Charles George Warnford Lock.

Economic mining: a practical handbook for the miner, the metallurgist and ... online

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ndtsof barium, is found native in large quantities, forming the species
of mineral termed barites or buytes, and commonly known as heavy-
sptr, on aocount of its weight (sp. gr. from 4* 3 to 4* 7). It is found
in Derbyshire and Shropshire, and often occurs in fine tabular crystals*
The massive variety found in the mountain limestone of the above
oountieA is sometimes called '' cawk " ; it is more frequently found in
white or reddish-white masses. In Saxony it occurs as the mineral
ttamgen^spaih, m a columnar form ; and at "Bologna a nodular variety
is found, called Bologna stone, which is notable for its phosphorescent
powers when heated.

The pure pigment is a heavy white powder, insoluble in water and
nearly insoluble in all other menstrua. It is prepared by heating the
native mineral, grinding it to powder, and well washing it, first in
dilute sulphuric acid, in order to remove any traces of iron, and after-
wards in water. It is then levigated or ** floated," the lightest
particles being the most valuable, and known as *' floats " ; the white
powder is afterwards thoroughlv dried. This process is employed at
several works in the neighbourhood of Matlock Bath, in Derbyshire,
hut much larger quantities could be produced in different parts of the
country if the demand for the article rendered its production more
profitable. The principal use of sulphate of baryta is to adulterate
white lead, and to form the pigment known as hlcmcfixe^ or permanent
white. For these purposes the native mineral, ground and washed as
described above, is commonly employed. The annual production in
the United Kingdom is 26,000 to 80,000 tons, valued at about 24«. a

Improvements in machinery and in the process of treating natural
barytes have overcome many of the objections which formerly existed
to its utilisation, and considerable attention is now being given to the
localities in the United States where it is found. The mineral, in
Older to be available for the uses to which it is put, must be fairly firee
fen quartz grains, the stain of iron rust, and otJier impuritiea If
the bar3rtes is stained to any extent, it is practically valueless, as a
good white colour is essential to its usefulness. Quartz grains or other
hard substances with which it is apt to be associated, injure the
aachmeiy in grinding. The purest barytes so far produced in
America comes from Missouri (where it is called '^tiff*' ), though a
very fair grade is now being mined in considerable quantities in

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Yirginia. The yearly output in the United States is about 30,000 to
35,000 tons, valued at 21«. a ton in the orude state, and ^. to |<Lalb.
when prepared.

Magnesite (see p. 329) being white, very heavy, opaque, and harm-
less, composed almost entirely of ma^esia carbonate, is becoming a
very important pigment as a rival to bskrytes.

Ochrea, — The large class of mineral pigments known collectively
as ochres or sienna earths possess considerable importance, notably on
account of their remarkable durability and their reasonable price.
They all consist essentially of an earthy base coloured by oxide of
iron or of manganese, or of both. Some authorities differentiate be-
tween ochres and siennas, and ascribe the latter name only to those
earths which contain manganese, but this seems to be an arbitrary
prooeeding, because the term sienna, or more properly Siena, is derived
solely from the name of the Italian province in which these minerals
are worked. They are of widespread occurrence, both geogranhically
and geologically, and the methods of mining and preparing tnem aie
not subjeet to much variation.

Thev are chiefly found in large quantities in the communes of
Oastel del Piano and Arcidosso. The yellow earths and bole found <m
the western slopes of Monte Amiata are true lacustrine deposits found
amid the trachytic rocks, of whidi it is principally composed. They
lie under, and are entirely covered by, the vegetable soiL Varying
in compactness and colour, they are termed yeUow earths when of a
clear ocbreous tint, and terra botare^ or bole, when of a dark chestnut
colour. Each deposit consists for the greater part of yellow earth,
beneath which bole is found in strata or small veins. The mineral
being very friable, its excavation is easy, and is generally oondncted
in open pits.

The different qualities are separated during the process, the bole,
which has the higher commer^al value, being the more carefully
treated. After the first separation the bole is further classed into first,
second, third, and intermediate qualities— 6oZe<to, fasda^ eerehiane, &c.
Its most important characteristio is termed, in commercial language,
punio di colore, or tint. The value of the bole rises as its tint deepens.
Thus bole of the third quality is lighter than that of the second, and
the second than that of the first. After the third quality comes the
terra guilla. The yellow earths, after excavation, are exposed to the
open air for about a year, by the pit side, without classification. The
bole, on the contrary, is placed in well-ventilated storehouses to dry
for about 6 months. This diversity of treatment is owing to the fut
that exposure to the elements brigbtens the colour of the yellow
earths, and raises their value, while it would damage the bole by
turning its darker tint first into an orange yellow, and, if continued,
into an ordinary yellow earth. It also loses in compactness and
crumbles up under exposure.

