MINERALS: THEIR BEHAVIOUR IN THE KILN.
THE SILICA GROUP.
Silica, the oxide of silicon, is found in brickmaking clays principally
in two conditions when not combined with other substances: in one of
these the free silica may be crystalline, when it is known as _quartz_;
in the other it may be hard, but not crystalline, as _flint_. We may
consider these in order.
QUARTZ.--When pure this mineral is perfectly white and transparent,
like ordinary window glass. It is exceedingly hard, and this property
is of much service as enabling us by the most elementary examination
to distinguish it from certain other minerals, which it is not unlike
at first sight. One of the latter is calcite, a crystalline form of
carbonate of lime, also white and transparent. Quartz and calcite
behave in a very different manner in the kiln, and as we shall see,
they are both rather common constituents of brick-earth. The difference
in hardness may easily be ascertained by the point of a good steel
knife; the steel will not scratch the quartz, but it will, easily, the
calcite.
When it has plenty of room wherein to crystallise, and is not hemmed
in, as it were, by other hard crystalline matter, quartz often forms
beautiful six-sided prisms surmounted by a six-sided pyramid, and,
rarely, pyramids are found at both ends of a prism. There are no
lines, or “planes of cleavage,” to interfere with the transparency,
either in the extremely minute forms of the mineral as investigated
by the microscope, or in the gigantic crystals occasionally found.
Regular crystals of quartz, although by no means rare in Nature, are
seldom met with entire in brick-earths. The most common form of the
mineral is in irregular aggregates with other minerals, as in the rock
granite, which is composed essentially, as previously mentioned, of
quartz, felspar, and mica. We have traced the history of the felspar
on the decomposition of that rock, and it may now be said that on
complete disintegration of the granite a great part of the quartz
present is simply resolved into fragments and dealt with by rain and
other transporting agents. For quartz is practically imperishable; it
is almost proof against the deleterious acids in the atmosphere, which
so readily attack many other common minerals. In dealing with it, all
Nature can do (at least at the surface of the earth) is to carry the
small quartz grains and pieces about from place to place; She can,
and does, in this process, reduce the quartzose fragments by causing
them to continually knock against each other and against other mineral
fragments and masses until the grains and pieces find a resting place;
She may put them in a mill and grind them to powder, but the quartz is
still there.
Another manner in which quartz occurs in Nature is as filling cracks
in rocks, but this is comparatively unimportant for our present
purposes. The purest quartz is known as rock crystal; but by far the
commonest kinds of the mineral are impure; they may contain iron,
schorl (a black needle-like crystal), and many other minerals. One of
the most interesting points about it, and which undoubtedly in certain
cases is of importance to the brick manufacturer as modifying its
melting properties, is the presence of myriads of extremely minute
so-called cavities, generally filled (or nearly filled) by liquids
of different kinds, the precise nature of which is not as well-known
as it might be, though in some instances it has been determined with
tolerable certainty. In some cases these inclusions are so numerous as
to obliterate the transparency of the quartz crystal, causing it to
present a frosted appearance. The fluids in these cavities may have
beautiful little crystals of other minerals, such as salt, floating
about--but it must be remembered that we are referring to something
infinitely little. These slight differences in the constitution of
minerals, however, have their influence in the kiln. For instance,
although the fluid present is usually water, that often contains carbon
dioxide, which acts as a species of flux to the quartz when present in
sufficient quantity.
In reference to the second form of silica present in brick-earths,
flint, that is of precisely the same chemical composition as quartz,
only that it is not crystalline, nor transparent, though thin pieces
of flint are translucent. Flint is by no means as common in Nature
as quartz; it is very hard, but brittle, and breaks with what is
termed a conchoidal fracture, from the fact that the fractured surface
frequently resembles the external appearance of the shell of a bivalve
mollusc. It occurs in a variety of ways; (1) often as hard lumps or
nodules running along in fairly regular layers in limestone rocks such
as chalk, and (2) occasionally filling up cracks or joints in such
rocks. It is hard to describe its origin in a few words, and we shall
not attempt it; all that need be noted is that it is frequently full of
the remains of extinct organisms of small size, which may, or may not,
constitute an impurity depending on the particular organism and its
present condition. When flint contains a fair proportion of iron it is
called chert--an extremely common constituent of brick-earths in some
localities--though that term refers to other rocks, such for instance,
as those made up almost exclusively of the siliceous spicules (hard
parts made of silica) of fossil sponges.
