These refined lubricating oils come either from the process of "running to cylinder stock," or from the redistillation of the "wax slop," and of the tar left in the still after cracking for kerosene is completed. These oils, in one way or another, form the basis of all grades of machine oil from the very lightest "spindle oil" to the heaviest grease. The processes of treatment differ only in minor details from those used for the lighter oils. Different cuts are made, and these cuts, together with varying methods of purification, bleaching and filtering, determine the particular grade pro duced. In general, however, the redistillation of the "wax slop" cut yields the major portion of the light and especially high-grade lubricating oils, while the heavier grades come from the cylinder stock.
Paraffin was once regarded merely as a by product of distillation, but it is now so widely used in industrial processes that in some refineries it is fully as valuable as any of the other products. Paraffin is obtained from the redistillation of either the residuum left in the tar process after cracking is completed, or from the "wax slop" cut in the cylinder-stock process. In either case the paraffin distillation is carried on in heavy steel stills at very high temperatures. The paraffin passes off in one long stream of distillate, the latter end of which may be almost pure wax. It then un dergoes the same chemical purification as the other products, the only difference being that the agita tor must be heated to prevent cooling and solidifi cation of the wax. The subsequent treatment, how ever, is much more complicated, consisting of a va riety of steps as follows: to a settling tank where the water is removed ; to a chilling tank where ammonia machines cause it to congeal and crystal lize; to a filter press which forces out any oil re maining, and leaves only solid paraffin ; to the melting tank to be converted into liquid paraffin again ; to the bone-black filter where all color im purities are removed ; and, finally, to the second chilling tank, where it is returned to the crystal lized form ready for the hydraulic presses, which convert it into cakes for shipment.
From this description it appears that only two of the important products of petroleum are regu larly obtained directly from the first distillation ; these are the illuminating oils and the cylinder stock, and both of these have to receive additional treatment subsequently. All other products are the result of a second distillation and chemical manipulations. The percentage of the different products obtained by refining varies immensely, depending both on the original character of the crude oil and on the special aims of the individual refiner. Illuminating oils run as high as seventy five per cent. or eighty per cent., and as low as
twenty per cent. to twenty-five per cent. Lubricat ing oils vary from nothing up to twenty per cent. or thirty per cent., and the residuum and waste may be as high as thirty per cent. of the whole volume of crude oil. The residuum, representing the com pounds which cannot be vaporized by ordinary means, is not, however, all loss, because, whether pitch, coke, or asphalt, according to the character of the crude oil, various methods of treatment and utilization are devised. Practically nothing is lost except moisture, solid impurities, and the varying amounts of uncondensed gases. Even the water used in washing the distillates is sent to huge set tling tanks to recover any oil which may have been included in it.
The most volatile of these distilled oils, the naph thas, are extremely inflammable liquids, the gases from which make violently explosive combinations when mixed with air. The presence of a very small percentage of the lighter naphtha oils in illuminat ing or lubricating oils is, therefore, a constant source of danger. if such oils are used explosions and fires are sure to occur. The danger is espe cially great in the case of naphthas present in kero sene : the most prolific cause of lamp accidents and fires in the early days of the industry. Continued complaints about the "deadly kerosene," as it was frequently called, led to the establishment of cer tain legal standards which all illuminating oils must meet. It has consequently become customary to subject all the distilled oils to standard tests in order to insure a uniform quality of the product. Testing is now fully as important a part of the re fining process as is distillation itself, since it is the only safeguard for the interests of both producer and consumer.
The lighter oils of the naphtha group are usually tested for gravity, odor, and acid impurity. The gravity test is made with the usual Baume hydrom eter, and on the basis of this test the oils are graded for commercial purposes as gasoline, naph tha, and benzine. The test for odors is made by simply saturating a cloth with the oil, as the oil evaporates from the cloth any foreign odors are readily detected. The presence of acid is revealed by testing with litmus paper, which immediately turns red if the acid has not been entirely removed. Benzines for special purposes, as in the manufac ture of paints and varnishes, also have to be free from any of the heavier oils. The test in this case is made by soaking part of a sheet of paper in the benzine, if heavier oil, like kerosene, is present, a grease spot shows as the volatile benzine rapidly evaporates; otherwise the whole sheet of paper presents the same appearance.