17. _Muriate of iron.__A Dictionary of Arts, Manufactures and Mines containing a clear exposition of their principles and practice

18. _Oxalate of iron_; _Humboldtite_, found by M. Breithaupt in the

lignite of Kolaw. It consists of protoxide of iron, 53·86; oxalic acid,

46·14; in 100.

19. _Titanate of iron_, consists of protoxide and peroxide of iron, 86;

titanic acid, 8; oxide of manganese, 2; gangue, 1 = 97. See _Black

Oxide_ of iron.

_Of the assay of iron-ores by fusion._--In the assays by the dry way,

the object is to separate exactly all the iron which the ore may

contain, with the view of comparing the result with the product of

smelting on the great scale. In order to succeed in this operation, we

must deoxidize the iron, and produce at the same time such a temperature

as will melt the metal and the earths associated with it in the ore, and

obtain the former in a dense button at the bottom of a crucible, and the

latter in a lighter glass or slag, above it. Sometimes the gangue of the

ores, consisting mostly of a single earth, as quartz, alumina, or lime,

is of itself very refractory, and hence some flux must be added to bring

about the fusion. The substance most commonly employed for this purpose

is borax; but ordinary flint glass may be substituted for it. Sometimes,

also, instead of adding borax, which always succeeds, lime or clay may

be added to the ore, according to the nature of its mineralizer; that

is, lime for a clay iron-stone, and clay for a calcareous carbonate of

iron; and both, when the gangue is siliceous, as occurs with the black

oxide.

The ore, pulverized and passed through a silk sieve, is to be well mixed

with the flux, and the mixture introduced into the smooth concavity made

in the centre of a crucible lined with hard rammed damp charcoal dust.

Were the mixture diffused through the charcoal, the reduced iron would

be apt to remain scattered in little globules through the crucible, and

no metallic button would be formed at its bottom. The mingled ore and

flux must be covered with charcoal. The crucible thus filled must be

shut with an earthen lid luted on with fire-clay; and it is then set on

its base, either in an air furnace, or on the hearth of a forge urged

with a smith’s bellows. The heat should be very slowly raised, not

employing the bellows till three quarters of an hour have expired. In

this way, the water of the damp charcoal (_brasque_) is allowed to

exhale slowly, and the deoxidation is completed before the fusion

begins; for by acting otherwise, the slags formed would dissolve some

oxide of iron, and the assay would not indicate the whole of the iron to

be obtained from the ore. At the end of the above period, the fire must

be raised progressively to a white heat, at which pitch it must be

maintained for a quarter of an hour, after which the crucible should be

withdrawn. Whenever it has cooled, it is to be opened, the _brasque_

must be carefully removed or put aside, and the button of cast-iron

taken out and weighed. The _brasque_ may sometimes contain a few

globules, which must be collected by washing in water, or the

application of a magnetic bar. The quantity of iron denotes, of course,

the richness of the ore. These assays furnish always a gray cast-iron;

and, therefore, the quality of the products can hardly be judged of,

except by an experiment on the large scale. The temperature necessary

for the success of an assay is about 150° of Wedgewood.

In the assays by the _humid_ way, we may expect to find manganese,

silica, alumina, lime, magnesia, and sometimes carbonic acid, associated

with the iron. 100 grains of the ore in fine powder are to be digested

with nitro-muriatic acid; which will leave only the silica with perhaps

a very little alumina. If an effervescence takes place in the cold with

a dilute acid, the loss of weight will indicate the amount of carbonic

acid gas expelled. The muriatic solution contains the iron, the

manganese, the lime, magnesia, and most of the alumina, with a little

silica. On evaporating to dryness, and digesting in water, all the

silica will remain in an insoluble state. If the solution somewhat

acidulated be treated with oxalate of ammonia, the lime will fall down

in the form of an oxalate; ammonia will now precipitate the alumina and

the oxide of iron together, while the manganese and magnesia will

continue dissolved in the state of triple salts (ammonia-muriates). The

alumina may be separated from the ferric oxide by potash-lye. The

manganese may be thrown down by hydrosulphuret of potash; and, finally,

the magnesia may be precipitated by carbonate of soda. 100 parts of the

red oxide of iron contain 69·34 of metal, and 30·66 of oxygen.

If phosphorus be present in the ore, the nitro-muriatic solution being

rendered nearly neutral, will afford with muriate of lime a precipitate

of phosphate of lime, soluble in an excess of muriatic acid.

When the sole object is to learn readily the per-centage of iron, the

ore may be treated with hot nitro-muriatic, the acid solution filtered,

and supersaturated with ammonia, which will throw down only the iron

oxide and alumina; because the lime is not precipitable by that alkali,

nor is magnesia and manganese, when in the state of ammonia-muriates.

The red precipitate being digested with some potash-lye, will lose its

alumina, and will leave the ferric oxide nearly pure. The presence of

sulphur, phosphorus, or arsenic, in iron ores, may always be detected by

the blowpipe, or ustulation in the assay muffle, as described under

FURNACE.

_Of the smelting of iron-ores._--We shall describe, in the first place,

the methods practised in Great Britain, and shall afterwards consider

those pursued in other countries, in the treatment of their peculiar

ores.

Iron is divided into three kinds, according to the different metallic

states in which it may be obtained; and these are called _crude_ or

_cast iron_; _steel_; and _bar_ or malleable iron. These states are

determined essentially by the different proportions of charcoal or

carbon held in chemical combination; cast iron containing more than

steel, and steel more than malleable iron; which last, indeed, ought to

be the pure metal, a point of perfection, however, rarely if ever

attained. It is impossible to assign the limits between these three

forms of iron, or their relative proportions of carbon, with ultimate

precision; for bar iron passes into steel by insensible gradations, and

steel and cast iron make such mutual transitions as to render it

difficult to define where the former commences, and the latter ceases,

to exist. In fact, some steels may be called crude iron, and some cast

irons may be reckoned among steels.

Towards the conclusion of the last century the manufacture of iron

underwent a very important revolution in Great Britain, by the

substitution of pitcoal for charcoal of wood, the only combustible

previously used in smelting the ores of this metal. This improvement

served not merely to diminish the cost of reduction, but it furnished a

softer cast iron, fit for many new purposes in the arts. From this era,

iron works have assumed an immense importance in our national industry,

and have given birth to many ingenious and powerful machines for

fashioning the metal into bars of every form, with almost incredible

economy and expedition.

The profusion of excellent coal, and its association in many localities

with iron-stone, have procured hitherto for our country a marked

superiority over all others in the iron trade; though now every possible

effort is making by foreign policy to rival or to limit our future

operations. In 1802, M. de Bonnard, now divisionary inspector in the

royal corps of mines of France, and secretary of the general council,

made a tour in England, in order to study our new processes of

manufacturing iron, and published on his return, in the Journal des

Mines, tom. 17., a memoir descriptive of them. Since the peace, many

French engineers and iron-masters have exerted themselves in

naturalizing in France this species of industry; and M. de Gallois, in

particular, after a long residence in Great Britain, where he was

admitted to see deliberately and minutely every department of the iron

trade, returned with ample details, and erected at Saint-Etienne a large

establishment entirely on the English model. More recently, MM. Dufrénoy

and Elie de Beaumont, and MM. Coste and Perdonnet, have published two

very copious accounts of their respective metallurgic tours in Great

Britain, illustrated with plans and sections of our furnaces, for the

instruction of the French nation.

The argillaceous carbonate of iron, or clay ironstone of the coal

measures, is the chief ore smelted in England. Some red hematite is used

as an auxiliary in certain works in Cumberland and Lancashire; but

nowhere is the iron-sand, or other ferruginous matters of the secondary

strata, employed at present for procuring the metal.

Among the numerous coal-basins of England there are two, in particular,

which furnish more than three-fourths of the whole cast iron produced in

the kingdom; namely, the coal field of Dudley, in the south of

Staffordshire; and the coal fields of Monmouthshire, in South Wales,

along with those of Gloucestershire and Somersetshire.

Dudley is peculiarly favoured by nature. There are found associated the

coal, the iron ore, the limestone for flux, and the refractory fire-clay

for constructing the interior brick-work of the furnaces. This famous

clay is mined at Stourbridge, and exported to every part of the kingdom

for making cast-steel crucibles and glass-house melting pots.

