SECTION II.
Of oils in general.
OILS of a nature fuited to this purpole have been the moll commodious and advantageous vehicle to colours hitherto dis-
covered j as well becaufe the undtuous con-fiftence of them renders their being Ipread and laid on more expedite than any other kind of vehicle ; as, becaufe when dry they leave a Ifrong gluten or tenacious body, that holds together the colours, and defends them much more from the injuries either of the air or accidental violence, than the vehicles formed oJ water. Several qualities are not, however) constantly found in the kind of oil propel for painting, which are, neverthelefs, indif penfibly requilite to the rendering them a perfedt vehicle for all purpofes; but the warn of fome of them can in many cafes be difpen* fed with; and one of them, we Shall firft mention, remedied by art in a great degree.
The principal and moll general quality tc be required in oils, is their drying well; winch, though it may be affifted by additions, is yet to be delired in the oil itfelf; as the effedt oi the pigments ufed in it are fometimes fuch as counteradt thole of the ftrongeft dryers, and occalion great delay and trouble from the works remaining wet for a very long time j and frequently never at ail becoming dry as it ought: and indeed there are fome parcels oi the oils which have this vice in an irremediable degree.
The next quality in oils is the limpidneis or approach to a colourlefs date, which is like wife very material; for where they partake of a brown or yellow colour, fuch brown or yellow intermixes itfelf neceflarilv with the teint of the pigments ufed in the oil, and of con- fequence depraves it. But fcelides the brown colour which may appear in the oil when it is ufed, a great increale of it is apt to fuc- ceed in time, if the oil be not good: and therefore this fhould be guarded againft as much as poflible, where it may be of ill con-, fequence.
Both thefe qualities are, however, greatly remedied by keeping the oils a long time before they be ufed ; and even linfeed oil, tho’ much the mod faulty in thefe refpe&s, is greatly improved by time; and fometimes rendered fit for almoft any purpofe whatever.
There are three changes that oils of the kind proper for painting are liable to fuffer in their nature, and which affeft them as vehicles, that are confounded by painters under one term, viz fattening; notwithstanding they are brought about by very different means, and relate to very different properties in the oils.
The firft is the coagulation before fpokeii of by admixture of the oils with fome kinds of pigments unduly prepared. This indeed is called the fattening of the colours; but the real change is in the oils; and the pigments are only the means of producing them. This, change is generally a feparation pf the oil into two different fubftances: the one a vifcid pitchy body, which remains combined with the pigment: the other a thin fluid matter, which divides itfelf from the colour and thicker part. This laft appears in very various proportions under different circumftances; and in fome cafes is not found at all, where the pigment happens to be of a more earthy and alkaline nature: for then only a thick clammy fubftance, that can fcarcely be (queezed out of the bladder, if it be put up in one, is the refult of the fattening. This fattening not only fucceeds when the oil and pigments are mixed together, and kept for any length of time in bladders or otherwife; but even fome- times after they have been fpread or laid Oil the proper ground: when, innead of drying, the feparation will happen j and one part of the oil will run off in fmall drops or ftreams, while the other will remain with the coloutf without (hewing the leaft tendency to dry.
The fecond is a change, which happens in oils from long keeping, by which they grow more colourlefs, become more ready to dry, and acquire a moré unftuous confidence. In this cafe the oils are faid to become fat; tho? they are in a very different ftate from that a- bove mentioned, which is caufed by unfuifr- able pigments: for when this change doe# not exceed a certain degree, it is, as I befortt (aid, every way a great improvement of dïtf oils.
The third is a change produced by artificial means, from expofing the oil a long time tqf the fun and air, (of the particular manner amt ufe of which we (hall fpeak more fully in it# proper place) whereby it is freed from it# groflèr and more feculent parts, and rendered colotjriefs and of a more thick and left fluid confidence, than can be produced by any o- ther treatment: but at the fame time made more reluctant to dry, particularly with ver- I milion, najjipn, lake, Pruffiaa blue, brown pink* and i^Wg’s yellow;, and indued with ether proper* ties that disqualify it She common. ufe as a vehicle in painting. Thefe qualities, never* thelefs, may be rendered advantageoufly fub- fervient to feme particular- perpofes: though the nature, and even the preparation of tat oil is lefs understood at prefent than one could imagine it poffible, with regard to a fubStance of to much cqnfequence, both to tome kinds of painting, and feveral other kindred arts. Oils in this State are called alio fat oils; tho’ it is a change that has not the lead: affinity with either of the others; but, on the contrary, differs oppofitely from both of them in ipme very eflential, circumstances.
In fpeakiog therefore of the fattening of oils or colours, attention Should be had to the.not confounding thefe three feveral kinds,, one with another; which can Scarcely be avoided iifc feme cafes, but by considering the occasion where the term is ufed, and judging from the circumstances which kind is meant.
Thefe are the feveral qualities by which oils aref rendered Suitable to, or improper for the purpofes of painting. When they dry quickly,, are. colourlefs, (especially through age,) and are Somewhat fat in the fecond of me above fenfest of that word, they are perfect with reSpeft to . the wants of painters: where they dry, tho,|r more Slowly, they may, neverthekfs, be improved, to a tolerable date hy additions; and where they are difeoloured, they, may ferve for-
feme
fome ufes ; but where, as is frequently found, they will dry only in a great length of time, or not at all, they are abfolutely unfit for this application.