This section is from the book "A Practical Treatise On Friction, Lubrication, Fats And Oils", by Emil F. Dieterichs. Also available from Amazon: A practical treatise on friction, lubrication, fats and oils.
The intermixing of fatty oils for purposes of adulteration, when the one oil can be bought in the market at lower price than the other, is not very detrimental in their use for manufacturing purposes, but when fatty oils have been adulterated with petroleum oils, the difference in value and the thereby lowered quality of the oil, demands investigation. Lard oil, cottonseed oil, neatsfoot oil and other fatty oils, are more or less adulterated with high-fire-test petroleum oils, and with such of them as have been especially-prepared and deodorized for that purpose. Such adulterations can he detected by the following simple tests:
When petroleum oil has been admixed in large proportion, the simple pouring of some of the oil on a dark ground, or, onto a smoked-glass plate, in a place where the sunlight can fall on it, will give sufficient proof of the presence of petroleum oil, by its bluish, fluorescent lustre. Its presence can also be ascertained by the use of the hydrometer. As the specific gravity of petroleum is so much higher than that of fatty oils, the hydrometer cannot fail to indicate by the so much increased specific gravity of the mixture above the well-known lower gravity of pure fatty oils, that petroleum is present and how much of it. Its presence in fatty oil compounds can further be ascertained by slowly heating a sample of the suspected oil in a porcelain dish over a spirit lamp, with a thermometer suspended in it, and by applying from time to time a lighted match to it and note the temperature at which the oil will ignite. Petroleum used for compounding with fatty oils will vaporize and ignite at from 110° to 300° F. while fatty oils require for ignition twice as much heat.
To ascertain with what proportion of petroleum a fatty oil has been adulterated, samples are prepared and mixed in various definite proportions. Mark on each sample the proportion of petroleum added to the oil, the specific gravity ascertained by the hydrometer, and the temperature at which it became ignited. By submitting a sample of the oil to be investigated to the same test, we can easily ascertain the proportion of petroleum it contains, by. comparing the result of the test with those marked on the prepared samples.
We can tell of the presence of petroleum in fatty oils, even in very small proportions, by placing a small quantity of the suspected oil on our tongue and subjecting it to the motion of mastication, the same as if we were tasting some other fatty nutriment, such as butter or lard. The alkaline saliva in the mouth will act upon and unite with the fatty oil of the sample but not with the petroleum oil it contains, and after the fatty oil has become absorbed by the system, the presence of petroleum will manifest itself by its remaining nauseating taste.
To prevent detection of adulteration of fatty oils by the hydrometer test, fatty oils are often mixed with petroleum and refined rosin oil, to equalize by the low specific gravity of the latter the lighter specific gravity of the petroleum. Their presence, however, is easily detected by taste and likewise by smell, when heated by friction between the hands, as before explained.
The comparative efficiency of oils for lubricating can be tested in many ways without the use of costly testing machines. To ascertain the gumming propensities of an oil we need only to spread some of it in a very thin layer over a glass plate, protect it against dust, and expose it to the sunlight or other slow, dry heat. In a short time the gumming propensity of the oil will be indicated by the tough and sticky appearance of what of the sample remains on the glass.
Viscosity is the degree of fluidity of an oil. The greater viscosity or body one oil possesses over another can be ascertained by allowing a given amount of the oils to be tested to drop out of the narrow end of a glass tube, or count the drops falling from them in one or more minutes and compare the difference. By varying the test at a colder or warmer temperature, the uniform consistency of the oils can likewise be ascertained and compared.
By pouring a drop of oil to be examined on a well-cleaned glass plate placed on an incline, alongside of another placed likewise, and noting the time it takes for each oil to reach a mark made on the bottom of the inclined plane, we can observe to some extent the greater viscosity or body and clinging power one oil possesses over the other. By increasing the temperature by means of an alcohol lamp or otherwise, we can likewise judge of the relative consistency one oil maintains toward another, or compare it with like tests made with oils of known efficiency.
For the cold test of petroleum and lubricating oils, the principal process usually followed by the oil trade without the use of the many expensive apparatuses in the market for that purpose is as follows: The oil is brought into 4-oz. glass bottles such as used for oil samples, and placed in an empty tomato can or other suitable vessel. A thermometer is placed in the oil through the mouth of the bottle and crushed ice or snow with common salt (chloride of sodium) in alternate layers is packed around the bottle up to the neck. The thermometer is examined from time to time as to the temperature at which the freezing proceeds and the degree noted at which the oil solidifies.