At Merthyr-Tydvil, the centre of the iron works of Wales, the iron-stone

is extremely plentiful, forming 16 beds, or rather constituting an

integrant portion of 16 beds of slate-clay. Sometimes it occurs in

pretty long tables adjoining each other, so as to resemble a continuous

stratum; but more frequently it forms nodules of various size and

abundance, placed in planes both above and below the coal seam. Eight

varieties of ore, belonging to different beds, have been distinguished

by the following barbarous names: black balls, black pins, six-inch-wide

vein, six-inch jack, blue vein, blue pins, gray pins, seven pins. The

bed containing the first quality of iron-stone is analogous to the black

ore of Staffordshire called _gubbin_; it is often cleft within like

_septaria_, and its cavities are sometimes besprinkled with crystals of

carbonate of lime or quartz. In the superior beds there are nodules

decomposing into concentric coats, of which the middle is clay. Crystals

of oxide of titanium are occasionally found in the middle of the balls

of clay iron-stone; to which the metallic titanium observed in the

inside of the dome of blast furnaces, may be traced. Both at Dudley and

South Wales, casts of shells belonging to the genus _unio_, are observed

on the iron-stone.

The average richness of the iron-stones of South Wales is somewhat

greater than those of Staffordshire. The former is estimated at 33 parts

of cast iron, while the latter rarely exceeds 30 parts in 100 of ore;

and this richness, joined to the superior quality or cheapness of the

coals, and the proximity of the sea, gives South Wales a decided

advantage as a manufacturing district.

The number of blast furnaces in the parish of Merthyr-Tydvil amounts to

upwards of 30. The cast iron produced is, however, seldom brought into

the market, but is almost entirely converted into bar iron, of which, at

Mr. Crawshay’s works, 600 tons are manufactured in a week. Numerous iron

railways, extending through a length of 220 miles, facilitate the

transport of the materials and the exportation of the products. That

concurrence of favourable circumstances, which we have noticed as

occurring at Dudley, prevails in an equal degree in South Wales.

The same economy which the use of coal has introduced into the smelting

of cast iron from the ore, also extends to its refinery into bars. And

this process would supersede in every iron work the use of wood

charcoal, were not the iron produced by the latter combustible, better

for many purposes, particularly the manufacture of steel. In some

English smelting works, indeed, where sheet iron is prepared for making

tin plate, a mixed refining process is employed, where the cast iron is

made into bar iron by wood charcoal, and laminated by the aid of a coal

fire.

Till 1740, the smelting of iron ores in England was executed entirely

with wood charcoal; and the ores employed were principally brown and red

hematites. Earthy iron ores were also smelted; but it does not appear

that the clay iron-stones of the coal-basins were then used, though they

constitute almost the sole smelting material at the present day. At that

era, there were 59 blast furnaces, whose annual product was 17,350 tons

of cast iron; that is, for each furnace, 294 tons per annum, and 5-1/8

tons per week. By the year 1788, several attempts had been made to

reduce iron ore with coaked coal; and there remained only 24 charcoal

blast furnaces, which produced altogether 13,000 tons of cast iron in

the year; being at the rate of 546 tons for each per annum, or nearly 11

tons per week. This remarkable increase of 11 tons for 5-1/8, was due

chiefly to the substitution of cylinder blowing machines worked with

pistons, for the common wooden bellows. Already 53 blast furnaces fired

with coke were in activity; which furnished _in toto_ 48,800 tons of

iron in a year; which raises the annual product of each furnace to 907

tons, and the weekly product to about 17-1/2 tons. The quantity of

cast iron produced that year (1788) by means of coal, was 48,800 tons,

and that by wood charcoal, was 13,100

------

constituting a total quantity of 61,900 tons.

In 1796, the wood charcoal process was almost entirely given up; when

the returns of the iron trade made by desire of Mr. Pitt, for

establishing taxes on the manufacture, afforded the following results:--

121 blast furnaces, furnishing in whole per annum 124,879 tons,

constituting an average amount for each furnace of 1032 tons.

In 1802, Great Britain possessed 168 blast furnaces, yielding a product

of about 170,000 tons; and this product amounted, in 1806, to 250,000

tons, derived from 227 coke furnaces, of which only 159 were in activity

at once. These blast furnaces were distributed as follows:--

In the principality of Wales 52

In Staffordshire 42

In Shropshire 42

In Derbyshire 17

In Yorkshire 28

In the counties of Gloucester, Monmouth, Leicester, Lancaster,

Cumberland, and Northumberland 18

In Scotland 28

---

227

In 1820, the iron trade had risen to the amount shewn in the following

table:--

Tons.

Wales manufactured, per annum 150,000

Shropshire and Staffordshire 180,000

Yorkshire and Derbyshire 50,000

Scotland, with some places in England 20,000

-------

Total 400,000

In a statistical view given by M. de Villefosse, of the French and

English iron works, he assigns to the latter, in 1826, 305 blast

furnaces, distributed as follows:--

In the principality of Wales 87

In Staffordshire 78

In Shropshire, Derbyshire, Yorkshire, &c. 84

In Scotland 56

---

305

Out of these, 280 were in activity at the same time; and if we suppose

their mean product to have been 50 tons a week, the total product would

have been, in 1826, 728,000 tons. But this estimate seems to be somewhat

above the truth; for, from the information communicated by Mr. Philip

Taylor to M. Achille Chaper, a considerable French iron-master, who, in

the summer of 1826, inspected two-thirds of the blast furnaces of Great

Britain, their product during this year was about 600,000 tons.

The preceding details shew the successive increments which the

manufacture of cast iron has received; and a similar progression has

taken place in its refinery into wrought iron. This operation was

formerly effected by the agency of wood charcoal in refineries analogous

to those still made use of in France. But when that kind of fuel began

to be scarce in this island, it came to be mixed with coke in various

proportions. The bar iron thus produced was usually hard, and required

much time to convert, so that an establishment which could produce 20

tons of bar iron in a week, was deemed considerable. At that time,

England imported annually from Sweden and Russia the enormous quantity

of 70,000 tons of iron.

Mr. Cort, to whom Great Britain is indebted for the methods now pursued

in this country, succeeded about that time, after many unsuccessful

experiments, in converting cast iron into bar iron, by exposing it on

the hearth of a reverberatory furnace to the flame of pitcoal. This

method, which possessed the advantage of employing this species of

combustible alone, likewise simplified the treatment, because it

required no blast apparatus. But this mode of refinery, consisting in

the use of a reverberatory furnace alone, did not produce altogether the

desired result. It was irregular; sometimes the loss of iron was small,

but at others it was very considerable; and there were great variations

in the quality of the iron, as well as in the quantity of fuel consumed.

Mr. Cort succeeded in removing this uncertainty of result, by causing

the puddling in the reverberatory furnace to be preceded by a kind of

refinery with coke. The intent of this operation was to decarburate the

iron, and to prepare it for becoming malleable. The metal took in that

case the name of _finery_ metal, called, for sake of brevity,

_fine-metal_.

He also substituted the drawing cylinders for the extension under the

hammer, an improvement which accelerated greatly the manufacture of bar

iron. The iron then yielded by the operation of puddling, was of a very

inferior quality, and could not be directly employed in the arts. In

order to give it more consistence, it was subjected to a second heating

in a reverberatory furnace; and whenever this method had arrived at a

high enough degree of perfection to afford products fit for the market,

it became exclusively employed in Great Britain. This new method of

transforming cast-iron into malleable iron, speedily gained such an

extension, that of late years, a single iron-work, Cyfartha in Wales,

manufactured annually more than twice as much as was made annually from

1740 to 1750, in the whole kingdom.

In surveying the improvements which the iron manufacture has received in

England in the space of the last 60 years, they are seen to be

resolvable into two; the first set relating to the smelting of the ores;

the other, to the conversion of the pigs into bar iron; hence naturally

arise two heads under which the subject of iron must be treated.

1. _Manufacture of cast-iron by coke and coal._--The cast-iron produced

by the English and Scotch blast furnaces is in general black and very

soft; but yet may be distinguished into several qualities, of which

three are particularly noticed.

No. 1. _Very black cast-iron_, in large rounded grains, obtained

commonly near the commencement of the casting, when an excess of carbon

is present; in flowing, it appears pasty, and throws out blue

scintillations. It exhibits a surface where crystalline vegetations

develope themselves rapidly in very fine branches; it congeals or fixes

very slowly; its surface when cold is smooth, concave, and often charged

with plumbago; it has but a moderate tenacity, is tender under the file,

and susceptible of a dull polish. When melted over again, it passes into

No. 2., and forms the best castings.

