344 C H E M 
this arifes partly from there being a more aftive add in one 
than in the other. - 
Sulphat of zink has hitherto been little ufed in dying. 
It makes the colours darker, but this arifes principally 
from the iron it contains. In general, the precipitates 
produced by fulphat of zink grow fomewhat darker by 
time, which proves that the oxyd of zink produces fonie 
combultion in colouring matters ; yet it combines with 
them but in fmall quantities. Hence it appears why it 
gives deeper colours than the oxyd of tin, though both 
are white. The aftion of verdigris in dying black arifes 
principally, perhaps entirely, from the uncombined 
oxydf which ferves to precipitate the iron not combined 
with the aftringent principle, by itfelf combining with 
fulphuric acid. Acetit of lead forms a plentiful precipi¬ 
tate with thefolutions of colouringfubftances; it darkens 
the colours, and makes them ftrong, but dull. Its 
greateft ufe is informing the principal mordant for painted 
cloths, in which it forms a combination of the bale of 
alum with the acetous acid. 
Fixed Alkalis. —Potafli is of extenfive ufe in dying 5 
it facilitates the diffolution of the colouring matters, 
and deepens the colours. As fixed alkali has a ftrong 
aftion upon fubftances of an animal nature, and diifolves 
them when caultic, much precaution is neceffary, both 
with regard to the proportion of this fait made ufe 
of in any procefs, and alfo to its more or lefs cauftic 
Hate. 
When the fait of foda is made ufe of, regard mull be 
had to the ftate it is in ; for, if in cryftals, it contains 
more than half its weight of water of cryftallization ; 
io that, when it comes to eftervel'ce, one part produces 
as much eft'eft as two parts of the cryftals. Soap is ufed 
in dying, principally indeed for cleaning the filk; white 
hardfoap, prepared with olive-oil, is the beft. 
Sulphur is ufed in dying, for an operation called fou- 
frage, or fulphuring. This operation is deftined to com¬ 
municate to filks which are intended to be white, and 
alfo to woollen fluffs, the greateft degree of wliitenefs 
they can receive. It communicates alfo an elafticity which 
produces the noife called rujlling. Butfulphured filk does 
not ealily take the dye ; previous to dying,- it mull be 
unfulphured, by foaking and drawing leveral times 
through the hand in warm water. 
The oxyd of arfenic was formerly much ufed in dying, 
particularly in painted cloths ; but its inutility is now 
acknowledged, and this poifonous fubftance is very little 
ufed. The arfenite of potafli is ufed alfo. Orpiment is 
alfo employed in fome of the precedes of dying, efpeci- 
ally with indigo. The quality of the water is an effential 
matter in the art of dying. Waters aft upon the colour¬ 
ing matters principally by the halts with an earthy bafe 
which they contain. Thefe falts are, the nitrats, mu- 
riats, and carbonats, of lime and magnefia, and the ful¬ 
phat of lime. 
Substances used in Dying.— Thefe are aftringents 
in general, and nut-galls in particular. The nut-gall is 
an excrefcence found on the young branches of the oak. 
There are different fpecies of gall-nuts, fome inclining 
to white, yellow, green, brown, red, afti-coloured, or 
black. They differ much in ftze ; and they are round 
or irregular, light or heavy, fmooth or rough, as it hap¬ 
pens. Thofe which are fmall, blackilh, granulated, 
and heavy, are the beft; they are called nut-galls of 
Aleppo, and are brought to us from Aleppo,' Tripoli, 
and Smyrna. The aftringents which referable nut-galls 
are, oak faw-duft, the nut-galls of our own country, and 
the myrobolans citrinus. 
The macerationof a fluff in a decoftion of nut-galls, 
is called by the French chemills engallage. This gives it 
a degree of weight of which it cannot afterwards be de¬ 
prived, and which cannot even be diminilhed beyond a 
certain degree by repeated waftiings. 
