THE MICRO-STRUCTURE OF BRICKS.
The reader may be tempted to enquire, What is the use of knowing the
micro-structure of a brick? We have anticipated the question to some
extent in dealing with the structure of brick-earths, but it may be
well to enlarge upon it here. In the first place, the study of the
minute structure enables the manufacturer to ascertain whether the
brick is thoroughly and homogeneously burnt. It tells him whether the
materials mixed together in the earlier stages of manufacture were
thoroughly incorporated or not, whereby, if need be, he can improve
that part of the process. In carefully examining what the average
manufacturer would call a well-burnt brick, the microscope assists us
in perceiving that it is often anything but well burnt, small local
patches--“tears”--of semi-vitrified matter being observed, which
should not exist, of course, in a perfectly homogeneous brick. And if
the brick is not homogeneous, it suffers in respect of its strength
as a whole, and in the majority of cases its colour is not uniform.
To arrive at the cause of this lack of uniformity is to indicate the
manner in which the manufacture of the brick may be improved, and the
microscope often enables us to arrive at a satisfactory solution of the
problem.
From a chemical standpoint we know that a high percentage of iron in
the average brick-earth is not conducive to the production of a good
brick. In the same manner by “rule of thumb” we learn that a high
percentage of lime prevents the manufacture of the raw material into
a fire-brick, unless, indeed, we are making basic bricks. The chemist
tells us also of the respective values of potash and soda. Too much
iron will cause the brick to “run”; salt has a similar effect; but
beyond this the chemist cannot go, except that in the broad sense he
explains what unions take place to produce such results.
The microscope, on the other hand, enables one to see exactly what
has taken place; the deleterious constituents are detected at their
work, and careful chemical investigation teaches us what to add to the
brick-earth to neutralise the effects observed; for it is only from
its effects that the artificial constitution of the brick-earth can be
properly regulated.
The same instrument is extremely useful in all questions concerning
the relations subsisting between a brick and the glaze upon it, the
cause and prevention of the cracking of the latter, and its general
quality from a physical aspect. And, speaking of cracks, we may
again draw attention to the influence these have on the strength and
durability of the brick: many of these minute fissures cannot be seen
by the naked eye. In a similar way can the microscope be made use of
in the manufacture of terra-cotta and faïence. The cracking of glazes
is one of the most troublesome features the high-class brick and
tile manufacturer has to deal with. If the character of the surface
of the brick is not suitable for “taking” the glaze, the maker knows
in a moment; the trouble is where the glaze takes readily and then,
some time after the operation is finished, it becomes covered with
“spider-web” cracks, unsightly and considerably detracting from the
value of the brick. The cause of the cracking is commonly attributed
to the composition of the glaze, and the manner in which the latter is
allowed to cool, and no doubt a great deal is due on both those heads.
At the same time, we know of many instances where the same glaze being
used under similar conditions on two different surfaces of bricks made
from one and the same brick-earth, the glaze cracks in the one case,
and hardly ever in the other. The direction of the cracks points to
their origin, and the character of the surface is brought in guilty.
And yet the average manufacturer would not detect any difference in the
quality of the surface--he could not, without a good lens or low power
objective, perceive the slightest discrepancy.
The ordinary glaze behaves very much like Canada balsam with
reference to surfaces on which it is laid, and something akin to what
petrologists call “perlitic” cracks is produced in the glaze. We can
make these cracks, and imitate the structure artificially, by suitably
distributing the Canada balsam over the surface of a piece of ground
glass, and in other ways. That direct relationship exists between
the cracks and the grain of the surface on which the preparation is
laid, is certain, for we may vary the distribution of the cracks by
varying the grain of the surface. An intelligent appreciation of the
disposition of cracks in glazes should be the means of preventing them
altogether, and not only with bricks, but with faïence and vitrified
work generally, the study may be best carried on by aid of the
microscope.
The microscope, also, may be made use of in identifying bricks in
case of dispute, though its applications in this respect are not so
important as in dealing with building stones.