No. 2. _Black cast-iron_, has a somewhat lighter shade than the

preceding, and may therefore on comparison be called blackish-gray. It

presents less large granulations than No. 1.; is tenacious, easily

turned, filed, and polished; excellent for casting when it approaches to

No. 1., and for the manufacture of bar iron when it has on the contrary

a shade somewhat lighter. If repeatedly melted, it passes into the next

quality, or

No. 3. _White cast-iron_; this is brittle, and indicates always some

derangement in the working of the furnace; it flows imperfectly, and

darts out in casting, abundance of brilliant white scintillations; it

fixes very quickly; and on cooling, exhibits on its surface irregular

asperities, which make it extremely rough. It is easily broken, and

presents a lamellar and radiated fracture; and is so hard that tempered

steel cannot act upon it. It is cast only into weights, bullets or

bombs, but never into pieces of machinery. When exposed to the refinery

processes, it affords a bad bar iron. It is owing probably to the

different nature of the cast-iron obtained in different counties in

England, that Staffordshire and Shropshire furnish the greater part of

the great iron castings, while Wales manufactures almost exclusively

malleable iron. The lower price of coals in Wales is perhaps the cause

to a certain extent of this difference in the results of these two iron

districts. It will be interesting at any rate, to describe separately

the processes employed in Staffordshire and Wales.

_The blast furnaces of Staffordshire_, in the neighbourhood of Dudley,

Bilston, and Wednesbury, are constructed almost wholly of bricks. Their

outer form is frequently a cone, often also a pyramid with a square

base. They are bound about with a great many iron hoops, or with iron

bars placed at different heights. This powerful armour allows the

furnaces to be built much less massively than they formerly were; and

admits of lighter and more elegant external forms. They are seldom

insulated; but are usually associated to the number of two or three in

the same line. A narrow passage is left between them, which leads to the

lateral openings where the tuyères are placed. At the front of the

furnace, a large shed is always raised. The roofs of these sheds present

in general circular profiles, and being made of cast or bar iron, they

display a remarkable lightness of construction. The cast-iron columns

likewise, which support the joists and girders, give additional

elegance.

[Illustration: 582]

In the Dudley field, the furnaces are almost always in the middle of the

plain, and an inclined rail-way must be formed to reach their platform.

These inclined planes, composed of beams or rails placed alongside of

each other, and sustained by props and cross-bars, as indicated in

_fig._ 582., are set up mostly against the posterior face of the

furnace. Two chains or ropes, passing over the drums of gins, moved by a

steam engine (commonly the same that drives the bellows), draw up the

waggons of wood or sheet iron _a a_, which contain the various materials

for supplying the furnace. To facilitate this service, the platform

round the furnace is sometimes enlarged behind by a floor; while a

balustrade, which opens when the waggons arrive at the platform,

prevents accidents. This projection is occasionally covered by a roof.

For a furnace of the largest size, the force expended by this lifting

apparatus, is not more than a two-horse power.

_Fig._ 582. is a vertical section through the furnace from front to

rear, or at right angles to the line of the lateral tuyères. The

erection of a pair of blast furnaces, of 40 feet high each, costs, in

the Dudley district, 1800 pounds sterling; and requires for building

each, 160,000 common bricks for the outside work, 3900 fire-bricks for

the lining or shirt of the furnace, and 825 for the boshes. The

dimensions of the fire-bricks are various; 5 kinds are employed for the

lining, and 9 kinds for the boshes. They are all 6 inches thick, and are

curved to suit the _voussoirs_.

The number of charges given in 12 hours is different in different

furnaces; being sometimes 20, 25, and even so high as 40; but 30 is a

fair average. Each charge is composed of from 5 to 6 cwt. of coak, (or

now of 3 to 4 cwt. of coal with the hot blast); 3, 4, and sometimes 6

cwt. of the roasted mine, according to its richness and the quality of

cast iron wanted; the limestone flux is usually one-third of the weight

of the roasted iron stone. There are 2 casts in 24 hours; one at 6 in

the morning, and another at 6 in the evening.

The height of the blast furnaces is very variable; some being only 36

feet high including the chimney, whilst others have an elevation of 60

feet. These extreme limits are very rare: so that the greater part of

the furnaces are from 45 to 50 feet high. They are all terminated by a

cylindrical chimney of from 8 to 12 feet long; being about one-fifth of

the total height of the furnace. The inside diameter of this chimney is

the same as that of the throat or mouth; and varies from 4 to 6 feet.

The chimney is frequently formed of a single course of bricks, and

acquires solidity from its hoops of iron, so thickly placed that one

half of the surface is often covered with them. At its lower end, the

mouth presents one or two rectangular openings, through which the charge

is given. It is built on a basement circle of cast-iron, which forms the

circumference of the throat; and a sloping plate of cast-iron _b_ is so

placed as to make the materials slide over into the furnace, as shown in

the figure.

The inside of the blast furnaces of Staffordshire is most frequently of

a circular form, except the hearth and working area. The inner space is

divided into four portions, different in their forms, and the functions

which they fulfil in the smelting of the ore.

The undermost, called the hearth, or crucible, in which the cast-iron

collects, is a right rectangular prism, elongated in a line

perpendicular to the axes of the tuyères. The sides of the hearth

consist in general of refractory sandstone (fire-stone), obtained mostly

from the bed of the coal basin, called _millstone grit_; and the bottom

of the hearth is formed of a large block of the same nature, laid on a

cast-iron plate.

The second portion is also made of the same refractory grit stone. It

has the form of a quadrangular pyramidal, approaching considerably to a

prism, from the smallness of the angle included between the sides and

the axis.

The third portion or lower body of the furnace is conical, but here the

interior space suddenly expands; the slope outwards at this part seems

to have a great influence on the quality of the cast-iron obtained from

the furnace. When No. 2. of the blackest kind is wanted for castings,

the inclination of this cavity of the furnace is in general less

considerable than when No. 2. cast iron for conversion into bar iron is

required. The inclination of this conical chamber, called the boshes,

varies from 55 to 60 degrees with the horizon. The diameter of this part

is equal to that of the belly, and is from 11 to 13 feet. The boshes are

built of masonry, as shown in _figs._ 583, 584.

The fourth part, which constitutes about two-thirds of the height of the

furnace from the base of the hearth up to the throat, presents the

figure of a surface of revolution, generated by a curve whose concavity

is turned towards the axis of the furnace, and whose last tangent

towards the bottom is almost vertical. This surface is sloped off with

that of the boshes (_étalages_ in French), so that no sharp angle may

exist at the belly. In some furnaces of considerable dimensions, as in

that with three tuyères, this portion of the furnace is cylindrical for

a certain height.

The following measurements represent the interior structure of two

well-going furnaces.

+---------------------------------------------+-------+-------+

| | No. 1.| No. 2.|

+---------------------------------------------+-------+-------+

| |_Feet._|_Feet._|

|Height from the hearth to the throat or mouth| 45 | 49 |

|Height of the crucible or hearth | 6-1/2| 6 |

| of the boshes | 8 | 7 |

| of the cone | 30-1/2| 36 |

| of the chimney or mouth | 8 | 12-3/4|

|Width of the bottom of the hearth | 2-1/2| 2 |

|Ditto at its upper end | 3 | 2-2/3|

|Ditto of the boshes | 12-2/3| 13-1/2|

|Ditto at one-third of the belly | 12 | 11-1/2|

|Ditto at two-thirds of ditto | 8-2/3| 9-1/2|

|Ditto at the mouth | 4-1/2| 3-2/3|

|Inclination of the boshes | 59° | 52° |

+---------------------------------------------+-------+-------+

[Illustration: 583]

The conical orifice called the tuyère, in which the tapered pipes are

placed, for imparting the blast, is seen near the bottom of the furnace,

_fig._ 583. at A. Nose tubes of various sizes, from 2 to 4 inches in

diameter, are applied to the extremity of the main blast-pipe. Under A

is the bottom of the hearth, which, in large furnaces, may be two feet

square. B is the top of the hearth, about two feet six inches square. A,

B, is the height of the hearth, about six feet six inches. B shows the

round bottom of the conical or funnel part, called in this country, the

_boshes_, standing upon the square area of the hearth. C is the top of

the boshes, which may be about 12 feet in diameter, and 8 feet in

perpendicular height. D is the furnace top or mouth (_gueulard_ in

French), at which the _materials are charged_. It may be 4-1/2 feet in

diameter. The line between C, D, is the height of the internal cavity of

the furnace, from the top of the boshes upwards, supposed to be 30 feet.

