134 • FURNACE. 
giiage is inaiieqiiate jufily, to defcribe. Such metal, in 
quantity, will remain fluid for twenty minutes after it is 
run from the furnacej and when cold, will have its fur- 
face covered with the beautiful carburet of iron, of an 
uneonimonly rich and brilliant appearance. When the 
furface of tlie metal is not carbureted, it is fmooth like 
forged iron, and always convex. In this flate iron is too 
rich for melting without the addition of coarfe metal, 
and is unfit to be iifed in a cupola furnace for making fine 
cartings, where thinnefs and a good Ikin are requifite, 
N° 4, or oxygenated crude iron, whenilTuing from the 
blaf^-furnace, throws oft'from all parts of the fluid furface 
a vaft number of metallic fpajrks : they arife from a dif. 
ferent caufe titan that exerted in the former inrtance. 
The extreme privation of carbon renders the metal fub- 
je6l to the contbination of oxygen fo foon as it comes into 
contafl with atmofpheric air. This truth is evidently 
manifefted by the ejedtion of fmall fpherules of iron 
from all parts of the furface: the deflagration does not, 
however, take place till the globule has been thrown two 
or three feet up in the air ; it then inflames and feparates, 
with a flight hifTnig explofion, into a great n\any minute 
particles of brilliant fire. When thefe are colledted they 
prove to be a true oxyd of iron, but fo much faturated 
with oxygen as to polfefs no niagnetic obedience. The 
furface of oxygenated iron, when running, is covered 
with waving flakes of an obfeure fmoky flame, accom¬ 
panied with a hiffing noife ; forming a wonderful contraft 
with the finC rich covering of plumbago in the other rtate 
of the metal, occafionally parting and exhibiting the iron 
in a rtate of the greateft apparent purity, agitated in 
iivimberlefs minute fibres, from the abundance of the 
carbon united with the metal. When iron thus highly 
oxygenated comes to rert, fmall fpecks of oxyd begin to 
appear floating upon the furface: thefe increafe in fize ; 
and when the metal has become folid, the upper furface 
is found entirely covered with a fcale of blue oxyd of 
various thickneffes, dependent upon the ftage of oxyge- 
nation or extreme privation of carbon. This oxyd, in 
Common, contains about 15 per cent, of oxygen, and is 
very obedient to the magnet. In place of a dark blue 
Imooth furface, convex and richly carbonated, the metal 
will exhibit a deep, rough, concave face, which, when 
the oxyd is removed, prefents a great number of deep 
pits. This iron in fufion ftands lefs convex than carbo¬ 
nated iron, merely becaufe it is lefs fufceptible of a rtate 
of extreme diyifion ; and indeed it feems a principle in all 
nietallic fluids, that they are convex in proportion to the 
quantity of carbon with which they are faturated. This 
iron flows dead and ponderous, and rarely parts in fliades 
but,at the diftance of fome inches from each other. 
The above is a flight fleetch of the appearance of the 
two extreme qualities of crude or pig-iron, when in a 
rtate of fufion; but there rtill remain two intermediate 
rtages of quality to be deferibed : thefe are, carbonated 
and carbo-oxygenated iron; that is, N° 2 and 3 of the 
inanufafturers. Carbonated iron exhibits, like N° i, a 
beautiful appearance in the runner and pig. The break, 
ings of the fluid, in general, are lefs fine; tlie agitation 
lel^s delicate ; though the divifion of the fluid is equal, if 
not beyond that of the other. When the internal ebul- 
litilon of the metal is greaiert, the undulating lhades are 
fmallert and mort numerous: fometimes they affume the 
fliape,of fmallfegments ; fometimes,fi,brated groups; and 
at other times minute circles, of a mellower colour than 
tire ground, of the fluid. The furface of this metal, ex- 
pofed,to,external air, w.hen: cooling, iS-generally^flightly 
convex^ and fulTof pumStures: thefe, in iron of a weak 
and fp.fible nature^ are commonly fmall in the diameter, 
and,ot;no great^deplh. In, firong, metal, the pun£lures 
qr,e ntneh wider and deeper. This.criterion, however, is 
not infallible, wJven pig-iron of different works is taken 
colleftiyely. At each individual work, that iron will 
be.ftrojigert whpfe.honeycombsare largeft and.deepeft. 
