REPORT OF THE SOCIETY 63 



The Open Tub Method 



This is a simple method of treatment and gives a better return than the 

 preceding one, but the apparatus needed for the cairrying it out is more exten- 

 sive. The posts are heated in a hot bath of creosote which makes the air in 

 the vessels of the wood expand, and converts into vapour the moisture that is 

 in the wood. As a result, part of the contained air and of the moisture is re- 

 moved. The posts are, after this, placed in a cold tub or copper, or allowed to 

 remain and get cool in the l)ath, which was previously heated but is now cooled. 

 The contraction of the air and the condensation of water vapour, which take 

 place at the reduced temperature, bring about a partial vacuum, and, as a result, 

 the preservative liquid is forced into the wood by atmospheric pressure. The 

 receptacles used, as well as the details of the method employed, depend on the 

 judgment of the operator and the cost of the material. 



An open tub, comparatively inexpensive, which can be employed on 

 all farms consists of a water-tight barrel, three pieces of iron piping and 

 the necessary fittings. One piece of piping passes into the barrel near the 

 bottom, a second higher up, while the third piece connects these two. A 

 number of posts are placed in the barrel, and creosote is poured in to a level 

 above the upper piece of piping. Then a fire is lit beneath the lower pipe, care 

 being exercised that the barrel does not take fire, for creosote is veiy inflammable 

 and a single spark may be the cause of the destruction of the whole. The heat 

 w'ill circulate slowh'^ and the tar will be raised to boiling point. The heat must 

 be maintained at this temperature, but not allowed to go above 220°F. The 

 loss due to evaporation will be considerable and will vary from 10 to 20 per cent, 

 according to the density of the liquid. When heating is discontinued the stakes 

 are left in this bath so long as any bubbles of air continue to rise to the surface. 

 The time necessary for treatment varies with the nature of the wood, the amount 

 of bark, and the dryness of the material. It may be anything from one to six 

 hours being a matter for the judgment of the man who is carrying out the work. 

 It is advisable to make a few preliminary trials to ascertain what length of 

 time gives the best results. When it is considered that the heating has proceeded 

 far enough, the fire is extinguished, and the creosote allowed to cool down, but 

 the stakes remain in the liquid for three or four hours longer. This time having 

 elapsed, they are placed in an empty barrel, but they are now upside down, as 

 compared with their position when being boiled in creosote. This is done so 

 that the excess of creosote which has not soaked fully into the wood may have 

 an opportunity of passing down into the portion which was previously uppermost, 

 and therefore not so well penetrated bj- the creosote. 



If it is preferred, however, the posts may be left standing just as they were 

 in the tub in which they were treated, and the surplus creosote collected and 

 used again. 



This "open tub" method permits of great variation in the details of treat- 

 ment and also certain variation in the plant used. 



The posts are sometimes lifted out from the tub containing the hot creosote 

 and placed in another receptacle containing cold creosote, where they remain 

 until a sufficient degree of penetration has been reached. If this method is 

 followed, the barrel with the hot liquid can be at once refilled with posts and the 

 operation started again immediately. In this way there will be an economy of 

 time. A wooden vessel sufficiently large to allow the posts to be laid in it on 