A, D, is the total height of the interior of the furnace, reckoned at

44-1/2 feet. E E is the lining, which is built in the nicest manner with

the best fire-bricks, from 12 to 14 inches long, 3 inches thick, and

curved to suit the circle of the cone. A vacancy of 3 inches wide is

left all round the outside of the first lining by the builder; which is

sometimes filled with coak dust, but more generally with sand firmly

rammed. This void space in the brick-work is for the purpose of allowing

for any expansion which might occur, either by an increase in the bulk

of the building, or by the pressure and weight of the materials when

descending to the bottom of the furnace. Exterior to E E is a second

lining of fire-bricks similar to the first. At F, on either side, is a

cast-iron lintel, 8-1/2 feet long, by 10 inches square, upon which the

bottom of the arches is supported. F, G, is the rise of the tuyère arch,

which may be 14 feet high upon the outside, and 18 feet wide. The

extreme size of the bottom or sole of the hearth, upon each side of A,

may be 10 feet square. This part and the boshing stones, are preferably

made from a coarse sandstone grit, containing large rounded grains of

quartz, united by a siliceo-argillaceous cement.

The bottom of the hearth consists, first, of a course of the said

gritstone; beneath which is a layer of bedding sand, having, in its

under part, passages for the escape of the vapours generated by damps;

the whole being supported upon pillars of brick.

[Illustration: 584]

_Fig._ 584. represents the hearth and boshes, in a vertical side

section. _a_ is the tymp stone, and _b_ the tymp plate for confining the

liquid metal in the hearth. The latter is wedged firmly into the

side-walls of the hearth; _c_ is the dam-stone, which occupies the whole

breadth at the bottom of the hearth, excepting about 6 inches, which

space, when the furnace is at work, is filled before every cast, with a

strong binding sand. This stone is faced outside by a cast-iron plate

_d_, called the dam-plate, of considerable thickness, and peculiar

shape. The top of the dam-stone, or rather the notch of the dam-plate,

lies from 4 to 8 inches under the level of the tuyère hole. The space

under the tymp plate, for 5 or 6 inches down, is rammed full, for every

cast, with strong loamy earth, or even fine clay; a process called the

tymp stopping. The area of the base of this furnace being 38 feet; its

extreme height is 55 feet.

The blast furnaces of Staffordshire have always two tuyères, at least,

placed on opposite sides, but so pointed that the blast may not pursue

directly opposite lines. In a furnace acting well in the neighbourhood

of Dudley, the one of the tuyères was 10 inches distant from the

posterior wall of the hearth, and the other only 4 inches. In other

furnaces with 3 tuyères; the side ones are placed, the one 16-1/2

inches, and the other 6-1/2 inches from the back. Three tuyères are

seldom made to blow simultaneously. The third is brought into action

only when the furnace seems to be choaked up, and when it becomes

necessary to clear it up by a powerful concussion. Too much pains cannot

be bestowed on the masonry and brickwork of a blast furnace, and on the

solidity of its foundation. In a soft ground it should rest on piles, so

driven that the channel left beneath for the drainage of the building

may be above any water level. Small passages should likewise be left

throughout the body of the work, for the transpiration of moisture.

The blowing machines employed in Staffordshire, are generally cast-iron

cylinders, in which a metallic piston is exactly fitted as for a steam

engine, and made in the same way. Towards the top and bottom of the

blowing cylinders orifices are left covered with valves, which open

inside when the vacuum is made with the cylinders, and afterwards shut

by their own weight. Adjutages conduct into the iron globe or chest, the

air expelled by the piston, both in its ascent and descent; because

these blowing machines have always a double stroke.

The pressure of the air is made to vary through a very considerable

range, according to the nature of the fuel and season of the year; for

as in summer the atmosphere is more rarefied, it must be expelled with a

compensating force. The limits are from 1-1/2 pounds to 3-1/2 pounds on

the inch; but these numbers represent extreme proportions, the average

amount in Staffordshire being 3 pounds. With this pressure a furnace

usually works, which affords 60 tons of cast iron in the week; and the

pressure may be 2-1/2 pounds on an average. The orifices, or nose-pipes,

through which the air issues, also vary with the nature of the coke and

the ore. In Staffordshire they are generally from 2 inches and 5 tenths

to 2 inches and 8 tenths in diameter.

The blowing machines of Staffordshire are always impelled by steam

engines. At Mr. Bagnall’s works, two blast furnaces, 40 feet high,

exclusive of the chimney or top, and two finery furnaces, are worked by

a steam engine of 40 horses power; and therefore the power of one horse

corresponds to the production of 2-1/2 tons of cast iron per weekly,

independently of the finery.

In South Wales, especially at Pontypool, there are slighter blast

furnaces, whose upper portion is composed of a single range of bricks,

each of which is 20 inches long, 4 thick, and 9 broad. The interior of

the chimney represents an inverted cone. These furnaces derive solidity,

and power to resist the expansions and contractions from change of

temperature, by being cased, as it were, in horizontal hoops, placed 3

feet, or, even in some cases, only 6 inches asunder. These flat rings

consist of four pieces, which are joined by means of vertical bars, that

carry a species of ears or rings, into which the hoops enter, and are

retained by bolts or keys. Instead of these ears, screw nuts are also

employed for the junction. Each hoop is alternately connected to each of

the eight vertical bars. The interior of these furnaces is the same as

of the others; being generally from 12 to 14 feet diameter at the belly,

and from 50 to 55 feet high. Though slight, they last as long as those

composed of an outer body of masonry and a double lining of bricks; and

have continued constantly at work for three years. In Wales also the

blast furnaces are generally somewhat larger than in Staffordshire;

because there the object being to refine the cast iron, they wish to

procure as large a smelting product as possible. But in Staffordshire, a

fine quality of casting iron is chiefly sought after, and hence their

furnaces have less height, but nearly the same width.

In a blast apparatus employed at the Cyfartha works, moved by a 90-horse

steam power, the piston rod of the blowing cylinder is connected by a

parallelogram mechanism with the opposite end of the working beam of the

steam engine. The cylinder is 9 feet 4 inches diameter, and 8 feet 4

inches high. The piston has a stroke 8 feet long, and it rises 13 times

in the minute. By calculating the sum of the spaces percurred by the

piston in a minute, and supposing that the volume of the air expelled is

equal to only 96 per cent. of that sum, which must be admitted to hold

with machines executed with so much precision, we find that 12,588 cubic

feet of air are propelled every minute. Hence a horse power applied to

blowing machines of this nature gives, on an average, 137 cubic feet of

air per minute. The pressure on the air as it issues, rarely exceeds two

pounds on the square inch in the Welsh works.

At the establishment of Cyfartha, for blowing seven smelting furnaces,

and the seven corresponding fineries, three steam engines are employed,

one of 90 horse-power, another of 80, and a third of 40; which

constitutes in the whole, a force of 210 horses, or 26 horses and 1/5

_per furnace_, supposing the fineries to consume one-eighth of the

blast. In the whole of the works of Messrs. Crawshay, the proprietors of

Cyfartha, the power of about 350 horses is expended in blowing 12

smelting furnaces, and their subordinate fineries; which gives from 25

to 26 horses for each, allowing as before one-eighth for the fineries.

As these furnaces produce each about 60 tons of cast iron weekly, we

find that a horse power corresponds to 2 tons and a tenth in that time.

Each of the furnaces consumes about 3567 cubic feet of air per minute.

These works have been greatly increased of late years.

The following analyses of the English coal ironstones have been made by

M. Berthier, at the school of mines in Paris.

+------------------+----------+----------+-------------------+

| | ore. | ore. | or _gubbin_. |

+------------------+----------+----------+-------------------+

|Loss by ignition | 30·00 | 27·00 | 31·00 |

|Insoluble residuum| 8·40 | 22·03 | 7·66 |

|Lime | 0·0 | 6·00 | 2·66 |

|Peroxide of iron | 60·00 | 42·66 | 58·33 |

| |

|On calculating the quantities of carbonate of iron, and me- |

|tallic iron, to which the above peroxide corresponds, we |

|have:-- |

| |

|Carbonate of iron | 88·77 | 65·09 | 85·20 |

|Metallic iron | 42·15 | 31·38 | 40·45 |

+------------------+----------+----------+-------------------+

The mean richness of the ores of carbonate of iron of these coal basins,

is not far from 33 per cent. About 28 _per cent._ is dissipated on an

average, in the roasting of the ores.