Carbo-oxygenated, or 3, pig-iron, .runs fmoothly. 
without any great degree of ebullition or difengagement 
of metallic fparks. Tlie partings upon its furface are 
longer, and at greater diftances from each otlier, than in 
the former varieties; the fhape tliey alUime is either 
elliptical, circular, or curved. In rooling, this metal 
acquires a confiderable portion of oxyd; the furface is 
neither markedly convex nor concave; the piinihires are 
lefs, and frequently vanifli altogether. Their abfence, 
however, is no token of a fmooth face fncceeding; in 
qualities of crude iron oxygenated beyond this, a concave 
furface is the confequence of the extreme abfence of car- 
bon; and, in proportion as this principle is abfent, the 
furface of the iron acquires roughnefs and afperity. It 
may Ire proper liere to mention, once for all, that al¬ 
though, for convenience, the manufadlurer has, from a 
juft eftimation of tlie value of the metal in a rubfeqiient. 
manufaflure, affixed certain numbers for determinate 
qualities of iron, yet it is difficult to decide at what de¬ 
gree of fatiiration of carbon each refpeftive term com¬ 
mences: fuffice it to fay, that the two alterative princi¬ 
ples, oxygen and carbon, form two diftinft claifes, that 
in which oxygen predominates, and that in which carbon 
predominates; the latter comprehends N° i and 2 of the 
inanufafturers, the former includes oxygenated, wliite, 
and mottled ; and the equalifation of thefe mixtures.form- 
carbo-oxygenated crude iron. 
N° I and 2, with their intermediate qualities, poflTefs 
furfaces more or lefs convex, and frequently with thin 
blirters: this we attribute to the pjefence of carbon, 
which being plentifully interfperfed betwixt and through¬ 
out the particles of the metal, the tendency which the 
iron has to flirink in cooling is entirely done away; it 
helps to dirtend the aggregate.of the mafs, and to give a 
round face, by gradually elevating the central parts of 
the furface, which are always laft to lofe their fluidity. 
Again, that quality of iron known by the name of N° 3, 
or carbo-oxygenated, is moft commonly found with a flat 
furface. If we flill farther trace the appearance of the 
furface of pig-iron, when run from the furnace, we fliall 
find N° 4, either with a white or mottled frafture, pof- 
feffed of concave faces rough and deeply pitted. Be¬ 
yond this it may be imagined that every degree of further 
oxygenation would be produftive of a furface deeper in 
the curve, and rougher, with additional afperities. The 
contrary is, however, the cafe: when crude iron is fo far 
debafed as to be run from the furnace in clotted lumps 
higiily oxygenated, the furface of the pigs is found to be 
more convex than that of the iron N° i, but then the 
fracture of fuch metal prefents an impure mafs covered 
on both faces with a mixture of oxydated iron, of a 
bliieifl) colour, nearly metallic. In fliort, this quality of 
iron is incapable of receiving fucli a degree of fluidity as 
to enable us to judge whether the convexity of its furface 
is peculiar to its rtate, or is owing to its want of divifion 
as a fluid, whereby the gradual confolidation of the metal 
is prevented. 
Thefe features fufficiently dirtinguifli .betwixt the va¬ 
rious qualities of crude iron after they-are obtained from 
the blart-furnace: they are, however, criterions not lefs 
infallible, whereby we can prejudge the quality of the 
metal many hours before it is run from the furnace. 
Thefe are the colour and form of the fcoria, the colour 
of the vitrid cruft upon the working bars, and the quan¬ 
tity of carburet which is attached to it. Thq variety of 
colour and form in the cinder almort univerfally indicate 
the quality of the metal on. the hearth. Hence, from a- 
long courle of experience, have arifen the following de¬ 
nominations: Cinder of fulphury iron ; Cinder of N® 1, 
N° 2, and N° 3; and Cinder of ballad iron. Although 
at different works, from local circumflances, the fame 
kind of fcoria may not indicate precifely the fame quality 
of iron, yet the difference is fo fmall,. that the following 
defeription of the various cinders may convey a very juft 
idea of their general appearance. 
When the fcoria is of a wifitilh colour and fliort form, 
a branching 
