288 



LUBRICATION— MINERAL OIL AS A LUBRICANT FOR MACHINERY. 



[1855. 



the manner just dosorlbed upon the metal itself, it is manifest that it 

 could not bo advantageously employed as a lubricant. 



Many of the oils and fats which are employed in greasing machinery 

 under'TO spontaneously a chemical change which endues them with the 

 power of oxidising metals, particularly copper, and consequently brass, 

 which is an alloy of that metal. Some fatty substances run more 

 quickly into this condition than others, and the change, when it has 

 proceeded far. is sufficiently marked to be at once recognisible on a 

 very cursory examination of the material. This change is popularly 

 known as rancidity : it consists in the spontaneous conversion of the 

 elements of the fatty substance into acids, which differ according to 

 the nature of the material itself, but which have in all cases sufficient 

 affinity for metallic bodies to establish that action upon the surtace to 

 which we have already alluded. In some instances the mere contact 

 of the fiitty body with a metal is sufficient to effect a change in it 

 which enables it to react secondarily upon the metal itself. An action 

 of this kind may be *en when a bright piece of lead is kept immersed 

 for some time in a vegetable oil, particularly olive oil: the whole sur- 

 face of the lead in this case will soon be covered with a thin deposit of an 

 unctuous substance, called by chemists margarate of lead ; the first 

 action of the lead being to promote the conversion of the margarine 

 contained in the oil into margaric acid, which then reacting upon the 

 lead, produces the substance described. With respect to the chemical 

 changes of the second kind in which the lubricant may be involved, 

 the circumstances are different. There the lubricant may be totally 

 inactive, so far as in its relation to the metal of the machinery is con- 

 cerned ; but, by exposure to air, or from some other influence, it may 

 have been so far chemically changed as to have become thickened or 

 rendered viscid and adhesive, or it might have acquired the property 

 of drying up after a while into a hard, resinous kind of matter : all 

 these changes would, of course, unfit any material for the purposes of 

 lubrication. A certain class of oleagnious bodies have naturally the 

 property of drying when exposed to the air, and many of those which 

 have not naturally this character are rendered thick and tenacious 

 under the combined influence of air and a raised temperature. None 

 of the various materials which are obnoxions to such changes can be 

 placed in the category of the substances suitable to the uses of the 

 practical engineer, except in the very roughest kind of machinery ; 

 and even then their employment is equivalent to a certain loss of power 

 in the prime moving force. 



From what has now been said, it appears, then, that a perfectly 

 good lubricant should combine these definite chemical and physical 

 qualities : — first, it must be incapable of exercising any chemical action 

 upon the metal exposed to its influence ; secondly, it must maintain 

 the integrity of its normal chemical constitution under the influence of 

 heat and atmospheric air ; and, thirdly, it must, for fine machinery, 

 possess sufficient unctuosity to enable it to interpose a homogenous 

 medium between the metallic surfaces without flying off, during the 

 rapid motion of any part of the work ; while, for heavy machinery, 

 it must, in addition to its freedom from all liability to chemical 

 change, possess that degree of consistency and tenacity which would 

 prevent it from being easily forced out from between moving surfaces 

 by pressni'e alone. There are few substances which combine these 

 qualities ; perhaps those in which they are most conspicions are sperm 

 oil and the oleine of solid fats: the former may be taken as a standard 

 by which we may judge of the lubricating power of all other fatty 

 materials. Having now, then, in some degree ascertained the points 

 in which the value of a good lubricant consists, we will inquire how 

 far the mineral oil can establish its claim to that title. 