The oil to be tested is placed in a porcelain dish over an alcohol or oil lamp. A thermometer is suspended in the oil to a short distance from the bottom. The lamp is lit and allowed to slowly heat the oil. A lighted match is from time to time passed over the surface of the oil and the degree on the thermometer noted when a flashing from the oil takes place. The heating is continued until the lighted match sets the surface of the oil on fire. The degree indicated on the thermometer when this takes place indicates the fire test.
The test often made with cylinder oil, by pouring some of it onto a cylinder chest, cannot be considered conclusive, as the temperature on the outside of a cylinder chest is dry and scorching, while the temperature inside the cylinder, where the oil is expected to perform its work, is moist. That an oil would show less tendency to evaporate in the dry and scorching heat on top of the cylinder chest, would only indicate its higher fire test, but high fire test is no criterion as to its qualities, as the temperature inside of the cylinder can never be a dry and scorching one. The fire test of an oil should always be in proper proportion to its heat-absorbing quality, or it will suffer decomposition and produce gummy deposits by its inability to vaporize fast enough under the fric-tional heat and carry its vapors diffused with the exhaust steam into space.
A very handy and simple way of testing oils is to place them side by side on white blotting paper and place this for a short time on a cylinder chest or a steam heater. The oil which penetrates the blotting paper quickest and spreads widest over it, is always the poorest and thinnest oil, as it shows by its lightness and the quick disappearance of its outer ring, that it is compounded from material of very light specific gravity. If by giving it some time, or by longer exposure to heat, the whole of the oil on the blotting paper disappears, the sample must be composed entirely of petroleum, and when an inner ring, with a well discernible line, is formed and remains longer, a proportion of paraffine-holding stock must have been compounded with lighter petroleum, and when a center ring, with decided outline and darker color, is formed, but no permanent trans-lucency is imparted to the paper, the compound must be made of a light hydrocarbon oil, with an addition of still-residuum stock, however, well refined or bleached. If the oil to be examined has been compounded from petroleum, with an addition of lard oil or tallow oil or other fatty matter, the blotting paper will retain its translucency in the center long after the petroleum, which at first spread rapidly over and through the paper, has disappeared. Petroleum penetrates blotting paper faster than fatty oils and spreads wider over it at first, but dries off rapidly and its translucency disappears; while that of fatty oils remains, as one of the principal characteristics of fatty oils is their faculty to render paper permanently translucent.
Tests made by the practical use of the oil are by far the best, but we must not allow ourselves to be deceived and be led to attribute too rashly any unsatisfactory results at first obtained, to the oil we are testing. If the oil previously used was an impure one and of a gumming quality, however satisfactory it may have appeared to work, it will have left its gummy deposits in crevices and joints and in the interstices in the metal, which is commonly regarded as a bearing which the oil has made for itself, and when the new oil to be tested is applied in too small a quantity, we fail to observe that such a small amount of purer oil has at first to contend with and dislodge the deposits, which may be the result of long standing and the accumulations of large quantities of the oil formerly used. It is this fact that misleads and often causes the better oil to be condemned, until a larger and longer application proves the correctness of this statement.
These same facts will appear when we apply for a test a poorer and impure oil, after having used an oil of purer and better quality. The poorer oil finding all the bearings or the cylinder clean and no gummy deposits to contend with will, for a time, appear to work equally as well as the good oil that was in use before.
Cylinders and bearings, as well as the feeding cups, should be carefully cleaned before testing or using a new oil, if we wish to be able to judge correctly as to its quality.
Objection is sometimes made by engineers to the stringy character of the "Valve-Oleum" oils, 7 but this stringy nature, while perhaps somewhat inconvenient under careless handling, constitutes the very life of the oils, secures their uniform consistency in cold as well as in warm weather, and enables them to cling tenaciously to the metal, absorb the frictional heat as fast as it is generated by the motion, and permits their body to be of light enough character to vaporize rapidly into space with the heat they have absorbed.
Unscrupulous parties, making imitations of the "Valve-Oleum" oils, use their stringy characteristics solely for the purpose of artificially thickening light and otherwise unsalable petroleum oils, but such mixtures do not stay together ; they separate and are not neutral and contain free fatty acids, and are but little more efficient for lubricating than the cheap petroleum distillates.
By the blotter test, the "Valve-Oleum" oils show their fatty characteristics by leaving permanent, translucent center spots on the paper; they show their uniform consistency by being slowly absorbed by the paper and spreading over it without separating, and they show their cleanliness by leaving no deposit of impurities on the paper.
To detect one part of oleate of alumina in 10 parts of mineral oil, heat the sample with distilled hydrochloric acid over a water bath and stir well. The hydrochloric acid absorbs the alumina and the fatty acid remains in the mineral oil. The mineral oil is drawn off and treated with soda lye which combines with the fatty acid and separates from the mineral oil.