Every ferruginous clay-stone is regarded as an iron ore, when it

contains more than 20 per cent. of metal; and it is paid for according

to its quality, being on an average at 12 shillings per ton in

Staffordshire. The gubbin however fetches so high a price as 16 or 17

shillings. The ore must be roasted before it is fit for the blast

furnace, a process carried on in the open air. A heap of ore mingled

with small coal (if necessary) is piled up over a stratum of larger

pieces of coal; and this heap may be 6 or 7 feet high, by 15 or 20

broad. The fire is applied at the windward end, and after it has burned

a certain way, the heap is prolonged at the other extremity, as far as

the nature of the ground or convenience of the work requires. The

quantity of coal requisite for roasting the ore varies from one to four

hundred weight per ton, according to the proportion of bituminous matter

associated with the iron-stone. The ore loses in this operation from 25

to 30 per cent. of its weight. Three and a quarter tons of crude ore, or

two and a quarter tons of roasted ore are required to produce a ton of

cast iron; that is to say, the crude material yields on an average 30·7

per cent., and the roasted ore 44·4 of pig metal. In most smelting works

in Staffordshire, about equal weights of the rich ore in round nodules

called _gubbin_, and the poorer ore in cakes called _blue flat_, are

employed together in their roasted state; but the proportions are

varied, in order to have an uniform mixture, capable of yielding from 30

to 33 per cent. of metal.

The transition or carboniferous limestone of Dudley is used as the flux;

it is compact and contains little clay. The bulk of the flux is made

nearly equal to that of the ore. To treat two tons and a quarter of

roasted ore, which furnish one ton of pig iron, 19 hundred weight of

limestone are employed; constituting nearly 1 of limestone for 3 of

unroasted ore. The limestone costs 6 shillings the ton.

Carbonized pitcoal or coke was, till within these few years, the sole

combustible used in the blast furnaces of Staffordshire.

The coal is distributed in circular heaps, about 5 feet diameter, by 4

feet high; and the middle is occupied by a low brick chimney, piled with

loose bricks, so open as to leave interstices between them, especially

near the ground. The larger lumps of coal are arranged round this

chimney, and the smaller towards the circumference of the heap. When

every thing is adjusted, a kindling of coals is introduced into the

bottom of the brick chimney; and to render the combustion slow, the

whole is covered over with a coat of coal dross, the chimney being

loosely closed with a slab of any kind. Openings are occasionally made

in the crust and afterwards shut up, to quicken and retard the ignition

at pleasure, during its continuance of 24 hours. Whenever the

carbonization has reached the proper point for forming good coke, the

covering of coal dross is removed, and water is thrown on the heap to

extinguish the combustion; a circumstance deemed useful to the quality

of the coke. In this operation the Staffordshire coal loses the half of

its weight, or two tons of coal produce one of coke.

As soon as the blast furnace gets into a regular heat, which happens

about 15 days or three weeks after fires have been put in it, the

working consists simply in charging it, at the opening in the throat,

whenever there is a sufficient empty space; the only rule being to keep

the furnace always full. The coke is measured in a basket, thirteen of

which go to the ton. The ore and the flux (limestone) are brought

forwards in wheelbarrows of sheet-iron. In 24 hours, there are thrown

into a furnace such as _fig._ 582., 14-1/3 tons of coke, 16 tons of

roasted ore, and 6-3/4 tons of limestone; from which about 7 tons of pig

iron are procured. This is run off every 12 hours; in some works the

blast is suspended during the discharge. The metal intended to be

converted into bar iron, or to be cast again into moulds, is run into

small pigs 3 feet long, and 4 inches diameter; weighing each about 2

hundred weight and a half.

The disorders to which blast furnaces are liable, have a tendency always

to produce white cast iron. The colour of the slag or scoriæ is the

surest test of these derangements, as it indicates the quality of the

products. If the furnace is yielding an iron proper for casting into

moulds, the slag has an uniform vitrification, and is slightly

translucid. When the dose of ore is increased in order to obtain a gray

pig iron, fit for fabrication into bars, the slag is opaque, dull, and

of a greenish-yellow tint, with blue enamelled zones. Lastly, when the

furnace is producing a white metal, the slags are black, glassy, full of

bubbles, and emit an odour of sulphuretted hydrogen. The scoriæ from a

coke, are much more loaded with lime than those from a charcoal blast

furnace. This excess of lime appears adapted to absorb and carry off the

sulphur, which would otherwise injure the quality of the iron. The

slags, when breathed on, emit an argillaceous odour.

A blast furnace of 50 or 60 feet in height, gives commonly from 60 to 70

tons of cast iron per week; one from 50 to 55 feet high, gives 60 tons;

two united of 45 feet, produce together, 100 tons; and one of 36 feet

furnishes from 30 to 40. A blast furnace should go for four or five

years without needing restoration. From 3-1/2 to 4 tons of coal,

inclusive of the coal of calcination, are required in Staffordshire to

obtain one ton of cast iron; and the expense in workmen’s wages is about

15 shillings on that quantity.

At the Cyfartha works of Messrs. Crawshay in South Wales, the average

price of the lithoid carbonate of iron, ready for roasting, is only

7_s._ 6_d._ a ton, and its richness is about 33 _per cent._ The furnaces

for roasting the ore in that country are made in the form of cylinders,

placed above an inverted cone. The cylindrical part is 6 feet high and

wide, and the cone is about 4 feet high, with a base equal to that of

the cylinder; towards the bottom or narrowest part of the inverted cone,

there is an aperture which terminates in an outlet on a level with the

bottom of the terrace in which the furnace is built. Sometimes, however,

all the roasting furnaces are in a manner combined into one, which

resembles a long pit about 6 feet in width and depth, and whose bottom

presents a series of inverted hollow quadrangular pyramids, 6 feet in

each side, and 4 deep. The bottom or apex of each of these pyramids,

communicates with a mouth or door-way that opens on a lower terrace,

through which the ore falls in proportion as it is roasted; and whence

it is wheeled and tumbled into the throat of an adjoining blast furnace,

on the same level with the terrace; for in Wales the blast furnace is

generally built up against the face of a hill, which makes one of its

fronts. The above roasting furnaces, which closely resemble lime-kilns,

after being filled with alternate strata of small coal and ore, are set

on fire; and the roasted ore is progressively withdrawn below, as

already mentioned.

The product of coke from a certain weight of coal is greater in Wales

than in Staffordshire, though the mode of manufacture is the same. At

Pen-y-Darran, for example, 5 of coal furnish 3-1/2 of coke; or 100 give

70; at Dowlais 100 of coal afford 71 of coke, and the product would be

still greater if more pains were bestowed upon the process. At Dowlais,

coal costs only 2 shillings a ton; at Cyfartha, it is worth from 2_s._

6_d._ to 5 shillings. About 2 tons of coke are employed in obtaining 1

ton of cast iron.

According to M. Berthier’s analysis, the slag or cinder of Dowlais

consists of silica, 40·4; lime, 38·4; magnesia, 5·2; alumina, 11·2;

protoxide of iron, 3·8; and a trace of sulphur. He says that the silica

contains as much oxygen as all the other bases united; or is equivalent

to them in saturating power; and to the excess of lime he ascribes the

freedom from sulphur, and the good quality of the iron produced. The

specimen examined was from a furnace at Merthyr-Tydvil. Other slags from

the same furnace, and one from Dudley, furnished upwards of 2 _per

cent._ of manganese. Those which he analysed from Saint Etienne in

France afforded about 1 per cent. of sulphur.

The consumption of coal in the Welsh smelting furnaces may be estimated,

on an average, at 3 tons per ton of cast iron; corresponding to 2·1 of

their coke. From this economy in the quantity of fuel, as well as from

its cheapness and that of the iron ore, the iron of South Wales can be

brought into the market at a much lower rate than that of any other

district. These blast furnaces remain in action from 5 to 10 years; at

the end of which time only their interior surface has to be repaired.

The lining of the upper part lasts much longer; for examples are not

wanting of its holding good for nearly 40 years.

