MINERALS: THEIR BEHAVIOUR IN THE KILN (_continued_).
CALCITE, ARAGONITE, &c.
Carbonate of lime may occur in a crystalline form, or as earthy
substances, and many varieties of it are found in clays used by the
brickmaker. The commonest are calcite, aragonite, and a white earth.
Calcite, known also as calc-spar, crystallises in the hexagonal system,
though true hexagons are not very common. It occurs principally as
rhombohedra and scalenohedra, with variations therefrom; also fibrous,
lamellar, granular, compact, nodular, and stalactitic. When pure,
calcite is colourless and usually transparent, but when mixed with iron
or other mineral colouring matter it commonly assumes yellow and brown
tints.
Aragonite is also a crystalline form of carbonate of lime, but is by no
means as common in Nature as calcite. It crystallises in the rhombic
system, which assists the mineralogist to distinguish it from the
last-mentioned mineral, from which it differs also in being harder and
of higher specific gravity. Aragonite may occur as globular masses,
or as incrusting other substances, or in the stalactitic form. It is
sometimes white, but more often yellowish, or grey, and it is not,
commonly, as transparent as calcite, whilst it often possesses one to
two per cent. of carbonate of strontia, or other impurity.
It is generally stated that carbonate of lime, when deposited from cold
solutions, crystallizes in hexagonal (calcite), and when from warm
solutions, in rhombic (aragonite) forms. No doubt, on the whole, that
is the case; but we ought not to forget that many marine organisms
make their hard parts of aragonite, which, under the circumstances, is
certainly not obtained from warm solutions. These crystalline forms of
carbonate of lime are both of them found in fossil shells and the like
in clays, and in not a few instances the calcareous constituent found
in the brick-earth is present almost exclusively in the fossils, which
are ground up with the rest in preparing the material for the moulding
machine.
When present as hard crystalline lumps or pebbles, they have been
derived from the destruction of limestones, and are then the greatest
nuisance imaginable to the brickmaker and the most dangerous
constituent at the same time. With proper machinery these hard lumps
may be ground down to fine particles, but they are even then only to be
admitted into the earth on sufferance. The best plan, without doubt,
is to remove them altogether from the raw earth. They are commonly met
with in what the geologist calls “boulder clay”--a deposit owing its
origin to glaciers and icebergs. Very often the pebbles alluded to
are not crystalline, but of an earthy character, as is the case when
made of chalk. In the semi-dry process of manufacture, it is next to
impossible to incorporate the ground-up particles of carbonate lime
sufficiently well to result in the production of such a homogeneous
earth as is desirable for making a first-class brick.
In sandy clays or loams, and in a few stiff clays used for brickmaking,
certain remarkable concretions called “race” are found, the deleterious
properties whereof are so well known to the average brickmaker that
he carefully avoids the particular strata in which they occur. It is
fortunate that these concretions have a habit of being confined to
narrow limits along definite horizons in the brickyard section, so that
they may be readily discarded in working. But that is not always the
case, and little nodules of “race” are usually more or less frequent
also in the beds above and below the horizons referred to. They are
composed wholly of carbonate of lime, and their general effect in the
kiln, and afterwards, will presently be explained. Other forms of
concretions are known as “septaria,”--tabular or rounded masses of
argillaceous limestone found in practically all stiff clays. These are
often of enormous size, and are disposed in regular lines which the
field geologist takes to indicate bedding planes in the clay--otherwise
often very difficult to make out. In certain stiff clays little pellets
of the same substance are found. The larger septaria have commonly been
cracked in various directions, the fissures being subsequently filled
with calcite.
Coprolites are impure varieties of phosphate of lime, and the term
should, properly speaking, be restricted to a substance of organic
origin,--the fossilised excrement of animals. But the name is now
loosely employed to designate phosphatic concretions in general, such
as are commonly found in stiff clays, in certain “greensands,” and in
other sedimentary deposits. The dark brown phosphate of lime has formed
on and often completely envelopes many fossils; in certain cases it
has in fact been utilised as a petrifying medium, in which form it
ordinarily occurs in the thick black clays of Peterborough, Cambridge,
the gault of Kent, Surrey, etc.
