THE MINERAL CONSTITUTION OF BRICK-EARTHS.
There cannot be any question that the applicability or otherwise, of
an earth for making good bricks, to a large extent depends on the
mineral constitution of that earth. A chemical analysis of a sample
of such earth will tell us how much silica, alumina, lime, iron,
etc., is present therein, and this information is frequently of great
value when given by a scientific chemist; but it does not tell us the
state in which those constituents exist in the earth--an essential
_desideratum_, if we are to understand the scientific aspects of the
question of burning in the kiln. Further, the size of the granules
and particles composing the earth is well worth knowing, as we shall
presently see. It is a great mistake to imagine that all clays are
essentially chemical deposits. The majority of them have been in part
derived from chemical disintegration, it is true; but the resulting
deposits contain so much also that is purely of mechanical origin, that
the behaviour of the whole is materially modified, from a metallurgical
point of view. Take one ingredient, for example--say, silica. That
may exist in a brick-earth in a variety of ways, both in a free and
combined state; but its behaviour in the kiln is largely dependent on
the particular form assumed, not only whether it is free or combined,
but as to how it is combined. In a certain sense, it is very doubtful
whether even in the best-burnt brick much of the raw material becomes
chemically combined; a sort of agglutination takes place locally, as is
clearly shown by the microscope; at such points true fusion undoubtedly
takes place, and there may be actual chemical combination. In the vast
majority of cases, however, such fusion or possible combination is of
an extremely partial and elementary character, whilst it hardly exists
in the average “rubber.” The microscope shows that even in the hardest
burnt brick there still remain enormous quantities of what may be
termed mineral grains, that have by no means succumbed to the burning
process. The edges of the grains may occasionally be seen merging into
the more or less vitreous ground mass in which they are embedded, but
beyond that they appear tolerably fresh, and their action on polarised
light remains unimpaired.
We did not intend to say anything yet concerning the microscopic
structure of bricks--that will be gone into in a subsequent chapter;
but we thought it useful to state the foregoing elementary facts in
order to endeavour to uproot a conviction that seems to be very firmly
grounded--viz., that the chemical composition of a brick-earth imparts
an accurate idea of the possible active agents, on the earth being
subjected to the kiln. As a matter of fact, some of these would-be
agents are imprisoned in the mineral grains and particles that have not
become involved in the partial melting or agglutination of the mass,
and might as well not be present in the earth for any work they may
accomplish either for good or for evil. There is greater probability
of the bulk of these grains and particles being of active service
when they are ground up exceedingly fine; but the clayworker’s idea
of “fineness,” as demonstrated by what passes through an ordinary
clayworking mill, and “fineness” in the sense here intended, are two
totally different things. We mean something that shall render the
particles so small as that they shall only be observable on being
magnified, say, 50 diameters. Hardly any clays used in brickmaking
are in bulk made of such small particles as this; there are a few, of
which the best terra-cotta and porcelain are manufactured, however,
but even these have to be very carefully prepared to exclude grosser
foreign particles. From what we have said, it will be gathered that
the terra-cotta and porcelain manufacturer is at the present time in a
better position to judge of the work done in the kiln or oven than is
the brickmaker. But that is simply a matter of education; the problems
presented to the average brickmaker are rather more complicated than to
the terra-cotta manufacturer, but they may be unravelled on sufficient
application, as we hope to point out.
Even under the most favourable conditions, however--when the particles
composing the mass require a ¼-inch objective for their elucidation--we
find that the best burnt brick is largely made up of them in an
unmelted condition. And we should be very sorry to get rid of them; for
if they disappeared, the stony attributes of the brick would disappear
also, and the general value of the substance would be deteriorated to
such an extent that it would be unsaleable as a building material.
The brick would nearest resemble a form of slag. All we now insist
upon is that in brickmaking a chemical analysis is only useful up to
a certain point, beyond which we must appeal to the microscope to aid
us, and this in conjunction with as perfect a knowledge as possible as
to the behaviour of earths of certain mineral composition when under
the influence of high temperatures. In many instances, the value of the
brick depends almost entirely on incapacity for fusion on the part of a
large proportion of the minerals of which the brick is made. Possibly,
a good all-round brick would be where the bulk of its mineral particles
were infusible at the temperature employed, and when the remainder
were fusible enough to partially run, so as to cement or agglutinate
the infusible particles firmly together. In order to bring about such
conditions artificially, or to arrive at them even approximately, we
must know at least three things, viz.--(1) the nature of the mineral
particles involved in the whole operation; (2) their behaviour
under high temperatures; and (3) a knowledge of certain branches of
metallurgical chemistry. Now, obviously, we cannot undertake to teach
even the spirit of what is involved in these three _desiderata_ in a
small book like this; but we can, and shall, attempt to do something
in that direction, and we must ask the reader’s indulgence to take
for granted observations to be occasionally made, in the inevitable
prospect of our not being able to explain them at sufficient length.
The following are the principal minerals found in clays used in
brickmaking, together with their more important attributes from our
point of view.
KAOLIN.
Pure clay is, theoretically, composed of this mineral alone, but pure
clay does not exist in Nature, except as a mineralogical curiosity.
