CLASS VIII. Of white colours.
Of white flake.
WHITE flake is lead corroded by V V means of the preflings of the grape; ^and confequently in fadl a cerufs prepared by ^Jie acid of grapes. It is brought here from ^ttaly; and far furpafles, both with regard to ^lie purity of its whitenefs, and the certainty its flanding, all the cerufs, or white lead, made here in common. It is u&d in oiL and varnifh painting-for all purpoie^ Wdiere a very clean white is required: but no kind of cerufs ought to' be ufed- iw water colours' for paintings that are intended to endure time ; as it wm g^hendiy turrt- black ;* and appear, at lengthy as if thefcad) itfelf'had beeft-ufed infteaid of anjf ptepwatietft’ &P to
White* flake isafuuilyhadcf thecoidtfrraeno iA* a prepared Attfe, under the name of fhdtar white : being lewgated' and' m&ed up with" f&rcft/, aWd' ntiorf frequently with white feitkL or riiueR wdrfb fophiftications. Whoever^ therefore, would1 be certain^ of ufingthisipigi*' merit pure, fliould procure (he white flake a* lump, as- it Is brought' over ; and levigated* themfelves : Wraftiing it over alfb; and, i# hr her neceflkry, in order to the- making it worfcf- ntore freely, they may grind if up afterwards with ftarch, in the proportion they fhalt flndr on trial moft fuitable to their purpofe.
The teft of goodnefs in white flake is the degree of whitenefs j which muft be diflin- guifhed by comparing it with a fpecimen of any other parcel .after it has been rendered of due finenefs. But where it is fufpedted to be adulterated by any other mixture than that of white lead, the fbphiflication, and proportion of the fpurious matter, may be afeertained by the fame means1 as are below advifed to- be ufbd for the1 examination of white lead.
White lead.
White, bad or cerufe is the corrofion on raft of lead, formed by means of vinegar.
It is made in our own country; and is much cheaper thou white flake; but inferior im white* n£f$, and die Other qualities winch render this pigment advantageous in painting. It is, how* ever,, the white employed' for all common pur** poles in oil painting; and alfo the body or fb- lidbaflsr of the- paint in many mixt colours, where; the. teiat.is-of a-lighter nature, or thee coloncedr pigments, will, bear diluting: with, white.
\ It is, made- by dipping,, or hrulhing, plate», of lead, in vinegar, or any- other cheap add r and,putti$g'thern in a cellar or any. cool damp, place.hut, as this is carried! on.as a large: ma*» nufedura» by thofe who are- concerned, inland: who can, confequently afford it at a, much, lower rate than any can prepare it for their own uie,. it-is- unneceflary to enter on a further detail wdth-jefpeft to the manner.
. There is-no previous preparation, neceflaiy, in. the. cafe- of white lead» to its, ufe; except waffling- over where it: is intended for. more delicate, purpofes ; but then- indeed it is always beft to fubftitute the flake white.
Not w-ithftanding the low price of white -lead, being: confumed in great quantities, it : is ior the moft part adulterated by the manufacturers^ OF w^olefele dealers.in it. The.com-mon fophiftication is with chalk or powdered talc ; as being the cheapeft ingredients with which it can be mixt without changing too much its appearance. This in a lefler degree is of no great moment; as they only diminifh the quantity of body or covering matter in the paint; but when in a greater proportion, they not only produce a great lofs by renderings a * larger quantity neceflary to do the fame work,: but deprave the paint highly with relpedt to its" other qualities.
The adulteration of lead white may be qioft cafily examined, by comparing a piece of any that is fufpedted with another piece, known to ‘ be pure, of equal bulk; and the difference of weight will fhew the fraud where the heterogeneous matter is in great proportion: as it1 will neceffarily be of a much lighter nature than lead : but, where the quantity of die a- dulterating matter is lefs, or the proportion of it would be more exactly known, the following method fhould be ufed.
“ Take an ounce of the white lead fu- ** fpedted; and mix it well with about half an “ ounce - of pearl-afhes, or of any fixt alka- “ line fait, and about a quarter of an ounce *f of charcoal dull; and, having put them “ into a crucible, give them a flrong heat.
“ The lead will by this .means be reduced to “ its metallic ftate : and, being weighed, will " fhew, by what it may fall fhort of the “ weight of an ounce, the proportion of the “ adulteration; about a tenth part being al-
“ lowed
.** lowed for the corroding acid which formed ** part of the white lead.” ,
Of calcined or burnt bartjhorn.
Calcined hartlhorn is the earth which makes tiie balls of horn, or indeed all other animal .fiibftances, rendered pure by the action of fire; which feparate from it all ialine and fulphure- ous fubftances. It.,is. of-thefirfl degree of whitenefs; and not fubjedt to Ipe changed by .tiie air or time; and is, on account of tiiefe qualities, almoft the, only white now ufed in water painting for nicer purpofcs ; white lead or flake, from the objection before mentioned with refpedt to their turning, black, being .greatly.difufed, by the more experienced painters. ...