Summing up the effects of carbonates and other kinds of lime in the
kiln, it may be at once said that when present in any other form than
as extremely minute particles, they are distinctly to be avoided. The
small pellets and large pebbles especially are to be avoided, for the
following reasons. Carbonate of lime is made up of lime and carbonic
acid; if a lump of this be subjected to great heat and thus calcined,
the carbonic acid is driven off, escaping by means of flues, the
open chimney, or kiln. The product is lime pure and simple--ordinary
builders’ lime. Everyone knows that on the addition of water builders’
lime becomes “slacked,” and eventually, after a fashion, “sets.”
Precisely the same thing occurs in the brick-kiln. The raw brick is
often composed of pieces of chalk or other limestone, in limestone
districts and in areas where boulder clays are largely employed for
brickmaking. On being subjected to the heat of the kiln these pieces
are promptly reduced to the condition of lime. During the process
of conversion considerable expansion takes place, and subsequently
contraction, leading to the formation of cracks radiating from the
fragments of limestone, the homogeneity of the bricks being at once
destroyed. Apart from this, when placed in the open air the lime
becomes slacked, and the quality of the brick is seriously impaired.
Lime is a highly refractory substance, strongly basic in character,
and forms fusible compounds with silica and other acid bodies. It is,
therefore, useful as a flux in many earths used in brickmaking, being
added to them expressly for that purpose, to the general improvement
of the brick. The celebrated Dinas bricks, for instance, are composed
of a highly refractory earth containing about 97 per cent. silica, the
remainder being lime, oxide of iron, alumina, alkali and water. To
render this material fusible and so as to make refractory bricks, from
1 to 3 per cent. of lime is added.
But what we more particularly desire to draw the reader’s attention
to at the present stage, is not the employment of lime in making
fire-bricks so much as its mixture with ordinary brick-earth, as in the
manufacture of malm bricks. Sometimes the mixture has been effected
by Nature, as is the case with true marls; but the brickmaker does
not care so much for these, as without considerable and expensive
artificial assistance they do not often make readily saleable bricks.
The common practice is, briefly, to grind chalk or similar earthy
limestone in the wet state, and then to introduce it to the brick-earth
with which it is thoroughly incorporated; and there are many ways of
doing this, which we shall not attempt to describe now. The object
of adding chalk to the brick-earth is twofold; in the first place
it assists in diminishing the contraction of the brick on drying,
_i.e._, before burning; and secondly, it acts as a flux in the kiln
by combining with the free silica, or the silicates, in the earth.
Undoubtedly the second is, theoretically, its chief function; but its
beneficial effects in that direction are largely marred by insufficient
burning, whereby a large proportion of the chalk is not actively
engaged, as may be seen on examining the majority of malm bricks with
the microscope. Indeed, the eagerness to save fuel, and to turn out the
bricks as rapidly as possible, often leads to the chalk particles being
utterly useless. And, if we may judge from conversations with several
brickmakers, it would seem that the real reason why the limestone is
used at all is unknown to them, except that it produces bricks of a
saleable colour. This question of colour is the all-predominating one
with most malm brickmakers.
We said just now that the fragments of limestone in the raw brick are
reduced to lime on being burnt; some of the latter, however, as may be
anticipated from our subsequent remarks, is engaged in forming a flux
wherever possible in the immediate neighbourhood of such fragments:
it is the “kernel” that is left which becomes “slacked,” and weakens
the brick. The object of utilising the smallest particles only of
the carbonate of lime is thus obvious; and if it were possible to
use ordinary builders’ lime instead of carbonate of lime, the result
would be better still. The difficulty in utilising builders’ lime is,
of course, its certainty of slacking during the preparation of the
brick-earth with which it would have to be thoroughly incorporated.
SELENITE.
