Many attempts have been made to perfect this process which is designed to treat susceptible timbers without the aid of cylinders. The apparatus required is comparatively simple and the results obtained are more or less satisfactory. The open-tank process is particularly convenient when small quantities of impressionable woods are to be treated, or where the treatment is to be localized as at the ends of posts or poles. It has not yet been widely adopted, however, nor is it likely that it will ever be extensively em ployed by those who treat the largest quantities of woods.
The timbers placed in an open reservoir are treated with hot, and then with cool baths of the anti septic solution. The changes of temperature may be obtained as follows: first, after the pieces have remained in the hot preservative for a sufficient time the fire may be withdrawn and the preservative allowed to cool; or second, the pieces to be treated may be trans ferred from a tank containing hot preservative to another tank con taining cool preservative; or again after the pieces have remained for a sufficient time in the hot pre servative the latter is replaced by the cooler liquid.
The time required for penetration varies with seasoning, species, and other factors, and in every instance, must be determined by actual test. It should be noted that sufficient penetration cannot be obtained from a hot solution only. On the contrary, the absorption usually takes place during that part of the process where the oil is cool. The prin cipal function of the hot bath seems to be to prepare the wood for treat ment. A general case would be as follows: The preservative solution is heated to from 190 degrees to 210 degrees Fahrenheit. Timbers are then placed in the warm solution in which they are permitted to re main for from two to six hours, after which they are placed in the cool solution in which they remain for from two to twelve hours.
The tanks employed in experiments upon large poles have been of two kinds. In some, the bottoms are inclined so that poles can be placed in and withdrawn from the tanks without the aid of derricks or other machinery. This form is convenient, but does not represent an economical design for permanent installation, because the large surface of oil favors the evaporation of the preservative, and because only a few poles can be treated at the same time.
Other designs provide cylindrical or rectangular treating tanks, in which the poles are placed vertically. Such tanks restrict the surface of the preservative exposed to the air and are thus more economical; but, these vertical tanks require derricks for handling the poles, and are usu ally intended to be operated in connection with the steam boilers em ployed to heat the preservative. A storage tank, an oil pump, and an emptying tank can be added to the equipment if the amount of work contemplated and the time required for treating each charge are of suffi cient importance to warrant their use.
The simplest kind of apparatus is sufficient where only a few short pieces of responsive wood are to be treated. That shown in the lower picture (see Fig. 77) is not likely to be satisfactory if used many times, because the connections between the pipes and wooden barrels cannot be prevented from leaking ultimately. A much better arrange ment includes a light iron tank, about the size of an oil barrel, fitted with a U-connection of two-inch pipe, which projects out for a sufficient distance from the side of the tank, and to which the heat is applied. The first cost of this device is greater than the cost of that shown in the second picture but it is more economical if permanency is desired.
It will be remembered that woods differ in receptivity, and that some kinds receive solutions much better than others. This is shown in some results reported by Kempfer, as follows:' A specification prepared by The American Telephone and Telegraph Company to guide the treatment of the butts of poles by the Open-tank Method is as follows:2 "Hot-oil Treatment. The poles shall be kept in the bath of dead oil of coal-tar maintained at a temperature of not less than 212°F. for cedar, chestnut, and partially seasoned and green loblolly pine poles, or not less than 200°F. for seasoned loblolly pine poles, and not more than 230°F. for seasoned poles, for at least five hours, partially seasoned poles, for at least eight hours, and for green poles, for at least ten hours.
"Cool-oil Treatment. At the completion of the hot-oil treatment, a sufficient quantity of cool oil shall be admitted to the treating tank to lower the temperature of the oil to at least 100°F. The level of the oil in the tank shall be maintained by means of an overflow outlet or an emptying pipe controlled by It.yalve. Poles shall be kept in the cool oil for at least eight ' • "If it is not possible to) lower the temperature of the oil as above described, the oil shall be;allowed to cool.by atmospheric exposure. In this case the poles shall be kept in the cooling oil until the temperature of the oil has dropped to at leak, 110°F., but-in no case for less than ten hours.
"The treated section of the pole shall not be exposed to the air during any portion of the treating process.
"Depth of Impregnation. All poles shall be treated so that the oil impregnation shall extend through the sapwood. One pole in ten shall be bored to ascertain the depth of penetration and such borings shall be made four (4) feet from the butt end. The bore hole shall be filled with hot dead oil of coal-tar immediately after the depth of penetration has been ascertained." The advantages and limitations of the open-tank process have been summarized by Kempfer as follows "The tests made by the Forest Service indicate that the sapwood of a great variety of species, including nearly all of our common native woods, when seasoned can be successfully impregnated by the open-tank pro cess. The heartwood of many species offers considerable resistance to impregnation and cannot be so well treated without pressure. However, with the exception of a few species having an unusually narrow sapwood, it is believed that the thorough treatment of the sapwood portion of round timber will afford good protection to the entire stick. Since poles are almost always used in the round, the open-tank process is especially well adapted to the treatment of this class of timber. The apparatus required is comparatively simple and inexpensive, especially where but few poles are to be handled, and if desired can be made portable.
The open-tank process is not adapted to the treatment of woods which are difficult to impregnate, nor to unseasoned or partially seasoned wood, and as regards economy of operation, has not justified itself in plants designed for the treatment of the entire pole. The large amount of oil lost by volatilization from open tanks and the difficulty of accur ately gauging and regulating the amount absorbed are other tages."