What is generally called pure clay is a white, or light-grey plastic
material, composed of kaolin with many other substances to a small
degree, from which it frequently has to, as far as possible,
be separated before being put to its highest uses in porcelain
manufacture. Chemically, pure kaolin may be regarded as a hydrous
silicate of alumina, viz.--silica = 46.3, alumina = 39.8, and water
= 13.9. Under the microscope, in reflected light, it is seen to be
made up of extremely minute, thin, six-sided plates, which are said
(doubtfully) to crystallize in the rhombic system; though, when
regarded with the naked eye, one would not suppose that it possessed
a crystalline structure, as it appears to be an earthy, unctuous
substance. It is commonly mixed with grains and small crystals and
fragments of quartz, which mineral will presently be described. Being
derived from the decomposition of felspars, the microscope reveals
the fact that in addition to the six-sided plates alluded to, a great
deal of opaque matter, as particles of mud, occurs in the substance
universally known as kaolin. It is very difficult to satisfactorily
state what this mud is; micro-chemically, its general character may
be brought out. There is no doubt, however, that in converting the
kaolin into china-ware, these particles are more active than the
minute kaolin crystals in uniting with other substances to form a
species of flux. The subject has been investigated to a very limited
extent, but from the foregoing observations it will be seen that the
proportion of amorphous mud particles to the minute crystals must be
an important factor in determining the nature of the fluxing material,
and of the quantity of this latter to be used. Correlatively, the
fusing point can be determined in the same manner. For, in itself,
kaolin is an infusible mineral, and before it can be made use of for
brickmaking, terra-cotta, or any kindred purpose, it must be rendered
artificially fusible by the addition of a fluxing substance. When,
therefore, we learn that kaolin is being used for these purposes, we
know, if used direct as it comes from the pit, that it must be impure
from a mineralogical standpoint, or that it is being mixed with other
substances. We say that kaolin is infusible (refractory); we mean at
any temperature used in the industrial arts, including brickmaking.
With the recent improvements in the electric furnace, the temperature
generated is so high that practically any mineral substance may be
melted; it is hard to speak of anything being infusible.
But the mineral matter called kaolin in ordinary clays, such as the
brown and blue London Clay, the Oxford Clay, “brick-earths,” etc.,
has very little in common with the more or less pure china-clay. The
microscope shows that in the vast majority of such clays scales of true
kaolin are few and far between, that opaque mud particles are more
frequent, and, above all, that pieces of highly decomposed felspar
(called “kaolinised” matter) are present. Eliminating all other and
foreign substances from the clay, the whole of what would commonly be
called kaolin and kaolinised matter, taken together, is of very varied
chemical composition, and might, indeed, be fusible in the ordinary
sense of that term. From this, the reader will perceive that the term
kaolin is very ambiguous and altogether too wide in its meaning. We
think it highly desirable, therefore, to describe kaolin as a true
mineral and not as a rock, reserving the term for the crystalline
plates. The mud particles referred to we may call “kaolinised
particles;” and the highly decomposed felspar “kaolinised matter.” To
sum up the relative fusibility of these substances, _per se_, we should
say that (1) kaolin crystals are practically infusible; (2) kaolinised
particles are either fusible, partly fusible, or infusible, depending
on the actual nature of the particles; and (3) that kaolinised matter
may be difficultly fusible or infusible. A mixture of (1) and (2) may
not be fusible, and could not be unless a great proportion of (2) of a
fusible character, so as to form a flux, were present. The reasons for
this will appear in considering the different kinds of felspar, next to
be described.
FELSPAR.
This mineral, a very common constituent of nearly all clays and
brick-earths is very variable in character, but may be separated into a
number of mineral species, each of which possesses a definite structure
and a more or less constant chemical composition. To show the range
of variation, the following kinds of felspar, with their chemical
composition, may be quoted:--[3]
_Chemical Composition of Felspars._
-------------+---------+----------+---------+-------+-------
| Silica. | Alumina. | Potash. | Soda. | Lime.
+---------+----------+---------+-------+-------
Orthoclase | 64.6 | 18.5 | 16.9 | |
Albite | 68.6 | 19.5 | | 11.8 |
Oligoclase | 63.7 | 23.9 | 1.20 | 8.1 | 2.0
Labradorite | 52.9 | 30.3 | | 4.5 | 12.3
Anorthite | 43.0 | 36.8 | | | 20.1
-------------+---------+----------+---------+-------+-------
Orthoclase felspar, in addition to the above, frequently has small
proportions of lime, iron, magnesia and soda. Amongst other things it
is an essential constituent of granite, and on the decomposition of
that rock is the first mineral to become affected. When attacked in
the open air by rain and the ordinary agents of denudation, granite
ultimately gives way by the dissolution of the felspar, and on being
removed, the felspathic matter may accumulate in convenient situations
to form kaolin. If we now compare the chemical constitution of
orthoclase felspar with that of kaolin as previously given, we notice
that the potash has disappeared in the decomposing process; it has
been dissolved and taken away by rivulets, and the like, or washed
by rain direct into the sea. We also observe that there has been a
re-distribution, so to speak, of the relative proportions of silica and
alumina--following well-known laws.
