200 R 0 L L I N 
the mould should always be placed at a considerable depth 
beneath the orifice where the metal is poured in, so that the 
pressure of a column of the fluid metal may be obtained to 
consolidate the casting, and render it free from those air¬ 
holes, or porous places, which will sometimes occur in metals 
cast without the pressure of a column. The long piece of 
metal which filled the aperture through which the metal ran, 
is left adhering to the roller, and is cut off afterwards. Case- 
hardened rollers must be used when it is required to have a 
very fair surface; viz. for such purposes as rolling iron to 
make thin plates for tinning ; also the large rollers for gold 
or silver, such as are used in the Royal Mint; rollers for 
making tin-foil, steel-plate for saws, and for many other pur¬ 
poses. These rollers are not hardened by a subsequent process, 
as in case-hardening wrought iron, but are cast in that state. 
This is effected by employing iron moulds: a cast-iron cylin¬ 
der of three inches thick, and its diameter equal to that of 
the roller, is bored out with great accuracy, and fitted with 
ends proper to form moulds for the necks required at each 
end of the roller; this is buried in the sand of the foundry, 
and when the metal is run into it, the rapid transmission of 
the heat through the iron mould causes the metal which is in 
contact with it to cool sooner than the other parts of the 
mass, and renders the surface of the roller very hard. In 
turning a roller of this kind, the centres must be chosen so 
that the circumference turns as true as it will admit, and 
then a very small quantify being taken off wall render it 
perfect: this care should be taken for two reasons; first, 
that less will be required to be removed to make it true, 
which is a difficult operation, as only the best steel tools will 
cut it; also, that if the metal is unequally reduced, or more 
on one side than Ihe other, the hardest part will there be cut 
away, and the roller will have a hard and a soft side, and 
soon wear out of the circular figure, and require a second 
turning. The less metal there is turned off a case-hardened 
roller the better it will be, because the hard part is only a 
case of slight thickness, and most hard at the surface. 
The operation of the rolling-mill is so simple, as scarcely 
to require any description: the metal is heated in reverbe¬ 
rating furnaces when it is in large masses, and for smaller 
pieces a kind of oven is used, in which the cokes are laid on 
the bottom or floor of the oven without any grate-bars, and 
therefore the draft of air being less rapid, it gives a slight, 
but very regular heat, which rises to a bright red, but no 
farther, and therefore it does not waste the iron by burning 
it to scales, as a greater heat and current of air will do. 
This oven is proper for heating plates, hoops, or small bars, 
to be rolled a second and third time: but for rolling large 
masses, a strong welding heat is requisite, that the metal 
may be consolidated, and all flaws or cracks securely closed. 
The reverberating furnace is used for this purpose. 
The iron, being heated in the furnace to the proper degree 
for the purpose which is intended, is taken out by a pair of 
pincers, the mill put in motion by drawing the shuttle, and 
the iron is presented to the rollers, which are previously ad¬ 
justed to the thickness of the piece which is to be passed. 
If this is not attended to, and the workmen attempt to 
reduce the iron too much at one time, there is danger of 
breaking some of the machinery, or of stopping the mill 
whilst the iron is only half passed through : this is a dis¬ 
agreeable accident, as it will require four or five men, with 
an enormous wrench applied to the nuts of the roller, 
to turn them back sufficiently to relieve the rollers, so 
forcibly are the screws pressed whilst the iron is passing 
through: this is indeed evinced by the circumstance of solid 
cast-iron rollers, of ten inches in diameter, being sometimes 
broken in the middle; and the necks of eight and nine inches 
are frequently snapped. When the iron is placed on the 
shelf or table before them, their motion will draw it through, 
and as they cannot recede from each other, because of the 
nuts of the bolts, the metal is reduced to the exact thickness 
of the space between them, increasing in length, but not at 
all in breadth : the iron is caught by another vcorkman 
behind the rollers, and returned over the top roller to the 
first man, who puts it through again, first giving the han- 
G - M I L L. 
dies of the nuts a small turn, to bring the rolls nearer toge¬ 
ther. In this manner it is repeatedly rolled, till it is reduced 
to any required length and thickness, but the breadth is not 
at all increased by rolling; and if it is required to increase 
the breadth, it is done by putting the iron obliquely through 
the rollers; or if a great increase is wanted, the iron is put 
through breadth-wise two or three times, till it is extended 
to the length of a gauge which the workman has marked 
upon the table in front. 
