164 I'KINC IPLES AND PRACTICE OF PRUNING 



138. Gas tar and pitch. Gas tar is the residual tar obtained from 

 coal used in gas making. This is produced in considerable quanti- 

 ties, and there being only a slight demand, it is sold at low prices, 

 ranging from 6 to 12 cents a gallon, exclusive of package. Pitch 

 is the pine tar derived from the distillation of pitch pine. Gas tar 

 handled in warm weather is fluid and very tenacious, with a strong 

 tendency to be completely absorbed by the wood. Upon this absorp 

 tion it is not clear that it gives a sufficiently continuous covering 

 over the wound to guard against subsequent openings. 



139. Forms of solid asphaltum. Besides the more or less impure 

 Trinidad asphaltum, used largely in street paving, there are avail- 

 able various types of asphaltum, which are essentially pure and 

 differ only in their melting points. All of these grades require 

 heating in order to apply them. 



For heating, we use a charcoal heating pot with tall bail of suffi- 

 cient spread to swing freely above a 10 to 12-quart galvanized bucket. 

 This neater is provided with openings below and vent opening 

 toward the top of the metal cylinder; also legs to keep free from 

 the ground. In such a case it is better to melt the asphaltum upon 

 some hotter lire, and merely to use the heating pot to maintain its 

 temperature in the orchard. The gasoline torch type of heater may 

 also be used and freely transported in the orchard. It will usually 

 require special modifications to enable one to use a sufficiently large 

 vessel for the asphaltum. 



The asphaltum is carefully melted until thoroughly liquid, in 

 which condition it is applied with a brush, preferably an old hearth 

 broom, a thin coating being run over the surface beyond the edge of 

 the living tissue. A second coating may be given after the first one 

 becomes partly cooled. The thickness of the coat should be de- 

 termined by the surface, as thin upon smooth surfaces as will give 

 complete covering. New hair brushes are usually destroyed in the 

 highly heated asphaltum. 



140. Advantages and disadvantages of solid asphaltum. The ad- 

 vantages of solid asphaltum are most appreciated when one must cover 

 a rough splintered wound, for the cavities may be tilled with the liquid. 

 The disadvantages of heating are usually great. Under ordinary 

 conditions it is not feasible to build a lire in the orchard, except in 

 moist weather, since there is danger of igniting the dead grass. The 

 troubles in maintaining liquid conditions are also considerable, since 

 to apply it effectively in a thin coating this asphaltum must be kept 

 hot and very fluid. Probably the most serious disadvantage is the 

 tendency to crack off during winter. There is danger also, where 

 the surface is not entirely dry, that the moisture will produce bubble 

 when the asphaltum is applied. This is dangerous, especially if bubbles 

 crack and expose the surface below. Vet, despite these disad 

 vantages, for large wounds melted asphaltum offers a rather higher 

 efficiency than anything else we have tried. 



141. Liquid forms of asphaltum. To make liquid asphaltum, use. 

 say, 10 pounds of solid asphaltum to 20 pounds of Varnolene, a 