Questions of durability may frequently be decided on appeal to that
instrument. Take a case in which a brick is known to contain a rather
high percentage of lime: if the lime were in a combined state, the
quality of the brick would not be materially affected; but assuming
it were not so employed, it is possible that in a short space of time
the brick would be thoroughly decomposed by atmospheric agencies. The
microscope tells us at a glance the state in which that and other
ingredients exist, in a well-burnt brick. We draw the line at bricks
intended for the “jerry” builder; they may well be left to take care of
themselves; we allude only to high-class productions in which science
may be some aid to the manufacturer.
And now as to the microscope--for we do not use an ordinary one in
such investigations. The best kinds of microscope are those used
by petrologists in the study of the minute structure of rocks and
minerals. The reader will find these fully described in works specially
devoted to the subject,[13] but we may say a few words thereon.
A common form of “Student’s” petrological microscope, as manufactured
by Swift of London, may be described as follows:--
_Eye Pieces and Objectives._--These need not be expensive, clear
definition being the principal object to aim at; the objectives should
be of low power, 2-inch, 1-inch and ½-inch objectives being plenty
for the purpose. Unless the reader desires to follow the subject
from a purely petrological point of view, to study the development of
trichites, globulites, skeleton crystals, etc., in vitrified bricks,
in such places as these latter have cooled from igneous fusion,
there is no occasion to resort to higher powers. We are far from
saying that the brickmaker of the present day would not derive any
advantage from studying this subject in its higher aspects, for the
origin of crystallization appeals strongly to the imaginative mind,
and is one of the most remarkable problems that Nature offers for our
investigation. But in an elementary treatise of this kind we cannot go
into the matter; and, as previously remarked, low power objectives are
sufficient for our present purpose. The eye-pieces should be fitted
with cross-wires, the use of which will presently be explained.
_The Stage._--In the instrument we are now describing this is
circular with a hole in the middle, and is so arranged as to revolve
horizontally on a collar about an axis, the centre of which comes
exactly underneath the centre of the objective. In other words, a
straight line drawn through the eye-piece down the centre of the barrel
of the microscope, and passing through the objective passes through
that axis. To assist in more accurately centreing than is otherwise
possible (depending on the lenses) with this cheap form of instrument,
a collar with adjustable screws is ordinarily affixed to the lower
part of the barrel of the microscope. The stage, with suitable clips
to hold the object to be examined, is graduated so that on its being
revolved it is easy to ascertain the number of degrees, at any period
of the revolution, through which it has been turned. Thus, it will be
observed that the object revolves with the stage. A pointer is placed
in a suitable position on the frame of the microscope to facilitate the
observation.
_The Polariscope._--This is an indispensable adjunct, for determinative
purposes it is often necessary to observe the object in polarised
light. Briefly, the polariscope consists of two parts--the analyser,
placed in the barrel of the microscope above the objective, and the
polariser, arranged underneath the revolving stage. The analyser is so
fitted that it may be shot in and out of the barrel in order that the
polariser alone may be used, or the latter may be removed, leaving only
the analyser in position, or both may be removed to enable the object
to be examined in ordinary light, either reflected or transmitted. The
lower nicol[14] is made to revolve, and the collar in which it is fixed
is broadly graduated and furnished with a pointer.
_Reflector._--An ordinary reversible and adjustable reflector is
arranged beneath all.
_Accessories._--For the more accurate determination of minerals, a
quartz wedge, a quartz plate, etc., are used by the petrologist, but
the description of these is beyond the scope of the present work.
For examination in reflected light it is highly desirable to have a
“bull’s-eye” condenser.
An ordinary microscope with a revolving stage may be readily converted
to petrological purposes, though it is better to have a special
instrument.
* * * * *
The object to be examined may be in the form of (_a_) a fragment of the
brick, or (_b_) a very thin slice of the same.