A more or less crystalline kind of silica is found, forming the
skeletons of minute aquatic plants, and these accumulating to some
depth, constitute the basis of such materials as Kieselguhr and the
diatom earth of the Isle of Skye, both of which, especially the former,
are used for making firebricks.
There is very little to be said concerning the behaviour of free
silica--quartz and flint--in the kiln. It is infusible except at
higher temperatures than are employed by the brickmaker. But, as we
have already remarked, the impurities often present in the minerals
form a species of flux which naturally brings them into the range
of fusible substances, though even then the temperature required is
far beyond what is usually attained in the majority of brickyards,
though it might be frequently arrived at in the manufacture of certain
fire-bricks. For all ordinary purposes, therefore, quartz and flint may
be regarded as infusible. In presence of much lime, iron, or similar
substances, however, both of them are readily melted, and it is part of
the science of brickmaking to know exactly how much lime, &c., to add
to yield the best results. Many brick-earths contain large quantities
of the calcareous and ferruginous substances alluded to, and are then
capable of being made into bricks direct, without any addition. But
although such natural brickmaking earths are frequently employed by the
manufacturer, nearly all of them could be made to yield a better brick
by a little artificial mixing. We must keep urging this point; there is
room for great improvement all round.
As with the majority of comparatively refractory substances, the
size of the grains and pieces of quartz and flint makes a difference
in their readiness to become fusible. The larger the grain the more
difficult it is to break down; fusion commences at the outside of a
quartz grain, the centre of which may at the same time be comparatively
unaffected. By arresting the fusing process, the microscope shows the
outside of the grain to have become softened (so much so as to affect
its doubly refracting properties), whilst the innermost parts still
retain their usual optical characters.
MICA.
The different varieties of mica are important as rock-forming minerals,
but they are not as often met with in brick-earths as is generally
supposed, except in insignificant quantity. Some of the purest clays,
however, contain a great deal of mica, derived almost directly from the
destruction of granite. The two commonest varieties of the mineral are
_biotite_ and _muscovite_.
BIOTITE MICA.--This mineral, usually known as ferro-magnesian mica,
is composed of silicates of magnesia, alumina, iron, and alkalies
in variable proportions. It occurs as six-sided plates or irregular
scales, usually of a bronze-black colour. Biotite weathers with
comparative facility, hence the reason why it is not more commonly met
with in brown and other impure clays.
MUSCOVITE MICA.--This is sometimes called potash- or alumino-alkaline
mica, composed of the silicates of alumina, alkalies, iron, and
magnesia; the proportion of silica ranges from 45 to 50 per cent. It
may usually be distinguished at sight from biotite by its silvery white
or light brown colour. When large enough, both the micas mentioned
may be split up into thin plates, muscovite yielding large transparent
sheets. Compared with all other constituents of brick-earth, the micas
are bright and of semi-metallic lustre. Muscovite is more durable
than biotite, and is much more frequently met with in brick-earths,
especially in the sandy varieties.
The influence of mica in the kiln is not of much importance in ordinary
brickmaking; in general its alkaline character renders it fusible,
though a high temperature is necessary at all times to effect that.
In china-clay mica is regarded as a nuisance, and in breaking down
the material it is separated in the washing process by running water,
the mineral collecting in depressions or basins, called “micas.” When
muscovite contains much fluorine, as it frequently does, it is very
undesirable in clays for high-class purposes. At the best of times the
proportion of iron in mica is sufficient to mar the quality of the
otherwise most excellent clays. In the kiln, or porcelain furnace, the
presence of mica (more particularly biotite) is apt to create yellow
and brown specks, or a species of mottling. It is highly satisfactory,
therefore, to note that these little shiny flakes may be easily floated
off by a moderate amount of care in washing, and thus separated from
the other constituents of the clay.
IRON.
Except in regard to white kaolin clays, nearly all earths used in
brickmaking contain more or less iron, which is usually present as
protoxide in many mineral constituents. The colouring matter of clays
is generally iron in some form, and blue clays weather into brown by
the alteration of that mineral. It is unnecessary for us to consider
the various minerals of the iron group; all we need do is to state the
mode of occurrence of iron oxides in clays and earths, to consider a
variety known as iron-pyrite, and the general effects of ferruginous
minerals in the kiln.
Iron may occur in clays simply as a stain, when it is usually not in
large quantity, or it may occur combined with some mineral or minerals
present--as for instance certain felspars and micas. The brown, yellow,
or blue appearance of the clay is due to it. In loam it may be found
also as a species of ochreous earth, and in thin bedded loams (as the
upper part of the Woolwich and Reading series of the London basin)
each layer frequently varies in the proportion of iron present. In the
more arenaceous parts of these loamy deposits, little grains of iron
sometimes make their appearance, as also in certain sands employed in
brickmaking; on careful examination, however, many of these grains are
found to be other mineral substances coated with iron. Certain horizons
in what are known as the Jurassic rocks contain great quantities of
ferruginous matter in little pellets.
