Transcriber’s Note:
In this Plain Text version of the eBook, italic text is indicated by
_underscores_ and chemical compounds on pages 76–77 are shown without
subscripts, e.g., H2O. Other Notes will be found at the end of this
eBook.
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THE SCIENCE OF BRICKMAKING
THE SCIENCE
OF
BRICKMAKING:
WITH SOME ACCOUNT OF
THE STRUCTURE AND PHYSICAL
PROPERTIES OF BRICKS.
BY
GEORGE F. HARRIS, F.G.S.,
Membre de la Société Belge de Géologie, Paléontologie et
Hydrologie; Lecturer on Geology, The Practical Applications of
Geology, and Mineralogy, in the Birkbeck Institution, London;
etc., etc., etc.
LONDON:
H. GREVILLE MONTGOMERY,
43, ESSEX STREET, STRAND.
1897.
[ALL RIGHTS RESERVED.]
PRINTED AT THE VICTORIA PRINTING WORKS,
STANSTEAD ROAD FOREST HILL S.E.
PREFACE.
The substance of this little work was first published as a series of
articles in the _British Clayworker_, in 1895–96, and I am indebted
to the courtesy of the Proprietor of that Journal for permission to
reproduce them.
An attempt is here made to furnish some information of an elementary
character on a special branch of technical education which has been
seriously neglected in this country. But the reader will understand
that it is only an elementary treatise. Its publication in serial form,
where each article must, more or less, be complete in itself, has to a
large extent determined the method of handling the subject, and I am
fully cognisant of the drawbacks of the work in that respect.
At the same time, it is hoped that the book will be useful to the more
advanced class of brickmakers and clayworkers generally, many of whom
have expressed a desire to see the articles in this form.
GEO. F. HARRIS.
Birkbeck Institution,
Bream’s Buildings, Chancery Lane, E.C.
1st February, 1897.
CONTENTS.
CHAPTER. PAGE.
I. FLUVIATILE BRICK-EARTHS 1
II. LACUSTRINE AND FLUVIATILE BRICK-EARTHS 17
III. MARINE BRICK-EARTHS 22
IV. THE MINERAL CONSTITUTION OF BRICK-EARTHS 28
V. MINERALS: THEIR BEHAVIOUR IN THE KILN 39
VI. MINERALS: THEIR BEHAVIOUR IN THE KILN (_Contd_) 49
VII. THE CHEMISTRY OF BRICK-EARTHS 58
VIII. THE CHEMISTRY OF BRICK-EARTHS (_Continued_) 75
IX. DRYING AND BURNING 94
X. THE DURABILITY OF BRICKS 103
XI. THE MICRO-STRUCTURE OF BRICKS 118
XII. THE MICRO-STRUCTURE OF BRICKS (_Continued_) 128
XIII. ABSORPTION 132
XIV. STRENGTH OF BRICKS 136
XV. ABRASION, SPECIFIC GRAVITY 146
THE SCIENCE
OF
BRICKMAKING.
CHAPTER I.
FLUVIATILE BRICK-EARTHS.
Let us go to Crayford, near Erith, or to Ilford, in Essex, and take a
superficial glance at some of the brickyards found at those places; in
particular, let us ascertain a little concerning the earths employed.
We find in one brickyard a series of stiff brown or bluish clays,
interstratified between sandy clays or “loams,” with thin brownish
partings. In another, the loam will become very sandy, and the earth
light, with a slight greenish tinge. A third has thin pebble or gravel
beds developed, or small stones sparingly scattered in the clays and
loams on certain horizons. A fourth contains, in addition to some of
the beds above described, a lime-clay or marl[1] with small pellets
of chalk. It will be noticed on entering the yards that these various
horizons, or beds, as they are conveniently termed, are disposed in
regular lines or layers, more or less horizontal; in other words, the
beds are “stratified.” On the face of the working being dug into, it
will often be found that these thin beds, a few inches or feet each
in thickness, vary in depth, and frequently disappear altogether, or
“thin out,” whilst, on the other hand, a bed only a few inches thick
may become as many feet, and new beds are found to be developed. A
pure sand may in like manner become loamy on being dug into, and, on
being further developed, pass insensibly into a stiff clay. And many
other changes take place into which we will not enquire at the moment.
