423 
PIGMENT. 
lure to the fun and air, ihat colour to which it changes will 
bid fair for being permanent, and therefore ought to be 
employed where it can be done. Of thefe changes the 
inflances are but very rare. One is the purple of the an¬ 
cients, which a'ffumed its colour by expofure to the fun, 
and confequently was exceedingly permanent. Another 
is in the lolution of filver; which, being mixed with 
chalk, the precipitate turns to purplifh black where it is 
expofed to the fun. A third is in lolutions of indigo by 
alkaline fubftances, which conftantly appear green till ex¬ 
pofed to the air by fpreading them very thin, upon which 
they become almoft inftantaneoufly blue, and continue fo 
ever after. Sometimes, though (till more rarely, a very 
remarkable change of colour happens, upon mixing two 
vegetable juices together. Almoft the only inftance of 
this we have on the authority of Mr. George Forfter, who 
informs us, that the inhabitants of Otaheite dye their 
cloth of a crimfon colour, by mixing together the yellow 
juice of a fmall fpecies of fig with the greenifh juice of a 
kind of fern. But the moft remarkable alterations of co¬ 
lour are effected by different metallic and faline folutions 
mixed with certain animal or vegetable fubftances ; and 
with thefe the colour-maker will be principally conver- 
fant. 
It is a common obfervation in chemiltry, that acids 
mixed with blue vegetable juices turn them red, and al¬ 
kalies green. It is equally certain, though not fo gene¬ 
rally known, that acids of all kinds generally tend to 
heighten red colours, fo as to make them approach to the 
fcarlet, or true crimfon ; and alkalies to darken, or make 
them approach to blue or purple. Mixed with yellow co¬ 
lours, acids alfo univerfally tend to brighten the yellow; 
and alkalies to turn it to an orange, and make it become 
more dull. But, though this is very generally the cafe, 
we are not to expefl that all acids are equally powerful 
in this refpefl. The nitric acid is found to heighten the 
moft of any, and the muriatic acid the lead of the mi¬ 
neral ones. The vegetable, as might be expefled, are lefs 
powerful than the mineral acids. Thus, if with a tinc¬ 
ture of cochineal, either in water or alcohol, is mixed the 
pure nitrous acid, it will change the colour to an exceed¬ 
ing high orange, or flame-colour, which it will impart to 
cloth. If fulphuric acid is ufed, a full fcarlet, inclining 
to crimfon rather than orange, is produced. With muri¬ 
atic acid, a true crimfon colour, bordering on purple, is 
the confequence. Alkalies, both fixed and volatile, 
change the colour to a purple, which is brighter with the 
volatile than the fixed alkalies. 
Here it is obvious, that whatever colours are produced 
by the mixture of different fubftances together, the per¬ 
manency of thefe colours can only be in proportion to the 
ability of fuch mixtures to refill the weather. Thus, 
iuppofe a high fcarlet or orange colour is produced by 
means of fpirit of nitre, it is plain that, were fuch a co¬ 
lour expofed to the air, it could remain no longer than 
the fpirit of nitre which produced it remained. In pro¬ 
portion, therefore,- as the fpirit of nitre was exhaled into 
the air, or otherwife deftroyed, the colour muft fade, and 
at be totally deftroyed; and thus, in proportion to 
the deftruflibility of the fubftances by which colours are 
produced, will be the difpofition of fuch colours to fade, 
or the contrary. In this refpefl alkalies are much more 
deftruflible than acids, and confequently lefs proper for 
the preparation of colours. With regard to acids, the 
nitric feems moft deftruftible, the fulphuric lefs fo, and 
the muriatic the lead of all. From the extreme fixity of 
the phofphoric acid and fedative fait, perhaps they might 
be of fervice in preferving colours. 