Some years ago, it was discovered that when the bituminous shales, 

 or schists — which abound in many localities in Europe — are exposed to 

 destructive distillation, they yield a very considerable quantity of a 

 tarry liquid, which, upon re-distillation, furnishes a volatile spirituous 

 fluid resembling coal naptha, and an abundance of oils, boiling at 

 varying but very high temperatures. At a later period it was found 

 that peat would yield, under similar treatment, oils of a very similar 

 character ; and, still later, a further source of these oils, in unlimited 

 quantity, has been discovered in the variety of coal, or more properly 

 shale, which has come into extensive use in gas-making under the 

 name of "Boghead coal." The peculiar character of these oils con- 

 sists in their containing a large quantity of the substance called 

 paraffins, which is held in solution by a thin oil of low specific gravity, 

 closely resembling, if not identical with, the oil first described by the 

 Continental chemist Reinchenbach under the n.ame of Eupion : this 

 oil, from its containing paraffine, is sometimes called, commercially, 

 " paraffine oil." In distilling the Boghead coal with the object of 



obtaining the oil, particular regard must be had to the temperature at 

 which the distillation is carried on ; for it is a remarkable fact, that 

 the nature of these pyrogenous products varies in an extraordinary 

 manner according to the temperature employed in producing them. In 

 order to obtain the maximum quantity of paraffine oil from the Bog- 

 head coal, the heat should not exceed, at any period of the distillation, 

 a dull red, and the process should commence with the lowest tempera- 

 ture at which the tar will distil over. This point being properly at- 

 tended to, the least quantity of gas and the largest quantity of oil 

 will be obtained : whereas if the temperature rise to a cherry or 

 bright red, the contrary will be the case, as a considerable portion of 

 the oil will be then converted into gas ; and not only so, but in the 

 place of paraffine, which is eminently the product of a low tempera- 

 ture, a different substance, napthaline, will be formed, and will, like 

 the paraffine, be held in solution in the fluid oil. In re-dittilling the 

 tar to obtain the paraffine oil, the nature of the product varies as the 

 process proceeds : fii-st comes over the thin eupion-like oil, which boils 

 at the lowest temperature: then comes, as the distillation advances, 

 more and more paraffine in admixture with the thin oil, until at length 

 the product solidifies on cooling, in consequence of its consisting 

 almost entirely of paraffine. All these products, excepting the solid 

 paraffine, are said to be excellent as lubricants under peculiar suitable 

 circumstances — this is, with regard to the character of the machinery 

 to which they are employed. 



The oils obtained in this process are of very low specific gravity, 

 ranging from '790 to -870. Sperm oil being -875, they are all more or 

 less unctuous to the touch, and at common temperatures are as fixed as 

 any of the organic oils or fats, boiling only at a temperature approach- 

 ing that at which the ordinary fats undergo partial decomposition, and 

 then distilling over unchanged. 



Having now seen how this mineral oil is obtained, the question arises 

 as to the characters and conditions which render it superior to other 

 substances as a lubricant, or indeed whether such is really the case. 

 A glance at the chemical constitution of this oil will perhaps enable us 

 to form an opinion on one part of the subject. 



Referring again to the characters of the ordinary fat oils, we shall 

 find that they consist essentially, in all cases, of certain chemical com- 

 binations of carbon, hydrogen, and oxygen, the proportions of which 

 vary slightly according to the nature of the oil ; but the specific char- 

 acter of oxygenated compounds belongs to all oils and fats of this class. 

 Such being, then, the chemical nature of these substances, it is obvious 

 that in the element oxygen they contain within themselves the prin- 

 ciple of oxidation, and that under the influence of an action which, 

 like fermentation, can establish a tendency to chemical change among 

 their own elements, compounds may be formed, acid jr otherwise, 

 without the intervention of external agents ; and these compounds 

 may, as we have already seen, possess chemical affinities which enable 

 them to attack and enter into combination with any oxidisable metal 

 with which they may be brought in contact. 



Independently, too, of the oils commonly employed in lubrication 

 containing oxygen which might be the means of effecting changes such 

 as we have mentioned, they are themselves all more or less susceptible 

 of oxidation from the influence of external agencies, and the moment 

 this oxidation takes place the normal character of the oil is lost. If we 

 turn now to the consideration of the chemical constitution of the 

 " mineral oil," we find that it difl'ers in one very important particular 

 from the oils or fats of org.anic origin. It contains, in fact, no oxygen, 

 being a compound of two elementary substances only, viz., carbon and 

 hydrogen. This oil belongs, indeed, to an extensive class of com- 

 pounds, called hydro-carbons ; and so entirely free is it from any 

 oxidising tendency or power, that substances having the most energetic 

 affinities for oxygen, and capable of taking it from any matter in 

 which it exists in combination or otherwise, are perfectly protected 

 from that action by being kept immersed in it. Thus, potassium 

 and sodium — metals whose oxidising tendency is so powerful that they 

 can be only preserved in the metaUio state with difficulty — can be kept 

 in the mineral oil entirely unacted upon, and maintaining their bril- 

 liancy of sui'face when freshly cut. There is another point to be con- 

 sidered : the hydro-carbons, and the mineral oil among the rest, have 

 not the slightest tendency to combine with oxygen themselves — at 

 least, under any ordinary circumstances. One of their great charac- 

 teristics appears, indeed, to be an intense internal conservative prin- 

 ciple or force which counteracts any liabilty to change among their 

 own elements, and which may be even communicated to organic sub- 

 stances placed under their influence, as many perhaps all, of these 

 substances possess a strong antiseptic power. The substance paraffine. 