Suffice it to say, that in such brickyards the strata are very locally
developed, though it follows from the circumstance of their existence
for so many years, that what changes have taken place, to some extent
compensate each other, so that the material is still an earth suitable
for making bricks. Again, certain beds of much economic value may be
more persistent than others, both in character and development. Having
noticed all these things, we perceive a couple of men digging with care
into the brick-earth, and presently they bring some objects to us which
we have no difficulty in recognizing as the remains of the lower jaw
of an elephant’s skull. Returning to the spot where they were exhumed,
the upper jaw and tusks also are uncovered. To the clay workers these
things are well known; in their time they have found many similar
skulls of animals in the brick-earth; but they know next to nothing
concerning them, or how they got there. Another expedition to the same
localities may yield the remains of rhinoceros, the musk sheep, grizzly
bear, hippopotamus, reindeer, and many other animals. A fine series of
the remains of these, obtained from the brick-earths of the valley of
the Thames at several points, is exhibited in the geological department
of the British Museum (Natural History), South Kensington, and more
or less complete skeletons obtained from the same source may be found
in other, and local museums. One of the most interesting points
concerning these remains is that so many of the animals represented
in the brick-earths are of extinct species--there are no species
included in this latter category of precisely similar kinds to animals
now living, Thus the elephant was different to modern elephants; we
know, from remains found elsewhere, that it was clothed with wool. The
same also with the rhinoceros. The reindeer no longer lives in this
country, being confined to northerly latitudes; whilst the musk sheep
is a denizen of the Arctic regions, and the hippopotamus is restricted
to the tropical or sub-tropical climes. But we might continue for a
long time expatiating on the character of the very numerous mammalian
remains found in our common brick-earths. What a curious assemblage of
animals! It is wonderful to contemplate the time when the reindeer and
musk sheep lived side by side with the elephant and rhinoceros on the
site whereon London now stands.
That is not all, however. In the same brick-earths and gravels, tools
(flint implements), fashioned by the hands of man, are also frequently
discovered, and in one place at Crayford, the spot whereon flint
implements were manufactured has been ’lighted upon. Each flake chipped
off has been collected and pieced together, and the shape of the
original flint has thus been determined. Clearly, from this evidence,
the earth from which millions of bricks have been made has formed
since primæval man (and with him the animals alluded to) inhabited
the valleys of the Thames and its tributaries. It is interesting, too,
to reflect on the circumstance that the materials upon which many of
these facts of great philosophical significance are based, have been
collected through the instrumentality of the workmen. Palæontologists
are proud to acknowledge that; their debt of gratitude to the
intelligent and persevering men can never be fully repaid.
Pursuing the matter still further, we discover a quantity of shells,
blanched and very frail--they seem to be deprived of much of their
original substance, so to speak; their entombment in the brick-earth
has taken all the natural colour out of them. Studying these, we soon
ascertain that they belong to land snails and mollusca which inhabit
fresh water. Living representatives of the same species are, with few
exceptions, found in Kent and Essex.
Putting all this evidence together, we come to the conclusion that the
brick-earths alluded to accumulated in the channel of a river; they are
found above the present level of the Thames, for the simple reason that
they have been elevated into that position partly by earth movements
and partly by the channel of the river being cut deeper by natural
causes, of which abundant proof will be adduced. The snails were washed
down from the land by freshets, or caught by the river in flood; the
elephant, rhinoceros, hippopotamus, and musk sheep were overcome,
perhaps, by floods, drowned, and subsequently covered up by the mud
of the swollen current. We can imagine that the savage hunter, in his
canoe, attacking the animals swimming in the river, loses his tomahawk,
or his frail bark may be upset, and he is striving to gain the shore
for dear life. Or, it may be winter time; the river is frozen over,
and he is cutting a hole in the ice with his flint chisel wherein to
fish; his hands are benumbed, and he loses his grasp of the tool; it
falls into the water, to be discovered in the brick-earth by one of our
intelligent friends. Truly, the revelations of the brickyard enable us
to construct a picture of one of the most interesting phases of the
past history of the Earth.
We have given an outline of the evidence upon which certain
brick-earths in the Thames valley are proved to be of fresh-water
origin--to have accumulated in quiet reaches of the river, and at
other convenient spots along its course--but we have used that as
an illustration only; phenomena of precisely the same character are
manifested in nearly all river valleys in this country, especially
those in which the bottom of the valley has only a slight gradient down
to the sea.