The fragment may be securely clipped and held in position on the
stage, the “bull’s-eye” condenser being brought into use to throw
a strong light on the part immediately under the objective. The
polarising apparatus is no use for this, and may be thrown out of
gear. A very low power should be employed. The observation may be
directed towards ascertaining how far the fragments composing the
brick are agglutinated, and their size may be noted. Anything like a
discolouration should be specially observed, and a minute description
jotted down. In bricks that have not been burnt very hard, and in
those that have merely been baked, we shall often be able to detect
particles of mineral matter which further investigation, after the
manner presently to be described, shows are opaque. Different forms
of iron, iron pyrite, fragments of clay that have merely been dried
in the process of baking, and minute pieces of chalk (now converted
into lime) are amongst the most prominent opaque substances met with
in common bricks. These may generally be differentiated and determined
at sight, and bricks thus composed are never of good quality, though
the ingredients have been ground very fine, and there may be nothing
superficially to find fault with. Their bad qualities are usually
brought out in the weathering. A great deal may, therefore, be learned
from a careful examination of fragments in this manner.
In regard to the examination of very thin slices, that is in the
majority of instances the most instructive, and, if we may say so, the
most interesting method of investigation, though it must always go hand
in hand with the other. The slice of the brick is so thin that the
bulk of the constituents is rendered transparent, or semi-transparent.
The preparation of such slices[15] is not difficult, but demands
some experience; those who have neither the time nor patience to
make them will find it convenient to send the fragments of brick to
Damon, of Weymouth, or some other first-class dealer in geological
and mineralogical specimens. The price charged, per slide, is usually
1s. 6d. At the same time, the student will find it eminently to his
advantage to prepare the slices himself. In the process he will learn
much that escapes attention when the work is done by another.
The thin slice mounted on a slip of glass is placed on the stage of
the microscope and firmly clipped, as with the fragment. The reflector
is brought into position, and a beam of light thrown through the
slice--the thin section is now being examined in transmitted light. At
first it will be convenient to study it with the polariser and analyser
thrown out of position. A certain proportion of the constituents is
found to be opaque, and should be examined in reflected light, as
above described. The remainder are more or less transparent, and some
of the grains will, possibly, be coloured. We notice the way in which
the whole of the fragments are bound together--say, by some opaque
mineral such as iron--or whether they seem to be partially or wholly
fused together. In the case of a vitrified brick, the latter phenomenon
is most usual, and we shall find that although crystalline fragments
have been melted, or partially fused, there is commonly a centre or
nucleus of each fragment in its original condition remaining, which
passes through insensible gradations from the crystalline to the
non-crystalline, or amorphous state. This latter circumstance may
be ascertained by using the polariscope. Ignoring the opaque matter
adverted to, we shall then see that what was transparent in ordinary
light appears, for the most part, to be opaque in polarised light.
Those portions which still let the light through are truly crystalline,
and by revolving the stage we notice that they frequently change
tint, becoming alternately light and dark. In that brick where the
particles are agglutinated by igneous fusion, we shall observe the
light decreasing in intensity from the crystalline portion (forming the
nucleus, as it were, of each particle) outwards, and where the crystal
fragment has been melted, so as to become fused to its neighbour, the
periphery, or rather what was originally the boundary of the fragment,
is quite dark. Polarised light cannot pass through non-crystalline
matter, and in being melted that portion of the crystal fragment
had passed from the crystalline to the non-crystalline stage. It is
very easy, therefore, to determine how far the fragments composing
a vitrified brick have been melted down and fused together; but to
observe the phenomena under the most favourable conditions, the brick
must be thoroughly well-burnt, and the section taken, by preference,
from near the outside surface of the brick.
In some instances, partial fusion is so well exemplified (especially
in bricks from fairly pure china clay), and the brick, after being
burnt, has been permitted to cool so slowly, that devitrification
has set in, when we are presented with aggregates of crystallites
closely resembling the “felspathic matter” of petrologists. That is
a circumstance which the maker should note well, for he has burnt
the brick to the best advantage, and it is not then so brittle as it
might have been had more “glass” made its appearance in the section.
Prolonged heat, just above the agglutinating point, has accomplished
this, and the microscope here clearly shows the advantage of allowing
the kiln to cool slowly, and to permit the lapse of several days in the
operation.