One of the greatest improvements ever made by simple means in any

manufacture is the employment of hot air instead of the ordinary cold

air of the atmosphere, in supplying the blast of furnaces for smelting

and founding iron. The discovery of the superior power of a hot over a

cold blast in fusing refractory lumps of cast iron, was accidentally

observed by my pupil Mr. James Beaumont Neilson, engineer to the Glasgow

gas works, about the year 1827, at a smith’s forge in that city, and it

was made the subject of a patent in the month of September of the

following year. No particular construction of apparatus was described by

the inventor by which the air was to be heated, and conveyed to the

furnace; but it was merely stated that the air may be heated in a

chamber or closed vessel, having a fire under it, or in a vessel

connected in any convenient manner with the forge or furnace. From this

vessel the air is to be forced by means of bellows into the furnace. The

quantity of surface which a heating furnace is required to have for a

forge, is about 1260 cubic inches; for a cupola furnace, about 10,000

cubic inches. The vessel may be enclosed in brickwork, or fixed in any

other manner that may be found desirable, the application of heated air

in any way to furnaces or forges, for the purposes of working iron,

being the subject claimed as constituting the invention.

Wherever a forced stream of air is employed for combustion, the

resulting temperature must evidently be impaired by the coldness of the

air injected upon the fuel. The heat developed in combustion is

distributed into three portions; one is communicated to the remaining

fuel, another is communicated to the azote of the atmosphere, and to the

volatile products of combustion, and a third to the iron and fluxes, or

other surrounding matter to be afterwards dissipated by wider diffusion.

This inevitable distribution takes place in such a way, that there is a

nearly equal temperature over the whole extent of a fire-place, in which

an equal degree of combustion exists.

We thus perceive that if the air and the coal be very cold, the portions

of heat absorbed by them might be very considerable, and sufficient to

prevent the resulting temperature from rising to a proper pitch; but if

they were very hot they would absorb less caloric, and would leave more

to elevate the common temperature. Let us suppose two furnaces charged

with burning fuel, into one of which cold air is blown, and into the

other hot air, in the same quantity. In the same time, nearly equal

quantities of fuel will be consumed with a nearly equal production of

heat; but notwithstanding of this, there will not be the same degree of

heat in the two furnaces, for the one which receives the hot air will be

hotter by all the excess of heat in its air above that of the other,

since the former air adds to the heat while the latter abstracts from

it. Nor are we to imagine that by injecting a little more cold air into

the one furnace, we can raise its temperature to that of the other. With

more air indeed we should burn more coals in the same time, and we

should produce a greater quantity of heat, but this heat being diffused

proportionally among more considerable masses of matter, would not

produce a greater temperature; we should have a larger space heated, but

not a greater intensity of heat in the same space.

Thus, according to the physical principles of the production and

distribution of heat, fires fed with hot air should, with the same fuel,

rise to a higher pitch of temperature than fires fed with common cold

air. This consequence is independent of the masses, being as true for a

small stove which burns only an ounce of charcoal in a minute, as for a

furnace which burns a hundred weight; but the excess of temperature

produced by hot air cannot be the same in small fires as in great;

because the waste of heat is usually less the more fuel is burned.

This principle may be rendered still more evident by a numerical

illustration. Let us take, for example, a blast furnace, into which 600

cubic feet of air are blown per minute; suppose it to contain no ore but

merely coal or coke, and that it has been burning long enough to have

arrived at the equilibrium of temperature, and let us see what excess of

temperature it would have if blown with air of 300° C. (572° F.),

instead of being blown with air at 0° C.

600 cubic feet of air under the mean temperature and pressure, weigh a

little more than 45 pounds avoirdupois; they contain 10·4 pounds of

oxygen, which would burn very nearly 4 pounds of carbon, and disengage

16,000 times as much heat as would raise by one degree cent. the

temperature of two pounds of water. These 16,000 portions of heat,

produced every minute, will replace 16,000 other portions of heat,

dissipated by the sides of the furnace, and employed in heating the

gases which escape from its mouth. This must take place in order to

establish the assumed equilibrium of caloric.

If the 45 pounds of air be heated beforehand up to 300° C., they will

contain about the eighth part of the heat of the 16,000 disengaged by

the combustion, and there will be therefore in the same space one eighth

of heat more, which will be ready to operate upon any bodies within its

range, and to heat them one eighth more. Thus the blast of 300° C. gives

a temperature which is nine-eighths of the blast at zero C., or at even

the ordinary atmospheric temperature; and as we may reckon at from 2200°

to 2700° F. (from 1200° to 1500° C.), the temperature of blast furnaces

worked in the common way, we perceive that the hot-air blast produces an

increase of temperature equal to from 270° to 360° F.

Now in order to appreciate the immense effects which this excess of

temperature may produce in metallurgic operations, we must consider that

often only a few degrees more temperature are required to modify the

state of a fusible body, or to determine the play of affinities dormant

at lower degrees of heat. Water is solid at 1° under 32° F.; it is

liquid at 1° above. Every fusible body has a determinate melting point,

a very few degrees above which it is quite fluid, though it may be

pasty below it. The same observation applies to ordinary chemical

affinities; charcoal, for example, which reduces the greater part of

metallic oxides, begins to do so only at a determinate pitch of

temperature, under which it is inoperative, but a few degrees above, it

is in general lively and complete. It is unnecessary, in this article,

to enter into any more details to show the influence of a few degrees of

heat, more or less, in a furnace, upon chemical operations, or merely

upon physical changes of state.

These consequences might have been deduced long ago, and industry might

thus have been enriched with a new application of science; but

philosophers have been and still are too much estranged from the study

of the useful arts, and content themselves too much with the minutiæ of

the laboratory or theoretic abstractions. Within the space of 7 years,

the use of the hot blast has been so much extended in Great Britain, as

to have enabled many proprietors of iron works to add 50 per cent. to

their weekly production of metal, to diminish the expenses of smelting

by 50 per cent., and, in many cases, to produce a better sort of cast

iron from indifferent materials.

[Illustration: 585]

The figures here given represent the blast furnace, and all the details

of the air-heating at one view. _Fig._ 583. is a vertical section of the

furnace and the apparatus; _fig._ 585. represents the plan at the height

of the line 1, 2. of _fig._ 583. The blowing machine, which is not shown

in this view, injects the air through the pipe A, into the regulator

chamber R, _fig._ 585.; the air thence issues by the pipe B, proceeds to

C, where it is subdivided into two portions; the one passes along the

pipe C D to get to the tuyère T, the other passes behind the furnace,

and arrives at the tuyère T´ by the pipe C E F.

These pipes are distributed in a long furnace or flue, whose bottom,

sides, and top are formed with fire-brick, where they are exposed to the

action of the flame of the three fires X, Y, Z. The flame of the fire X

plays round the pipe B at its entrance into the flue, and quits it only

to go into the chimney H; that of the fire Y acts from the point D to

the same chimney, passing by the elbow C; that of the fire Z acts

equally upon F and H, in passing by the elbow E.

[Illustration: 589]

_Disposition of the fires and furnace._--_Fig._ 586. represents, upon a

scale three times larger than _fig._ 585., the section of the fire X,

of which the plan is seen in _fig._ 585., and the elevation in _fig._

583.; as also in the outside view of the blast furnace, _fig._ 589.

[Illustration: 586]

The grate is at L; the fuel is introduced by the door P, _fig._ 583.;

the flame rises above the bridge I K, and proceeds along the vaulted

flue towards the chimney H. Through a length of about 13 feet including

the grate, the furnace is on each side supported by oblong plates of

cast iron, which are bound together by 4 upright ribbed or feathered

bars, also on each side; these bars _n_ being bound together by iron

rods furnished with screw nuts at their ends (_figs._ 583, 585, 586.)

Beyond this distance, the outside of the furnace is mere brickwork.

The fires Y and Z have exactly a like disposition with the above.

[Illustration: 587]

_Fig._ 586. indicates the dimensions and the curvature of the arch above

the grate, near the bridge; _fig._ 587. represents the section of the

furnace and of the pipe beyond the cast-iron casing.

I find that the furnace is only about 3 feet wide at the bottom, and

that the elevation of the arch above the bottom is no more than 30

inches. Perhaps it might be made a little wider with advantage; the

combustion would be more vigorous and effective; and if the sides also

were a little thicker, the heat would be better confined.

The distance from the fire-place X to the chimney H, is 43-1/2 feet.

Y to the point C, is 13 ----

Z to the chimney, is 29 ----

including the turn of the elbow E.