It is not neceflary, that this earth Ihould be - produced firom the horn of flags : foe any other horn, or indeed any other animal fubftance of the more folid kind, will equally ‘Well produce it. The common manner of preparation of this matter is to calcine, in an open furnace, the coal of the horn remaining after the diftillation of the fpirit of hartlhorn: but what is fold for calcined hartlhorn at pre- fent is more frequently the earth of bones: aind if there be no further iophiftication prac- tifed, this fubftitution is not in the leaft detrimental. It is, however, I am afraid, too ufual to mix chalk or lime with the animai earth : which, by their alkaline power, change the I colour
cofouf of die vegetable pigments ; and Fraftrate often the labour of die painter, without his being able to guefs at the caufe of his mifcar- riage. Whoever is defirous to prevent this inconvenience, and to have the burnt hartfhom perfectly pure, may prepare it eafily themfelves in the following mahner.
Take horn, or bones, and bum them bi ■** any commoh. fire till thfey become a cod, n or are calcmied to fome degree of whftehefi. ** Then, having freed diem Carefully froth “ any other coal or filth,; reduce them to a “ grofs powder ; and put them upon a veSftl *® made, inform of a common earthen cfiih, u of ground cnrciblesand Sturbridge clay, uttd u well dried : and procure this to be placed <{ in a tobacco-pipe maker's or potter's fttf- ** nace, during the time they keep their pipes " or pots in the fire. The earth of the horn ** or bones being thus thoroughly' calcined, it M muft be very well levigated with water; . “ and it will he yet further improved by being <c carefully wafhed over.
The perfection of calcined hartfhom lies Ik its whitenefs and finenefs j which may be dr- ftinguifhed by the fight and touch ; and in purity alfo, which is not fb eafily difcovered; but may be known neverthelefs by the pouring oil of vitriol upon any fulpeCted quantity ; which will not produce any ebullition with pure calcined horn or bones; but will immediately excite an apparent fermentation with lime or chalk ; the commoh matter with
which
>w^ch they are, ^Iterated, .if ftny .hc aiixt •With thfftBr • : r! •>
- Gif peari nvbite. , . • :•: "
*•;v '. .. ■ . . >• f , .-->r . •»
;;. .pefvrl-whfte is fhf poywjpr of pparjs, or ,th* %er pfcrfoof pjfor-ihpUs;. b\* ;I fupp^rthe W rarely been ^fed on.^fiponfit ^f. ith« of: %. psjuh> ^
gtftm tbp le^bpt^r for ;tfys, j^ypofp than; the
.;TRw whte.,i$ pfpd in rnwi^H^ paintings* mt.fgrW mod* better wij4 %!v^e^j? ce* Jen#* than %be» •white le^ w troy. white. :.;t:The piethod^of prepviogd>e pes*rl white* fe .tq ^Jce ‘^he afters 4? they an? fonnd PR^the fa-$09#, pajejnedby the fin* > or o<h^#P *• dry frelh ones by the fire till they will pow*- der eafily, (avoiding however carefully luch heat as may in (hp lpaft burn thpm, or change their colour); to fcrape off1 from thefe fliells *U the outward .or other parts- that may not & .of the m«ft perfect whitenels ; and‘, to levigate therq well with, water, on the a^wafy frte powder over till thoroughly" fine.
. Of titty. VibiH ir Spanrjb white, ;.
The troy white or Spanifh white is chalk
neutralized by the addition of water in which
alum is diflolved, and afterwards walhed over.
It is ufed by fome in water colours as a white,
and may be thus prepared.
“ Take
** Take a pound of chalk ; and foak it well “ in water. Then wafh over all the fine part} te and, having poured off the firft water, add ** another quantity in which two ounces of alum is diflolved. Let them ftand for a day “ or two, Itirring the chalk once in fix or eight ** hours ; wafh then the chalk again over, tUl “ it be rendered perfectly fine ; and pour off ** as much of the water as can be feparated from the chalk by that means, taking off w the remainder of the diflolved alum, by tc feveral renewed quantities of frefh water. ** After the laft is poured off, put the chaik dt into one of the cullender filters, with a ft linnen cloth over the paper ; and, when the ** moiffure has been fufficiently drained off ** from it, lay it out in lumps to dry on « “ proper board.”
Of egg-ß>e!l white.
Égg-fhell white is ufed by fome in water colours; and preferred to flake or the troy white. It may be thus prepared.
<c Take egg-fhells ; and peel off the inner ** fkins. Then, levigate the fhell to proper “ finenefs ; and wafh over the powder.”