In addition to the punto di colore, the size of the pieces influences
the commercial value of the bole, which increases with their volumeu
Thus the classification is holo pezzo, holo grapolino, and hoh polpere.
The yellow earths are clashed as giallo in pezzo, gtaUo eomtiMiiBe, aid
giallo impalpabile, the impalpable being worth more than the oommon

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yellow. The production of the Siena earths is estimated at about
600 tons per annum, of which amount about 50 tons are calcined, and
the rest sold in the natural condition. The value of the trade is
estimated at from 40002. to 60002.

The European trade in these earths is very large. Bouen exports
ffm» 5000 tons yearly, and Havre about 1500 tons.

Similar deposits occur in America, where they are known as
^ paint-beds," and the earths are called '* metallic paints." A promi-
nent example is the paint-bed at Lehigh Gap, Carbon County, Penn-
sjivania, which was originally opened as an ironstone mine. The
mineral proved valueless metallurgically, but remnrkably useful as a
pigment, since it contains about 28 per cent, of hydraulic cement,
which hastens the drying and causes the paint to set .without any
addition of artificial dryers, thereby making it eminently fitted for
all outdoor application.

Along the outcrop of the paint, the beds are covered by a cap or
overburden of clay, and by the decomposed lower portion of the Mar-
cellos slate, which is 50 ft thick at the Butherford shaft.

Beginning with the Marcellus slate, the measures ooour in the
following descending order : —

0. Hydratdio cement (probably Upper Helderberg), very hard and

b. Blue clay, about 6 in. thick.

e. Paint-ore, varying from 6 in. to 6 ft. in thickness.

J. Yellow clay, 6 ft. thick.

e. Onskany sandstone, forming the crest and southero side of the

East of the Rutherford shaft the sandstone forms the top-rock of
the bed. This is due to an overthrow occurring between the Ruther-
ford tunnel and shaft.

The paint-bed is not continuous throughout its extent. It is
f&alted at several places ; sometimes it is pinche i out to a few inches
uid agHin increases in width to 6 ft. A short distance south of Bow-
iQan*8 there is a fault striking north-east in the Marcellus slate,
»hich has produced 'a throw of about 200 ft. The measures dip from
10^ to 90\ The dip at the Rutherford shaft is about 79° south,
whereas at the tunnel it is 45° north. The ore is bluish -grey, re-
lembling limestone, and is very hard and compact. The bed is of a
ighter tint, however, in the upper than in the lower part, and this
8 probably due to its containing more hydraulic cement in the upper
ttnita. The paint-ore contains partings of clay and slate at various

At the Butherford shaft there are fine bands of ore, alternating
^th clay and slate, as follows— Sandstone (hanging-wall), clay, ore,
ilate, ore, clay, ore, clay, ore, slate, ore, cement, slate (foot-wall). These
cartings, however, are not continuous, but pinch out, leaving the ore
^thout the admixture of clay and slate. Near the outcrop the bed
>ecomeB brown hematite, due to the leaching out of the lime and to
vjmplete oxidation. Occasionally, streaks of hematite are interleaved
^th the paint-ore. In driving up the breasts, towards the outcrop,
lie ore is^ found at the top in rounded, partially oxidised and weatn*

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ered masses, called '* bombshells," covered with iron oxide and sitr-
rounded by a bluish clay. In large pieces the ore shows a dedded

The method used in mining is a variation of panel-work. Nearly
the same system of working is employed by all of l^e companies who
have developed their mines either by means of tnnnels or shafts.
Tunnels are preferred whenever equally convenient, because they in-
volve no expenses for pumping and hoisting machinery, fuel« repairs
to machinery, Ac.

The following description of the operation of the Butherford mines
is typical of all the workings in the vicinity.