[Illustration: 588]

_Distribution of the pipes._--At B, the pipe is 18 inches diameter

outside, and one inch thick of metal, and it tapers to C; from C to D

and from D to C the pipes are only 11 inches in external diameter, and

three-fourths of an inch thick; they are 5 feet long, and are united by

two kinds of joints; the ordinary ones, and those of compensation, to

give play for the expansion and contraction. One of these is seen

between B and C, one between C and D, one between C and E, and a fourth

between E and F. These pipes and their adjustment are seen more at large

in _fig._ 588.; U V is one of these pipes, its widened mouth receives

the extremity M of the preceding pipe. These pieces are truly bored and

turned to fit each other, and slide out and in like telescope tubes, by

the effect of dilatation and contraction of the pipes with changes of

temperature.

At certain distances castors or friction-rollers of cast iron are placed

to carry the pipes, which roll upon oblong plates of cast iron laid upon

the floor of the flues. These castors are shown at _a_, _b_, _c_, _d_,

_e_, _f_, _g_, _fig._ 585.; one of them is shown separate upon a larger

scale at G, _fig._ 587., as also the plate or rail S, on which it runs.

The tuyères T T´ are adjusted into the pipe behind them; this is truly

bored, so as to allow the thick end of the tuyère to slide tightly

backwards and forwards in it, like a piston in the barrel of a pump; a

diaphragm moreover prevents the tuyère from being drawn or forced

entirely out of its tube. At the side of this tube there is a small

orifice, which may be shut or opened at pleasure with a stopcock or

screw-plug: it serves to try the degree of heat of the air-blast; if a

lead wire does not melt when held at this hole, the temperature is

reckoned too low; being under the 612th degree of Fahrenheit. The

nozzles are 2 inches in diameter.

Near the fire-places of the air-heating furnaces the pipes are at a

cherry-red heat; and lest they should be burned, they are there coated

with a lute of fire-clay, as shown near K _fig._ 586. By this means the

air is kept up at the heat of 350° C, or 662° F., a little above the

boiling point of quicksilver.

_Quantity of air and pressure._--The blowing-machine belonging to the

above blast-furnace is moved by a water wheel of 22-horse power, the

pistons are 4 feet in diameter, have a 3-1/2-feet stroke, work double,

and expel 1200 cubic feet of air in the minute; or 600 cubic feet for

each nozzle. The pressure of the air is equivalent to no more than 2 or

2-1/4 inches of mercury; formerly with cold air it amounted to 3-1/2

inches. This furnace yields, upon an average, 5-1/4 tons of cast iron

daily, and consumes 1-1/3 cwt. of coke for each cwt. of cast iron

produced; being 7 tons of coke _per diem_.

The consumption of the three flue fires is 30 pounds of small coal, for

100 pounds of cast-iron produced, which may be reckoned equivalent to 15

pounds of coke; hence altogether each ton of cast iron requires for its

production 1-1/2 tons of coke.

The same furnace worked with the cold blast, the same pressure and the

same ores, produced only 3-1/2 tons of cast iron daily, with an

expenditure of 2·55 of coke for 1 of cast iron; in which case the coke

amounted to 9 tons daily.

The returns by the hot blast compared with those by the cold, are

therefore as the numbers 3 and 2, which shows an advantage by the former

plan of 50 per cent. The consumption of fuel in the two cases is as 8 to

9, being a saving in this article of about 11 per cent. Coke is used on

account of sulphur in the coal.

_Hot-blast heated by the flame of the furnace mouth._--This system is

mounted in Staffordshire. The heating apparatus is there set immediately

upon the mouth of the furnace; and is composed of 2 large cast-iron

cylinders of the same length, the one within the other, leaving a space

between them. This annular interval amounts to 16 inches, and it is

closed at top and bottom: but the innermost cylinder is open at both

ends, and forms, indeed, the vent of the chimney or furnace. It carries

nine rows of pipes, three in each row, which cross its interior, and

open into the annular space.

The flame of the furnace passes between the intervals of the cross

pipes, heating them, and also the two upright cylinders with which they

are connected. The air of the blowing machine arrives by a vertical

pipe, which is placed at the back of the furnace; it enters into the

above annular space, and thence circulates, with more or less velocity,

through the 27 cross tubes, upon which the flame is continually playing;

lastly, it is drawn through to the bottom of the annular space; the two

tubes which conduct it to the two tuyères, pass down within the

brickwork of the furnace, and thus prevent the dissipation of its heat.

Below this heating apparatus there is a door for putting the charges

into the furnace.

The above arrangement does not seem to be the best for obtaining the

greatest possible heat for the blast, nor for favouring the free action

of the furnace; but it illustrates perfectly well the principle of this

application. A serpentine movement in a long bent hot channel would be

much better adapted for communicating heat to so bad a conductor as air

is known to be.

In the month of July, 1836, I paid a visit to Codner Park and Butterly

works, in Derbyshire, belonging to the eminent iron-masters, Messrs.

Jessop and Co., where I was kindly permitted not only to study the

various processes of the manufacture of cast and wrought iron, but to

inspect the registers of the products of cast iron in their blast

furnaces for several years back. It appeared that in the year 1829, only

29 tons of cast iron were made weekly in each of the blast furnaces at

Codner Park. They were then worked with coke, and blown with cold air.

Each ton of iron required for its production, at that time, 6·82 tons of

coals, made into coke for smelting; with 2·64 of roasted iron ore

(carbonate), called mine; and 0·87 of limestone, the _castine_ of the

French.

In 1835 and 1836, the same furnaces turned out weekly, 49 tons of cast

iron each; and every ton of iron required for its production only 3 tons

of coal (not made into coke); 2·72 tons of mine; and 0·77 of lime.

In 1829, and for many years before, as well as one or two after, each

ton of coals is said to have cost for coking the sum of 6_s._, whence

the 6·82 tons of coals then converted into coke for smelting one ton of

iron, cost fully 40_s._ in coking alone, in addition to their prime

cost. The saving in this respect, therefore, is 40_s._ upon each ton of

iron, besides the saving of fully half the coal, and the increased

produce of nearly 60 per cent. of metal per week. The iron-master pays

the patentee 1_s._ upon every ton of iron which he makes, and at the

prices of 1836, he lessened his expenses by, at least, 30_s._ or 40_s._

per ton by the patent improvement.

The following tabular view of the progression in the management and

results of the hot blast, is given by M. Dufrénoy, after visiting the

various iron works in this country where it had been introduced.

“At the Clyde iron works, near Glasgow; in 1829, when the combustion was

effected by the cold air blast,--

Coal.

Tons. cwt. lbs.

There were consumed, for smelting; 3 tons of coke,

equivalent to 6 13 0

---- for the blowing engine 1 0 7

--------------

Total coal per ton of iron 7 13 7

Limestone 0 10-1/2 0

In 1831, with the hot blast at 450° F., coke being

still used in smelting,--

There were consumed, for smelting; 1 ton 18 cwt. of

coke, equivalent to 4 6 0

---- for heating the air, 5 cwt. }

---- for the blowing engine, 7 cwt.} 0 12 4

4 lbs. }

--------------

Total coal per ton of iron 4 18 4

Limestone 0 9 0

In July, 1833, with the hot blast at 612° F., raw

coal alone being used for smelting,--

There were consumed, for smelting 2 0 0

---- for heating the air 0 8 0

---- for the blowing engine 0 11 2

--------------

Total coal per ton of iron 2 19 2

Limestone 0 7 0

“At the last period the use of hot air had increased the make of the

furnaces by more than one third, and had consequently produced a great

saving of expense in the article of labour. The quantity of blast

necessary for the furnaces was also sensibly diminished; for a blowing

engine of seventy-horse power, which, in 1829, served only for three

blast furnaces, was now sufficient for the supply of four.

“On comparing these several results, we find that the economy of fuel is

in proportion to the temperature to which the air is raised. As for the

actual saving, it varies in every work, according to the nature of the

coal, and the care with which the operation is conducted.

“This process, though it has been four years in use in the works near

Glasgow, (which it has rescued from certain ruin) has scarcely passed

the borders of Scotland; the marvellous advantages, however, which it

has produced, are beginning to triumph over prejudice, and gradually to

extend its use into the different English iron districts. There are

one-and-twenty works, containing altogether sixty-seven blast furnaces,

in which hot air is used. The pig iron run out of these furnaces is

generally No. 1., and is fit for making the most delicate castings. This

process is equally applicable to forge pigs for the manufacture of bar

iron; since in order to obtain this quality of iron, it is only

necessary to alter the proportion of fuel and mineral. In the forges of

the Tyne iron-works, near Newcastle, and of Codner Park, near Derby,

pigs made in furnaces blown by hot air, are alone used in the

manufacture of bar iron.

