materials such as fruits, if sections 

 over 20 |.i in thickness are desired. Paraffin of excellent quality and 

 stated melting point can be purchased from biological supply houses, 

 but at rather high cost. Canning paraffin can be used for preliminary 

 infiltration, and the more expensive hard paraffin used for the 

 final embedding. Canning paraffin requires no preparation; the 

 pieces may be put into the oven tank where melting takes place 

 readily. 



Bulk paraffin can be purchased in 10-lb. slabs at low cost from 

 petroleum refining companies. This bidk paraffin usually contains 

 considerable dirt but it can be purified easily. Heat a quantity in a 

 pan until it just begins to smoke, then keep over a small Hamc for 

 at least i/^ hr. Avoid heating the paraffin to the ignition point. Pour 

 the paraffin into a tall metal container, such as a tall cofl^ee can, and 

 permit it to cool in a warm place. This permits particles of dirt to 

 settle. Cool until the surface begins to s()lidif\, then decant into the 

 oven tank. The smoking hot wax can be filtered rapidly through dry 

 filter paper. Use a coarse filter paper and kecj) the sides of the metal 

 funnel warm with a small bunsen flame. 



Each ptnchase of paraffin should be tested \)\ casting a test block 

 in a paper boat. The paraffin test block should contain no l)ul)bles, 

 opacjue spots, streaks or internal fractures. When the chilled block is 

 broken, the fractine should sho^v a grainless or finely granular surface. 

 The paraffin should slice into thin curled shavings, not into brittle 

 granules. Keep a test block at a temjx'raturc of 30 to 35°C. for 

 24 hr.; bubbles and opaque crystalline spots should not a])pear. 



Cast blocks of good paraffin should remain free Iroiii iiittinal 

 defects indefinitely, especially if stored at a constant, low temperature. 

 Occasionally, one encounters old blocks thai are ahnost as clear as 

 glass. Some such waxes have adequateh fine grain and ma\ cut \ery 

 well. In other cases, the impact of the knife causes opaque fracturing 

 of the wax, as when ice is struck ^vith an axe. The wax has ob\iously 

 crystallized into a coarse texture during storage. WMien a block of 

 such parallni is melted slowly in the o\en and recast, the new block 