The Butherford tunnel is 6 ft high and 600 ft long. The gang-
ways are driven along the foot-wall of the cement side, 6 ft. high, lu^
are heavily timbered and lagged at the top and on the clay side. The
sets of timbers are Z\ ft. apart, and usually of 9-in. timber. Tfae
width at the top is 3]^ ft., with a spread of 5 ft at the bottom, tlie
extra width being cut from the clay. Where the cement-rock is firsu
the collar is hitched 6 in. into it and supported by a leg cm tiie day
side. The cost of the timber is 54 cents (2«. dJ.) per set, including
the lageing. The monkey gangway, which carries the air along tbd
top of &e breast ft'om the air-shaft, is 2\ ft. hi^, 1^ ft wide at the
top, with a spread of 2^ ft. at the bottom. Wooden rails with a
gauge of 18 in. are spiked to the cross-tie&

The gangway is not driven continuously, but after being driyeii
about 55 ft. on either side of the shaft, the breasts are started 25 ft froa
the shaft, a pillar being left to protect it. The breast is then opened
up to the face of the gangway, and when one ore-breast is worked out,
the gangway is driven {Jiead about 30 ft., and a new breast is opend
and worked out before commencing a third. The air-hole is fii^
driven to the surface, then the breast is opened to its fall width oi
6 ft The thickness of the bed of ore here varies from 4 to 6 ft. de
pending upon the thickness of the partings of clay and slate. Tin
clay aim slate are left on the bottom, which is made sloping to allon
the ore to roll down to the shute ; this is 6 ft. wjde and 4 ft. long anc
heavily timbered. Small props or sprags are hitched into the o^nent
and wedged with a lid on the clay side to prevent &lls of rock.

The holes are drilled by hand in the day-partings. They vaiy i
depth ft-om 1 to 4 ft, and the charge of dynamite is varied
spondinglv, according to the amount of ore it is desired to
down. The loose ore is wedged down with crowbars and pioks^ \
is then freed from any adhering clay and thrown down the shvte.
is there loaded into boxes holding about \ ton each, which are posL
to the shaft on a truck. The ore-boxes have 4 rings at the came
to which are attached 4 chains, suspended from &e wire hoisting
rope. At the top of the shaft the boxes are detached and placed i
a truck, which is run to the dump ; 30 cars, averaging 15 toiw,
extracted in a day of two shifts, the day-shift working 9 boors
the night-shift 11. The pay of the miners is 5«. per shift,
cost of mining the ore averages 7«. per ton.

The ore, as it comes from the mines, is free from refuse, great <
having been taken to separate slate and clay from it in the

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It is hauled in 2-ton wagons to kilns, which are situated on
Idll-aide for oonvenienoe in charging. The platform n]^n which
le ore is dumped is built from the top of the kiln*to the side of the
ilL The ore is first spalled to fist-size and freed from slate, and is
wn carried in buggies to the charging-hole of the kiln.

The slate, when burned, has a Ught yellowish tint, which would
lymge the colour of the product. Figs. 90 to 92 represent a front
bnSion of the kiln, and two sections at right angles to each other,
lie kiln is 22 ft high and 16 ft square on tiie outside. The interior
\ cylindrical, 5 fL £am., witJi a firebrick lining a of the best quality,
"he interior lining slopes from the fireplace 6 to the door c, by which
be charges axe withdrawn ; this facilitates the removal of the cal-
bed ore. The casing <i is of sandstone, b\ ft. thick, and tied together
|ilh the best white-oak timber s. When charged, a kiln holds 16
MS of ore, and the kiln is kept constantly fcdl. llie heat passes
torn the fixei^aoee &— of which there are two, placed diametrically

- Fioc 90, 91, 92.— Knj( fob Bubkino Paint Obb.

ffMMite each other — through a checker-work / of brick into the
entre of the charge. The c£arge enters at g and is witiidrawn by a
)fxx c in the front wall, 2 ft. long and 18 in. high. The ashpit is at
• The fixe is k^t at a cherry-red heat, and about 1 cord of wood is
mroed every 24 hours.

The kiln works continuously, calcined ore being withdrawn and
ireah oharees made without interruption. The ore is subjected for 48
hows to ute heat, which expels the moisture, sulphur, and carbon
lioxide. About 14 tons of calcined ore are withdrawn every 3 hours
huing the day. The outside of the lumps of calcined ore has a light
bown colour, while the interior shows upon fracture a darker brown.
Sreat care is necessary to regulate the heat so that the ore is not over
bined. When this happens, the product has a black scoriaceous ap-
pearanoe, and is unfit for the manufacture of paint, being extremely
bard to grind.

The calcined ore is carried from the kiln in wagons to the mill,
whmre it is broken to the size of grains of com in a rotating crusher.