Stuff thus prepared may be combined with any other 
colouring matters, and the colours thereby acquire fix- 
S T R Y. 
ity, if they have it not of themfelves, jo that the aftrin- 
.gent principle communicates its ftrength to the triple 
combination, or whatever combination may be formed 5 
but the colour ufually becomes darker by the combina¬ 
tion. If fluff's already dyed are to undergo this opera¬ 
tion, it ipuft be done, cold, or the colour will fuifer a 
change. 
The maceration is made with different proportions of the 
nut-galls, or other aftringents, according to the quality 
of the aftringents, and the effefts required. The fol¬ 
lowing are general direftions for the procefs.: Boil the 
bruiled nut-galls for two hours in a quantity of water 
proportioned to that of the material to be macerated; let it 
cool till you can juft bear your hand in it. Divide it in¬ 
to equal parts,, as near as may be, to macerate the fluffs 
in portions of about a pound each ; and pour the re¬ 
mainder upon the whole. Leave it thus for four-and- 
twenty hours, if intended to be dyed with madder, or 
of a black colour; for other colours, ten or twelve 
hours are enough. Then prefs the cotton or wool, and 
dry it. 
Op Indigo. —This is a colouring matter refembling. 
fecula. There are two fpecies of the indigo-plant, the 
true, and the bafe. When the indigo-plant is nearly 
ripe, it is cut and put into ftone troughs, with water to 
ferment, which it loon begins to do. When left to be 
quite ripe before it is cut, it gives a more beautiful co¬ 
lour, but it yields lels ; if over-ripe, it yields ftill lefs, 
and the indigo is of a bad quality. 
Berthollet, in his work upon dying, has defcribed the 
procefs ufed in Ameiica and in Africa for extrafting it. 
During the operation, there is a vegetable fermentation, 
and a difengagement of ammoniac and carbonic acid 
gas : this latt faft is certain, fince by putting extinguilh- 
ed lime diluted with water, or milk of lime, into the 
troughs, the precipitation will be haftened. Bergman, 
who has a good dilfertation upon indigo, ftates, that 
it owes its colour to iron : but this cannot be correft, 
fince, if it were lo, indigo would befoluble in alkalis. 
If to an alkali be added any lubftance greedy of oxy¬ 
gen, efpecially fulphure of arfenic, and the whole be 
poured over indigo, it is eafily dilfoived by lofing its 
oxygen. This faft, which was difcovered by a manu- 
fafturer, has thrown great light upon the hiltory of in¬ 
digo. This is now the praftice of dyers who ufe indigo : 
they deprive the colouring matter of its oxygen, which 
they reftore afterwards by expofing the fluffs to the air. 
Berthollet eftablilhed this after a great quantity of ex¬ 
periments. He knew that indigo contained much car¬ 
bon and hydrogen ; fo that, if any concentrated acid, 
except the fulphuric, be poured upon indigo, it caufes 
it to pafs through feveral colours in fucceffion, and at 
laft to become black, becaufe its hydrogen combines 
with the oxygen of the acid, and the carbon remains 
predominant. 
Befides hydrogen and carbon, indigo contains a pretty- 
large quantity of oxygen, a little azot, and a fmall por¬ 
tion of iron ; but its proportion of carbon is greater 
than that of any vegetable lubftance ; which caufed Ber¬ 
thollet to remark, that thofe colouring matters which, 
contained molt carbon in their compofition, are alfo 
richer in colour, and give the molt halting tints. 
Hence we deduce, 1. That indigo in its natural ftate 
contains oxygen. 2. That, till deprived of oxygen, it 
cannot,combine with lime or alkalis. 3. That thel’e fub- 
ltances which can deprive it of this portion of oxygen, 
render it foluble in alkalis and lime. 4. That this folu- 
tion is decompofed, and the indigo returns to its natu¬ 
ral ftate, by contaft with the atmofpheric air, from 
which it attrafts the oxygen it had been deprived of. 
Powdered indigo, digefted in alcohol, gives firft a yel¬ 
low colour, then red, and laftly brown. Water feparates 
from this dye a brownilh refinous matter. Ether afts 
upon indigo nearly in the fame manner; but oils, either 
fixed or volatile, have very little effect upon it. 
The 
