SECTION IV.
Of particular dryers.
Of drying oil.
Drying oil is formed of linfeed oil, prepared by the means of boiling, fomer-times with the addition of other fubftances, and fometimes without. The fubftances add-ed to oil, in this preparation, are very various t there being many different recipes in the hand* of different perfons j fome of which prefcribes a lefs number of ingredients, and others al- moft every kind. Thefe fubllances are, white vitriol, r fugar of lead, feed lac, gum maftic, gum fandarac, gum animi, gum copal, umbo*, colcothar, litharge, and red lead. But the firft eight of thefe articles being dearer j when, great quantities of drying oil are prepared for common purpofes, tney are ufually omitted j and the others, or fome of them, only employed : and indeed if the linfeed oil be good, and boiled for a proper time even alone, it will have nearly the fame properties, as if the molt efficacious of thefe fubllances be added to it. I will, however, give two of the molt approved recipes for preparing drying oil, with the addition of the ufual ingredients. The one for the kind fuppofed proper for the more nice and delicate painting: the other for com- *mon work.
“ Take of nut or poppy oil one pint, oi “ gum fandarac two ounces, of white vi- “ triol and fugar of lead each one ounce <{ Boil the whole, till the lolid ingredients b* “ diffolved, and the mixture be of the colou. “ of linfeed oil.”
This oil will dry fall; and, mixed with an equal quantity of nut oil, and the proporttions of oil of turpentine each perfon may find them convenient to ufe, will render any other o capable of drying well without the leaft haz&r of fattening, however difbofed to it: and it may be, therefore, ufed for all nicer purpofes, where common drying oil would be injurious by the brownnefs of its colour.
Drying oil may be well prepared for coarfer work according to the commonly approved method, in the following manner.
** Take of linfeed oil one gallon, of litharge ** of gold or filver one pound, of white vi* *** triol half a pound, of fugar of lead, gum ■** Arabic, and umber, each a quarter of a “** pound. Boil them fo long as the difcolour- ing the oil, which is the gradual confe- quence of the boiling, will permit it: for ** the oil muff not be burnt till it approach ^ * too near to blacknefs.”
The drying oil prepared in a fimpler manner, as by thofe who make it for fale, will arot differ much from the above in its qualities, although it be prepared as follows.
“ Take linfeed oil one gallon, red lead one * pound and a half. Boil them fo long as the colour will bear it."
This laft may be much improved by the addition of gum fandarac;. and will then be perhaps more ufeful than the more complex ^-nd expenfive compofition above given. Indeed a gallon of linfeed oil, in which two pounds of gum fandarac, and one pound of gum Arabic has been diffolved, is the drying oil I would recommend for common purpoles i and it need pot be boiled near fo high as that fofd in the ftiops 5 for, as the gums give it a very confiderable body, the colours may be brought to a proper ftate for working by a little of dais with a larger proportion of fpii it of turpentine, either with or without the addition of unprepared oil; and confequently the proportion of brown given to the colours is lefs, than where a greater quantity of the oil muft be ufed.
The drying oil of any kind may be boiled in an earthen pipkin, or iron pot: but great care muft be taken in the operation, if it be performed within any building, that the matter do not boil over; which, when it happens, greatly endangers the firing fuch building. It is therefore much better, efpecially where any larger quantities are made, to have an iron furnace that can be moved about, or to build an occafional one of bricks; which, few this purpofe may be done without mortar; and need only be a cylinder of bricks, in which a frame with bars to lhpport the fewel is fixed, with a hole about eight inches above the bars for feeding the fire, and another in the high- eft part of the hollow for venting the fmoke.
When the drying oil is taken off the fire, it may while yet hot be ftrained through flannel; but if umber, or any ingredients which will not diilolve, are added to it, it fhould firft ftand to fettle, that the clearer part may be poured olf from the grofler: and, after ftraining, if any fiediment appear, the oil fhould be again decanted olf from it, and the foul part heated and palled again through the flannel.
Of oil of turpentine.
Gil of turpentine is diftilled from turpentine. It is an ethereal oil, which quickly exhales in the air j and if mixt with linfeed, nut, or poppy oils, in flying off carries with it the more volatile part of fuch oils, and caufes them to dry much fooner than they would otherwife. On this account it is very generally ufed as a dryer to mix with the other oils: for which purpofe it has greatly the advantage over drying oil, with regard to colour, as it is perfectly transparent and white.
It is ufed without any other preparation than mixing it, either alone or together with drying oil, with the other oils and colours: and it is not fubjed: to any adulteration, except the dif- folving crude turpentine or refin in it j which do not greatly injure it with regard to this application of it: fuch adulteration may, however, be perceived, when in a greater proportion, by a flight degree of yellow colour and glutinous confiftence, the unfophifticate being quite limpid and without the lead tenacity; ana every degree may be difeovered by evaporating part of what is fufpe&ed with a very gentle lieat, when a refinous or terebinthinate fub- jftance will be left behind, if any fuch have; been mixt with the oil.
Of fugar of lead.
Sugar of lead is a chemical preparation of lead, by dilTolving it in vinegar, and reducing the fait, formed, to chryftals by evaporation: hut it is to be had fo conftantly, and at (b much lefs cxpence at the (hops of chemifts and druggifts, than it can be made in (mall quantities, that it is needlefs to give any more particular recipe for the making it for the purports of painting. It is fuppofed to have a quality, when mixt with oils, of rendering them much more prone to dry: and on that account is very frequently ufed, as well in the making drying oil, as in the common ufe of the oils; where it is ground up with them together with the colours. It requires no other treatment, but to it well commixt with the oil with which it is ufed: and it is not lubjedt to any adulteration if it be obtained in its chryftalline Jtbnn and. not powdered, for there the appearance alone is luttscient to fhew whether it is clear from any mixture of heterogeneous matter.
It is. novwithtfendmg, the general confidence which painters have in this fublhtnce as a dryer, dubious whether the ufe of it, except in the preparation of drying ett. utlvos m any materi*! degree the etui i ibr though it may af'f: the beat in predating that change in oh> which renders them dry-urg oil, yet it dees not rteexitorihr redew. tha any Luca C- feci eccoed. ce it withe cc heat i though a: is very probable, that the fuppofition of dais lias been the occafion of its being introduced into pra&ice, as a dryer that will operate on oils by barely mixing it with them.
Of white vitriol.
White vitriol is a mineral fubftance found in Hungary, and many other places. It is ufed in the fame manner, and for the fame purpofes, in painting, as fugar of lead ; with which it is .generally joined in practice, both in the mak-ing drying oils, and the admixture with the common oils and colours. It is not liable to be j&dultcrated, on account of its low price: but it lias not been unufual to fell common green vi-triol or coperas calcined to whitenefs in the placie of it: and as they are very different fubftances, this ought to be guarded againft. Thole who are acquainted with the appearance of both may eafily diftinguifh them by view ; but o-thers may make themfelves certain they are
not impofed upon in this particular, by put-ting a fmall piece into the fire on a poker, or by any other method; when, if the parcel in quel-tion be true white vitriol, no rednefs will come upon it; but, if it be the green vitriol calcined to whitenefs, it will turn red by this greater heat, and have the appearance of Indian red, or brown oker.
What was (aid of the dubioufnefs of the efficacy of fugar of lead as a dryer, except in the preparation of drying oil, holds equally good with refpedt to white vitriol : though from a ftrong perfualion of the contrary, it is ufed very generally, and in a large proper?* tion.