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The broken ore is carried by eleyatora to the stook-bins at the top d
the building, and thence by shutes to the hoppers of the mills, whidi
grind it to the necessary degree of fineness. Elevators again carry i
to the packing-machine by a spout, and it is packed into '
holding 500, 300, or 100 lb. each.

Ochres owe their colour to hydrated oxide of iron, besides wliii
body they contain clayey matter ^silicate of alumina), earthy matf
barytes, carbonate and sulphate of calcium, Ac, dependent upon
locality from whence they are obtained ; thus Derbyshire ochres
tain mostly calcareous earthy matters, barytes, gypsum, dbc, whii
Oxford ochres and French ochres contain clayey matter; Welsh c '
are variable, and usually contain a good deal of silicious matter.

The annual production of ochres (and umbers) in the United JUng|
dom reaches 10,000 to 20,000 tons, with a value of about 40«. a ton.

The yearly output of ochres (and metallic paints) in the United
States is about 40,000 tons, having an estimated value of about 5t)i
a ton.

SnudU. — This pigment has not maintained its position in cm
petition with artificial ultramarine. Formerly it was very largely
used to correct the yellow tone of cottons, papers, and pottery. I
has a pale violet-blue tint, which, however, is not constant in arti
ficial bght. Being a silicate it is very permanent, and proof againsi
the action of acids, alkalies, and sunlight, besides being inert wba
mixed with other pigments. It can be used with either water or d
as a medium, but is not a successful paint owing to its weak colonricj
power. It is virtually a double siHcate of cobalt and potash, or
cobalt glass, containing a few impurities, of which the chief
aluminium, iron, and lead oxides. The colour varies somewl
according as these impurities fluctuate, and the finest ground mm\
is always the palest. It is hardly ever adulterated, and the di
point to secure is that it be ground to the finest possible degree. J

Its manufacture is most extensively and successfully carried on a
Saxony. The raw materials used are cobalt speiss (an arsenide a
cobalt and iron^, potash, and sand The ore is broken up into coa
venient sized pieces and roasted at red heat in a reverberatory fnniid
provided with a tall shaft for discharging the sulphurous and arsenici
fumes at a high altitude. When the evolution of these fumes h^
ceased, and the mass begins to assume a pasty consistence, the roaetd
ore is removed from the furnace, cooled, reduced to a fine pulvemW
condition (then known as *'zafire") and passed through a silkd
sieve. Should it be necessary, the cobalt ore is first spalled and huA
picked to remove the ores of foreign metals which are associated wi^
it ; and then reduced to a very fine state in an edge-runner or morti
mill, and freed from earthy impurities by washing. The ooneentiatfl
ore is then dried and dead-roasted in small charges at a time (abo)!
4 cwt.) in a specially designed reverberatory furnace such as shovi
in Fig. 93, of which a is the hearth on which the ore is spread ; h, tk
fireplace, the products of combustion from which pass over the.orecj
the hearth, and thence into the flues c, which repeatedly circle rood
the furnace so as to provide abundant opportunity for the aisenioi
oxide derived from the combustion (oxidation) of the arsenic in tl

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to oondense ; this highly poisonous arsenions oxide is coUeoted in
lid form from the flues at oonvenient intervals by means of the
B (2. The ore is charged and discharged at the door e. The
ting should not be carried to such a point that the whole of the
hur and arsenic are removed when making smalts, as by leaving
»rtion of these substances in the ore at this stage the ultimate
fication is better accomplished.

rhe next stage is to fuse the roasted ore with potash and silica so
) form a blue glass. The proportions in which the ingredients
mixed depend upon the depth of colour in the zaffre operated
a and the tint desired in the finished smalts; hence it is iJways
nuined by a preliminarv experiment, and is then most carefully
sred to, each material being accurately weighed out. Only the
potash can be used, as it must be quite free from soda, and iron


Fio. 98.— Kiln fob Boastino Ck)BALT Obs.

iker metal ; the effect of soda is to render the blue greenish tinted,
rtz affiords the requisite silica, and is hand-picked to ensure
iom from alumina, iron, and lime, which import dulness into the
or, and then ground to a fine powder in €Ln edge runner milL The
r weighed quantities of the several ingredients are intimately
^ in wooden or cement-lined vessels, so as to preclude the possi-
7 of any metallic iron finding its way in ; and as a further pro-
um against this risk a little white arsenic is often added so that
iron may be carried down in the regulus which is formed during
^lsion in the crucible.