Iron, in large proportion, acts as a flux to other constituents when
the brick-earth is subjected to great heat in the kiln, and on that
account must be carefully watched. But, to the average brickmaker,
the ferruginous constituent is far more interesting as a colouring
medium. At a later stage we shall have something to say concerning the
colouring of bricks, &c., but it may now be remarked that red bricks,
in practically all cases, owe their colour to the effects of firing
on iron. It is a great mistake to imagine, however, that a large
percentage of iron in a clay will necessarily produce a good red tint.
In the first place, a great deal depends on the way the clay has been
mixed or prepared; and in the second, the method of burning and the
temperature employed, taken in conjunction with the general composition
of the earth, are all important. This much may be said, however,
that without the iron (or some mineral colouring matter possessing
similar properties in the kiln) a red brick would not result. An even
colour is the effect of thorough and homogeneous incorporation of
the iron with the brick-earth; that may have been brought about by
natural processes, but it is most frequently obtained in the careful
preparation and mixing of the clays. A very essential point is that
the earths must be of such a character as to withstand the requisite
heat in the kiln without becoming vitreous, or twisting or warping.
It must not be forgotten that a certain proportion of the iron, under
great temperatures, may be carried away out of the kiln in union with
other things, in the form of vapour. To successfully treat a raw earth,
so that all these points may be taken into account, and to produce a
thoroughly uniform red brick, that shall not vary in tint from kiln
to kiln, is a matter requiring considerable skill and attention,
though fairly good bricks of that character have been produced by
sheer accident in burning natural earths fairly rich in thoroughly
disseminated iron oxides.
Two minerals commonly met with in earths used for brickmaking
are pyrite and marcasite, both of which are of the same chemical
composition, namely, iron disulphide. We may first consider them
separately, for they are of great importance to the brickmaker.
Iron pyrite occurs as regular cubic crystals, or irregular streaks,
or as nodules or lumps; in clay, the last-mentioned is its commonest
form. It is a good petrifying medium, so that it is frequently
associated with organic remains, as is exemplified in almost any yard
where stiff clay is being worked. The nodules, on being broken open,
ordinarily exhibit a radiating structure of brassy lustre and extremely
beautiful appearance, though often marred by brown iron stains due to
decomposition of the mineral. In the refuse of slates, now so largely
used in several parts of the world for brickmaking, pyrite is most
frequently found as fine cubic crystals of a durable nature.
Marcasite, on the other hand, crystallizes in a different manner (in
the rhombic system of mineralogists), but is chiefly found in fibrous
masses or dirty-brown nodules, the last-mentioned being common in
clays. When bright it is paler in tint than pyrite, though this is not
a constant character. It occurs abundantly in almost all sedimentary
rocks diffused as minute particles, but sometimes in irregular layers.
Sir Archibald Geikie states[5] that this form of the sulphide is
especially characteristic of stratified rocks, and more particularly
of those of Secondary and Tertiary age. That it is not abundant in
Primary rocks is not to be wondered at when we consider its liability
to rapid decomposition; indeed, for it to be preserved at all it must
be well shielded from atmospheric agents by Nature. Exposure even for
a short time to the air causes it to become brown, free sulphuric acid
is produced, which may attack surrounding minerals, sometimes at once
forming sulphates, at other times decomposing aluminous silicates and
dissolving them in considerable quantity. It plays even a larger part
than pyrite as a petrifying medium, at any rate in the younger rocks.
Both pyrite and marcasite are abundant in many other rocks than those
of special interest to the brickmaker; the former, in fact, is almost
universal in its occurrence.
It will be convenient to consider the behaviour of these two minerals
in the kiln together, as the difference between them from that point
of view is practically _nil_. Under the action of the intense heat
met with there, they become partially decomposed; oxide of iron and
basic sulphides of iron remain. When, at a subsequent period, bricks
containing these substances are exposed to the action of the weather,
oxidation takes place, sulphate of iron and sometimes of lime are
formed, which on crystallizing expand with considerable force and split
or crack the brick. From this it is evident that sulphide of iron
in any form is not to be tolerated in brick manufacture, and if the
earth used in the first place contains much, it must be removed in the
preparing process. If permitted to remain, it is impossible to obtain
either a durable, or a good coloured brick.