“In the side of the tuyère pipe a small hole is made, by means of which

the heat of the air may be ascertained at any moment. This precaution is

indispensable, it being of importance to the beneficial use of hot air,

that it be kept at a uniformly high temperature. With a proper apparatus

the air is raised to 612 degrees Fahr., which is a greater heat, by

several degrees, than is necessary for the fusion of lead.”

“At Calder works the consumption of fuel has diminished in the

proportion of 7 tons 17 cwt. to 2 tons 2 cwt. There has also been a

great diminution of expense in limestone, of which only 5-1/2 cwt. are

now used, instead of 13 cwt., which were used in 1828. This decrease

results, as I have already said, from the high temperature which the

furnace has acquired since the introduction of hot air.

“The quantity of blast has been reduced from 3500 cubic feet per minute,

to 2627 cubic feet; the pressure also has been reduced from 3-1/4 to

2-3/4 lbs.”

_Of the refinery of cast iron, or its conversion into bar-iron, in

England._--This operation is naturally divisible into three distinct

parts. The first, or the finery properly speaking, is executed in

peculiar furnaces called _running out fires_; the second operation

completes the first, and is called _puddling_; and the third consists in

welding several iron bars together, and working them under forge

hammers, and between rolls.

1. The _finery furnaces_ are composed of a body of brickwork, about 9

feet square; rising but little above the surface of the ground. The

hearth, placed in the middle, is two feet and a half deep; it is

rectangular, being in general, 3 feet by 2, with its greatest side

parallel to the face of the tuyères; and it is made of cast iron in four

plates. On the side of the tuyères there is a single brick wall. On the

three other sides, sheet iron doors are placed, to prevent the external

air from cooling the metal, which is almost always worked under an open

shed, or in the open air, but never in a space surrounded by walls. The

chimney, from 15 to 18 feet high, is supported upon four columns of cast

iron; its lintel is four feet above the level of the hearth, in order

that the labourers may work without restraint.

The number of tuyères is from two to three; they are placed at the

height of the lip of the crucible or hearth, and distributed so as to

divide its length into equal parts; their axes being inclined towards

the bottom, at an angle of from 25° to 30°, so as to point upon the bath

of melted metal as it flows. The cast-iron nose-pipe is encased, and

water is made to circulate in the hollow space by means of cylindrical

tubes; being introduced by one tube, and let off by another, so as to

prevent the tuyères from getting burned in the process.

Two nozzles are usually placed in each tuyère, to render the blast

constant and uniform; and for the same end, the air impelled by the

bellows, is sometimes received at first in a regulator. The quantity of

air blown into the fineries is considerable; being nearly 400 cubic feet

per minute for each finery; or about the eighth part of the consumption

of a blast furnace.

[Illustration: 590 591]

The finery furnace, or running out fire, is represented in _figs._ 590.

and 591. It is a smelting hearth, in which by first fusing and then

cooling gray cast iron in a peculiar way, it is converted into white

cast iron, called fine iron, or fine metal, of the quality of forge pig,

for making malleable iron by the puddling process. The furnace resembles

the forge hearth employed in Germany and France for converting forge pig

into wrought iron; but it differs, particularly in this, that the fused

iron is run out into an oblong iron trough, for sudden congelation.

_a_ is the air-chest, in communication with the blowing cylinder, or

bellows; the air being conducted through at least two blast pipes to

the fire, and sometimes through even 4 or 6 pipes. _b_ is the side of

the furnace, corresponding to the tuyère plates, in which are the

openings for the blast pipes. All the sides of the furnace are hollow,

and are kept cool by the circulation of water through the cavity between

them. _c_ is the front wall of the furnace, having a strong cast-iron

plate containing the tap holes for running off the melted metal. _d d_

is the exterior wall of the furnace, which corresponds to the

_contre-vent_ and ash-hearth of the French refining forge. _e_, is the

top plate upon which the coke is piled up in store. _f f_, _f f_, iron

props of the chimney, (not shown in this view). _g_, cast-iron trough

into which the fine iron is run off in fusion; which is sometimes made

in one piece, but more usually in separate plates joined together.

Beneath this mould a stream of water is made to flow. _h_ is the bottom

of the hearth, covered with sand.

In the finery process, the hearth or crucible of the furnace is filled

with coke; then six pigs of cast iron are laid horizontally on the

hearth, namely, four of them parallel to the four sides, and two in the

middle above; and the whole is covered up in a dome-form, with a heap of

coke. The fire is now lighted, and in a quarter of an hour the blast is

applied. The cast iron flows down gradually, and collects in the

crucible; more coke being added as the first quantity burns away. This

operation proceeds by itself; the melted metal is not stirred about, as

in some modes of refinery, and the temperature is always kept high

enough to preserve the metal liquid. During this stage the coals are

observed continually heaving up, a movement due in part to the action of

the blast, and in part to an expansion caused in the metal by the

discharge of gaseous oxide of carbon. When all the pig iron is collected

at the bottom of the hearth, which happens commonly at the end of two

hours, or two and a half, the tap hole is opened, and the _fine_ metal

flows out with the slag, into the loam-coated pit, on a plate 10 feet

long, 3 broad, and from 2 inches to 2-1/2 thick. A portion of the slag

forms a small crust on the surface of the metal; but most part of it

collects in a basin scooped out at the bottom of the pit, into which the

fine metal is run.

A large quantity of water is thrown on the fine metal, with the view of

rendering it brittle, and perhaps of partially oxidizing it. This metal

suddenly cooled, is very white, and possesses in general a fibrous

radiated texture; or sometimes a cellular, including a considerable

number of small spherical cavities, like a decomposed amygdaloid rock.

If the cast iron be of bad quality, a little limestone is occasionally

used in the above operation.

Three samples of cinder, analyzed by Berthier, gave,

Silica 0·276; protox. of iron, 0·612; alumina, 0·040;

phosp. acid, 0·072, Dudley.

---- 0·368 ---- 0·610 ---- 0·015;

puddling of Dowlais.

---- 0·424 ---- 0·520 ---- 0·033;

ditto.

The remarkable fact of the presence of phosphoric acid, shows how

important this operation is to the purification of the iron. The charge

varies from a ton and a quarter to a ton and a half of pigs; and the

loss by the process varies from 12 to 17 per cent.

The _fine metal_ is broken into fragments, and sent to the puddling

furnace after the product of each operation has been weighed. The coal

consumed in the fine metal process is from 4 to 5 hundred weight for the

ton of cast iron. About 10 tons may be refined _per diem_, a quantity

somewhat greater than the supply from a blast furnace; but the fineries

are not worked on the Sundays; and therefore a smelting furnace just

keeps one of them in play. Whatever care be taken in this process, the

bar iron finally resulting is never so good as if wood charcoal had been

used in the refinery; and hence in making sheet iron for the tin plate

manufacture, wood charcoal is substituted for coke in one Welsh

establishment. The cast iron treated with charcoal, gets into clots or

lumps in the finery furnace, which are lifted out, set under the hammer,

and flattened into thin cakes.

The main effect of the finery process, is probably the separation of the

plumbaginous part of the charcoal, which is disseminated through the

gray cast iron in a state of imperfect chemical combination. When that

is removed the metal becomes more homogeneous, having no crystalline

carbon present to counteract its transition into pure iron; much of the

silica and manganese are also vitrified together, and run off in the

finery cinder.

[Illustration: 592]

2. The _puddling furnace_, is of the reverberatory form. It is bound

generally with iron, as represented in the side view, _fig._ 592., by

means of horizontal and vertical bars, which are joined together and

fixed by wedges, to prevent them from starting asunder. Very frequently,

indeed, the reverberatory furnaces are armed with cast-iron plates over

their whole surface. These are retained by upright bars of cast iron

applied to the side walls, and by horizontal bars of iron, placed across

the arch or roof. The furnace itself is divided interiorly into three

parts; the _fire-place_, the _hearth_, and _flue_. The _fire-place_

varies from 3-1/2 to 4-1/2 feet long, by from 2 feet 8 inches to 3 feet

4 inches wide. The door way by which the coke is charged, is 8 inches

square, and is bevelled off towards the outside of the furnace. This

opening consists entirely of cast iron, and has a quantity of coal

gathered round it. The bars of the fire grate are movable, to admit of