The “petrified water” of the brickmaker. It is a crystalline form of
gypsum--a hydrous sulphate of lime, occurring in large quantities in
the commonest clays used in brickmaking. Large and beautiful crystals,
some of them radiating from a central point, are found in the London
Clay, Kimeridge Clay, Oxford Clay, &c. By expelling the water from
selenite, or gypsum, plaster of Paris may be prepared. In the kiln,
therefore, it is important that this constituent be as finely ground as
possible, so as to localise the effects of the anhydrous sulphate on
being moistened subsequently. In hard burnt bricks, no doubt, a great
deal of it is effectively used as a flux to other constituents of the
clay; but in by far the larger quantity of bricks this sulphate is
reduced to fine powdery particles easily picked out as being softer and
lighter in tint than the remaining constituents. The weather-resisting
qualities of the brick are naturally, not improved when much baked
selenite is present; and the colour of the whole is apt to become
variegated--that is, in a fairly soft brick.
DOLOMITE.
Dolomite is, chemically, composed of the carbonates of lime and
magnesia in about equal proportions. It is found as rhombohedral
crystals, the faces of which are often curved; also in granular and
massive conditions. Its prevailing colour is light yellow both in
crystals and rock masses, but, as with most other minerals, impurities
occasionally make it assume other tints, principally red and green.
Carbonate of iron is frequently present, sometimes to such an extent as
to entirely alter the character of the substance. As separate crystals
dolomite has very little interest for us, though rarely it may take
the place of calcite or aragonite in the fossils of brick-earths and
clays. But in its massive condition, as magnesian limestone, it is of
increasing importance to the brickmaker. For many years it has been
utilised in the manufacture of basic bricks, though at the present
moment the market in these materials is attentively looking at the
possibilities of the next mineral to be described.
MAGNESITE.
Magnesite is pure carbonate of magnesia--that is, magnesia = 47.6, and
carbonic acid 52.4 per cent. It usually occurs massive or fibrous,
but sometimes granular, and its fine rhombohedral crystals are well
known. Like dolomite, its prevailing tint is yellow or light brown,
but, when very pure, is as white as snow. It is usually associated
with serpentine rocks. In the kiln it is highly refractory, and
behaves very much in the same way as lime--forming fusible compounds
with silica and silicates. For the higher grades of basic bricks it
is at this moment largely exploited in the few localities where it
occurs in paying quantities. A few years since, investigation to
determine the best basic refractory material was actively prosecuted
in Germany, and magnesia, preheated at the highest white heat, was
awarded the palm. Magnesite, when calcined, yields magnesia, which,
however, still contains the impurities that might have been present in
the raw material. An average percentage composition of the magnesite
of commerce shows it to contain magnesia 45, carbonic acid 50, lime
1.5, protoxide of iron 1.6, the remainder being silica, alumina,
and protoxide of manganese. The presence of silica in magnesite is
an objection, because it is liable to have a fluxing effect at high
temperatures.
Magnesite has been found in paying quantities in California, Styria,
and recently in Greece. In Eubœa, in the last-mentioned country,
the mineral occurs in lodes which, near Krimasi, are worked on two
levels 30 to 40 feet from the top, and dipping at an angle of about
70 degrees. The general average of the lode gives 88 per cent. of
carbonate of magnesia, and the substance is peculiarly suitable for
the manufacture of basic bricks. A novelty with the raw material is
that the proprietors sell either by guaranteed degree, or degree of
analysis, the former being 95 per cent. of pure magnesia, whilst the
latter often gives as much as 97.8 per cent. In inferior grades the
principal increase is in the proportion of silica.
SALT.
Chloride of sodium, or common salt, is present in many natural clays,
especially (in England) in that formation known to geologists as the
Trias, developed largely in Cheshire. The influence of a salt-bearing
bed is, naturally, not confined to the immediate vicinity of the
formation; salt being so readily soluble in water, it comes forth from
the rocks in springs, which, flowing over loams and other similar
absorbent earths, impart a saline character to them. In this manner
otherwise useful earths for brickmaking are rendered absolutely unfit
for the purpose. Salt is one of the most powerful fluxes known; when
mixed even in very small quantities with clay it becomes impossible
to make a good brick of the substance. But we must recur to this
matter at a later period in another connection. The fluxing property
is sometimes taken advantage of by mixing salt with sand in moulding,
or in employing a sand already saline, as when dredged from the sea,
or obtained between tide-marks. A species of glaze is produced on the
brick by the action of such moulding sand.
We may ignore the presence of a number of minerals such as rutile,
augite, and hornblende in brick-earths, as they only exist therein in
such small proportion, and have no appreciable effect in the kiln.