As all colours, whether derived from the animal or ve¬ 
getable kingdom, muft be extradited either by pure water 
or fome other liquid menftruum, they cannot be ufed for 
the purpofes of painting till the colouring fubftance is 
united with fome earthy orfolid matter capable of giving 
it a body, as the workmen call it; and, according to the 
nature of this fubftance, the colour will be tranfparent or 
otherwife. This bafis ought to be of the moft fixed and 
durable nature; unalterable by the weather, by acids, or 
by alkalies. It ought alfo to be of a pure white colour, 
and eafily reducible into an impalpable powder. For this 
reafon all earthy fubftances fliould be avoided, as being 
afted upon by acids; and therefore, if any of thefe were 
added to heighten the colour, they would not fail to be 
deftroyed, and their effeft totally loft. Precipitates of 
lead, bifmuth, &c. though exceedingly fine and white, 
ought alfo to be avoided, as being apt to turn black by 
expofure. The only fubjefl to be chofen in preference to 
all others, is oxyde of tin, prepared either by fire or the 
nitric acid. This is fo exceedingly refraflory as not only 
to be unalterable by alkalies, acids, or the fun and wea¬ 
ther, but even by the focus of a very large burning mir¬ 
ror. It is befides white as fnow, and capable of being 
reduced to an extreme degree of finenefs, infomuch that 
it is made ufe of for polifhing metallic fpeculums. For 
thefe reafons it is the moft proper bafis for all fine colours. 
For coarfe ones, the white precipitate of lead, mentioned 
under the article Chemistry, will anfwer very well. It 
hath a very rtrong body, i. e. is very opaque, and will 
cover well; may be eafily ground fine, and is much lefs 
apt to turn black than white lead ; it is befides very 
cheap, and may be prepared at the fmall expenfe of 3d. 
per pound. 
If what we have juft now obferved is attended to, the 
general method of extrafling colours from any vegetable 
or animal fubftance, and fixing them on a proper bafis, 
muft be very eafily underftood. For this purpofe a quan¬ 
tity of oxyde of tin is to be procured in proportion to 
the quantity of colour defired. This muft be well rub- 
bed in a glafs mortar, with a little of the fubftance de- 
figned for brightening the colour, as alum, cream of tar¬ 
tar, fpirit of nitre, See. after which it muft be dried, and 
left lor fome time, that the union between the two fub- 
llances may be as perfefl as poffible. If the colour is to 
be a very fine one, fuppofe from cochineal, the colouring 
matter muft be extrafled with alcohol without heat. 
When the fpirit is fufficiently impregnated, it is to be 
poured by little and little upon the oxyde, rubbing it 
conftantly, in order to dillribute the colour equally 
through all parts of the oxyde. The fpirit foon evapo¬ 
rates, and leaves the oxyde coloured with the cochineal. 
More of the rinflure is then to be poured on, rubbing 
the mixture conftantly as before; and thus, with proper 
management, may very beautiful colours, not inferior to 
the bell carmine, be prepared at a moderate expenfe. If, 
inftead of cochineal, we fubftitute brazil-wood, turmeric, 
logwood, See. different kinds of red, yellow, and purple, 
will be produced. For the coarfer colours, aqueous de- 
coftions are to be ufed in a fimilar manner; only, as thefe 
are much longer in evaporating than the alcohol, very 
little muft be poured on at a time, and the colours ought 
to be made in large quantity, on account of the tediouf- 
nefs of the procefs. 
Hitherto we have confidered only the effefls of the 
pure and fimple falts, viz. acids and alkalies, on different 
colours ; but by combining the acids with alkalies, earths, 
or metals, thefe effedls may be varied almoft in infinitum; 
neither is there any rule yet laid down by which we can 
judge & priori of the changes of colour that will happen 
on the admixture of this or that particular fait with any 
coiouring fubftance. In general, the perfefl neutrals aft 
weakly; the imperfefl ones, efpecially thofe formed from 
metals, much more powerfully. Alum and fal ammo¬ 
niac confiderably heighten the colour of cochineal, bra¬ 
zil, turmeric, fuftic, madder, logwood, Sec. The fame 
thing is done, though in a lefs degree, by common fait, 
Glauber’s fait, nitre, and many other neutrals. Solu¬ 
tions of iron in all tiie acids ftrike a black with every one 
of the above-mentioned fubftances; and likewife witli 
flmmach, galls, and other aftringents. Solutions of lead, 
or faccharum laturni, univerfally debafe red colours to a 
dull purple. Solution of copper changes the purple co- 
3 lour 