Of the remaining felspars the commonest for our purposes is oligoclase,
a mineral found in nearly all British “granites” in a greater or less
degree. That contains a higher percentage of alumina than orthoclase,
and there is a fair proportion of soda and little lime, but much
less potash. The lime-soda felspar, labradorite, and its near ally,
anorthite, are not often met with in a recognisable form in clays.
If present, they are generally as “kaolinised matter,” too highly
decomposed to exhibit their characteristic optical properties.
It is pretty generally stated, and too often assumed by some, that pure
china-clay is derived from the direct decomposition of rocks containing
“orthoclase” felspar. Yet, this cannot really be so, if we reflect on
the mineral composition of many of the rocks, which, obviously, have
yielded the china-clays in question. Take the china-clays of Devon and
Cornwall; they have undoubtedly been derived from the “granites” of
those counties. To some extent, as previously remarked, the orthoclase
is attacked, and provides the material of which china-clay is made. But
in the “West of England,” we have yet to learn that some of the other
felspars are not also involved in the process. If we examine a fresh
piece of granite from the flanks of Dartmoor, or from the neighbourhood
of Liskeard, or St. Austell, we find no difficulty in recognising a
fair proportion of triclinic felspar (one or more of those mentioned
in the table except orthoclase) in it. There is a difference in
the composition (and therefore the commercial applicability) of a
china-clay derived from a rock containing orthoclase alone, and one
from a rock having orthoclase and one or more triclinic felspars
in addition. The latter minerals are more easily decomposed than
orthoclase, especially the lime and lime-soda varieties. We should
not have raised this point only that, by reason of the granites
being to some extent mechanically as well as chemically decomposed,
a large proportion of “kaolinised particles” and “kaolinised matter”
is introduced into certain china-clays, which render them different
in their behaviour under intense heat from those china-clays in which
orthoclase alone has been principally concerned. In other words,
great practical advantages accrue from an accurate knowledge of the
constitution and origin of the china-clays in question. Two clays of
the same chemical composition often behave in a different manner in the
kiln; the cause of this is frequently to be found in the prevalence of
“mechanical fragments” of felspar in one of the clays; and the absence
of these, but the presence of “kaolinised particles” of the same
chemical composition, in the other.
Another point to which we may draw attention is the erroneous
supposition that granites which have yielded china-clay have in all
instances been reduced to the condition in which we now find them by
the action of atmospheric agents of denudation alone. Granites, as a
matter of fact, yield very slowly to the action of the atmosphere,
and taken as a whole no building stone is as durable as they. How
comes it, then, that they have decomposed to such an extent as to
have formed extensive deposits of china-clay in a very short space of
time, geologically? We think the answer is to be sought, at any rate
in some instances, in the alteration the rock as a whole has undergone
in certain situations, whereby it became more easily decomposable.
Take the rotten china-stone of the neighbourhood of St. Austell, for
example. In that material we clearly see a stone from which the “life”
has been sapped, and instead of a bright, sparkling, porphyritic
granite, as it once was, we now notice only a ghost of its former self.
The large orthoclase felspars may be seen in it as skeletons, the
mica is reduced to mere iron-stains (when present at all), whilst the
quartz is also slightly affected. This altered and comparatively rotten
material (although sometimes hard enough to be used as building stone)
extends to an enormous depth from the surface; it has not been bottomed
in some parts of the district. Such an extensive transformation could
not possibly be due to ordinary agents of denudation which do their
work at and near the surface of the rock only. It seems to arise from
an enormous regional alteration, acting underground to an unfathomable
depth, and which may not be unconnected with the mineral veins so
common in, and in the immediate vicinity of the workings.[4]
Yet another thing to be remembered is that, under certain conditions,
as near St. Austell, china-clay has been formed _in situ_, and has
therefore not been deposited by the action of running water, as have
the majority of china and other clays. Mr. Collins remarks that this
china-clay is very irregular in its occurrence. It seems to be formed
of various granite masses decomposed in place; it often occupies
considerable surface areas, and extends to a depth unknown. He remarks
that at Beam mine, and also at Rocks mine, both near St. Austell,
china-clay was found to a depth considerably exceeding 60 fathoms from
the surface. This china-clay, in its natural condition, is very much
the same as china-stone; but the decomposition has proceeded further,
the felspar being completely changed into clay; and nothing more is
necessary for extracting the clay than the disintegration of the whole
mass by a stream of water directed upon it, when the clay is carried
away in suspension and collected at convenient spots. Thus there is
every gradation between the true crystalline orthoclase and triclinic
felspars, through china-stone into china-clay formed _in situ_, so
into china-clay deposited from water by natural or artificial means,
and into a pure clay containing a large proportion of kaolin crystals,
“kaolinised particles” and “kaolinised matter.” But although we can
state that much, a great deal yet remains to be done in connecting
mineral structure with chemical composition of the purer clays, and
in defining the various grades scientifically, in order that full
advantage may be derived from them in a commercial sense.