Fhese crucibles are of refractory earthenware quite free frrom lime,
meesore about 18 in. across at top, gradually diminishing to
B* at bottom, so that an ordinary clunrge is about f cwt. They
placed in rows in a furnace which generall}* bears a close re-
Uanoe to a glass furnace, the operation being very similar. The

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IB shon

of the



pots h






pots, 1


a whii

Fig. 94.— Kiln fob makino Smalts, ^

glass. The fused mass is repeatedly sampled, and when it hasbfli
quite homo^neous, and the regulus or speiss oontaining ^i
antimony, bismuth, arsenic, copper, nickel, sulphur, and other iiB]
ties has completely separated itself and collected at the bottom fk
pots, the blue glass is ladled out and dropped at once into cold n
by which it is disintegrated and rendered very brittle, ready fa
subsequent grinding. The regulus is then drawn off from tbt
through holes provided for the purpose, and removed by the orii
after which the pots are ready for another charge. They oidiii
remain serviceable for about 6 months.

The grinding needs to be done with great thoroughnees^ i
accomplished partly by stamps and partly by edge-mnnCT nn
the presence of water. The particles as reduced are floated off l|
water to a series of settling tanks communicating one with aM
The portion which settles in the first of the series is too

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ad 18 retnmed to the edge-runner for further grinding ; while

vrtion in the last of the series possesses such a weak coToor that

rejected, or put into the crucible to undergo a second fusion.

sleeted portions are dried ready for the consumer.

i&eri. — Theee form a large class of natural earths of a bro¥m

, differing widely in the proportions of their chief constituents,

seelj allied to the ochres and siennas in general composition,

ring their colour mainly to the presence of hydrated oxides of

sd manganese, the latter prevailing in the umbers to a greater

I than in the ochres and siennas (see p. 814).

Is or veins of umber of varying thickness and extent are found

ay places, especially in connection with magnesian limestone

4te). Apparently they are often derived from decomposition

I rock, perhaps due to the infiltration of carbonated water,

has aeted upon the calcium and magnesium carbonates in the

nite, and left the silica and the iron and manganese as oxides,

ing the bulk of the umber. Usually these beds of umber are

the surface, though covered by an overburden of v^etable soil,

the operation of working them may be called quarrying rather

mining, bein^ of a superficial and simple character, often only

inting to smafi pits.

LB no umber is a definite body, but rather a mixture of various
t%n<y#, so the composition of every kind is peculiar to itself, and
wide differences are noticeable. Even the same bed will not
Bsaiily produce always the same class of umber. The fcdlowing
«8 show the extent to which the proportions of the several
edients may vary : —


Water p:iTen off at 212<'F 4 to 65

Water in oombinatioii 5 „ 11)

Silica 4J „ 291

Maoganeee dioxide 7 „ 27

Ferric oxide 6 „ 36

Wloinm carbonate is sometimes present to the extent of 21 to
r cent., and at other times is quite absent, its place being taken
to 1 per cent, of lime (calcium oxide); some of the English
9TS contain about 2 per cent, of calcium sulphate (gypsum) in
don to the carbonate. Alumina may occur to the amount 01 2^
l\ per oent^ or may be wanting altogether. In a sample of
lyshire umber analysed by Hurst &ere appears to have been over
»r cent, of barium sulphate (barytes), which looks suspiciously

Jmoet every variety of shade may be found in umbers. The
Bat and richest in colour — a warm violet-brown — is the so-called
:ey nmber, mined in Cyprus, and formerly shipped viA Constan-
»le ; this is of very fine quality and commands the higher price
&e mmrket. A readish-brown Irish umber, known as Gappagh
n, obtained from the Cappagh mines in Cork county, is much
med among artists, both for water-colour and oil painting, and
dally for the latter when it has been subjected to a preliminary
cation at a temperature of about 170^ F. Heated to the boiling

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point its oolonr changes to a rich red, resembling bnmt sieni
Ck>mish timbers are of fairly good quality. Derbyshire Tunben a
poor, and incline to a reddisn tint, l^sides being gritty. Sometia
they are adulterated with a little lampblack, which renders the to
more like that of Turkey umber, and thus deceives the nnwi

There are three conditions in which umbers come into commen
(1) as raw lump, being the mineral just as it is mined; (2)a8n
powdered, when it has been ground very fine and levigated or wai
in flowing water, whereby the particles get assorted according
their several degrees of fineness ; and (2t) as burnt, being the po«i
after it has been subjected to calcination in a closed furnace. Sq

Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 35 of 76)