Black-plates, such as are used for making the boilers of 
steam engines, tanks, or other large vessels, in wrought iron, 
are, when large, and of considerable thickness, rolled 
from the blooms, or half blooms, which are made under 
the fcrge-hammer. These blooms, which are also called 
slabs, are nearly the length of the intended plate; their 
breadth about one-half or one-third as much as their length, 
and of a thickness to contain as much metal as will make 
two, three, or four plates. These pieces, when heated to a 
white heat, are presented breadth-wise to the rollers, and 
passed through several times at the same heat, until what was 
the breadth of the bloom, being extended two or three times 
as great, becomes equal to its length. The thick square 
plate, thus formed, is now cut up by the shears across into 
two or three pieces, of about the same size as the first, but 
in a direction which will make what was the length of the 
first piece to be the breadth of the second. These pieces 
being heated and rolled again, become extended to the size 
of the required plates: the reason of thus dividing the opera¬ 
tion is, that the rolling only extends the metal in the direc¬ 
tion in which it moves, and not at all in breadth : by this 
means, the particles of iron being drawn by the sides of one 
another, acquire something of a fibrous texture, or an 
approach thereto, which is desirable in bars, rods, and hoops, 
but not at all in plate, as it should be equally strong, in 
either direction ; therefore, by rolling it first one way, and 
then the other, the grain, as far as it is produced at all, is in 
both directions. There is no doubt that better plate would 
be made, if the slabs or original pieces were cut to the pro¬ 
per proportions of length and breadth, and of a thickness 
to form only one piece; then rolling it alternately length 
and breadth ways every time it is passed between the rolls, 
and continuing this till the plate is finished, at one heat; 
a better grain or texture will be thus acquired, because, in 
the former method, it will be weaker one way, having some¬ 
thing of a grain in the direction of the last rolling. 
Plates which are to be tinned, are made from the best 
English iron, and some of the very thinnest from foreign 
iron : the bars are drawn out, by the forge-hammer, to five 
inches broad, and half a inch thick, and are cut into lengths 
of eleven inches by the shears; these are heated in an oven, 
and passed breadthways through case-hardened rolls : this is 
repeated till they are extended to twice the length of the 
intended plates : the pieces are then folded, and set on edge 
in the furnace till properly heated, when they are rolled 
double, the fold being put in first; they are thus extended to 
twice the length of the folded plate. Now two men, with 
strong tongs, tear the two leaves asunder at the fold, and 
fold each again separately, putting one into the other, like 
two sheets of paper; in this state they are heated, and rolled 
four thicknesses together, the next time eight, and so on, 
till the plate is reduced to the required thickness: in the 
very thinnest plate, such as is used for tagging laces, sixteen 
leaves are rolled together. In folding the plates, care is 
taken every time to put a new surface of metal outside, 
otherwise, those which were constantly reduced by the pres¬ 
sure of the adjacent leaves, would, at length, become 
grained on the surface; but by continually gaining new 
surfaces, which are smoothed by the immediate contact of 
the rollers, those which are laid against them are also rendered 
smooth. A small quantity of oil is sprinkled between the 
leaves, when they are first put in, and instantly spreading 
over the whole surface, prevents any adhesion; the plates 
are dressed square by the shears every time before they are 
folded, to remove those parts which, by projecting over the 
edges of the upper leaves, are not so much reduced. 
After 