The brickmaker may ask: What is the practical bearing of these
observations? What difference does it make to us whether the earths
we use are of fresh-water, lacustrine, or marine origin? All the
difference in the world, from the points of view of structure,
composition and suitability of the earths, and especially of their
distribution over the face of the country. How much easier it is to
value an extensive brickmaking property when you feel perfectly certain
as to whether the face of earth as shown in the pit will die out on
being worked into for a few yards, or whether it will be persistent
throughout the whole of the property to be valued. Better still, when
your knowledge enables you to state definitely whether the quality of
earth now being worked in a pit is likely to continue the same, or
whether it will get better, or worse. The disposition of the earths, in
some instances, is so clear that no brickmaker with an eye to business
could fail to trace their extent over his property. But this is not
often the case, for the earths being used are for the most part covered
by a superficial mantle, or overburden, which masks the true character
of the beds beneath. A very slight acquaintance with the principles of
geology overcomes these difficulties as a rule; and we are about to
lay down the elements of these principles, so far as they apply to the
immediate subject in hand. By seeing why it is the beds of brick-earth
vary in structure and composition we shall be in a better position to
make forecasts of their general behaviour.
In regard to fluviatile deposits, it goes without saying that every
river flows along a general depression more or less pronounced, called
a valley, and that this valley is bounded physiographically by a ridge,
except in the region of its entrance to the sea or lake, or, if a
tributary, of its joining a main stream. The watershed of a river and
its tributaries includes and comprises what is technically termed the
“river basin.” All valleys are, in the end, the result of denudation
taking place in them. In other words, on the birth of a valley a very
slight depression or other physical feature determined its general
direction for the time being, but the little rivulet once being
formed proceeded, through the medium of the “agents of denudation,”
to carve out its channel more clearly, and eventually to eat into the
rocks over which it flowed, until a large valley had been formed. The
“agents of denudation” in river valleys may be summarised as rain,
snow, ice, heat, and wind, and their general effect on rocks is called
“weathering.” We need not stop to enquire into the precise methods
adopted by these agents in accomplishing their work; it suffices at
present to say that the rock destroyed or broken up is removed by the
running water constituting the rivulet, stream, or river, as the case
may be. Some of the material is chemically dissolved in the water,
whilst another and larger proportion is taken away in suspension, or
is said to be dealt with mechanically by the river. The agents of
denudation do their work very slowly, as a rule, and yet no one who
stands on London Bridge and contemplates the swollen stream laden with
muddy sediment passing under it after a few days’ rain, could say that
they are not doing their duty effectually. To give some idea of the
quantity of sand, gravel, and mud removed from the land through the
medium of rivers, we may remark that the Mississippi discharges into
the Gulf of Mexico annually a mass of earthy matter equal to a prism
268 feet in height with a base area of one square mile. In regard to
denudation by chemical means we may say that the Thames carries past
Kingston 19 grains of mineral salts in every gallon of water, or a
total of 1,502 tons every 24 hours, or 548,230 tons every year; this is
not taking into account the muddy sediment, gravel, &c., annually sent
down to the Nore, which must be infinitely greater in quantity.
Enough has now been said to show that stupendous quantities of mineral
matter derived from the destruction of the land are sent down to the
sea by natural agencies, and we may at once state that a very large
proportion of this, which finds a resting-place in and about the
mouths of the rivers and their backwaters, is material suitable for
brickmaking at places where it is obtainable. Enormous quantities of
muddy sediment, sand and gravel, however, never reach as far as the
sea with great rivers. This material is arrested at sundry convenient
spots, and, as a rule, forms excellent brick-earth.
[Illustration: Fig. 1.--Formation of Brick-earth in a river valley.]
See Fig. 1, which represents part of a river of slow current with
three bends, A, B, C. The water is flowing in the direction indicated
by the arrows; and it is part of the mechanics of such a river that in
rounding a bend its velocity is greatest (and its eroding power also)
at the outer portions of the curves approximately indicated by the
arrow points. The water “wheels round” such portions of the curves,
and “marks time” at the points _x_ _x_ _x_, and, indeed, its progress
may be altogether arrested for a time at the latter places. Now the
transporting power of a river is its velocity, and, naturally, the
greater the velocity, the coarser will be the fragments or particles
of rock carried along. It is interesting in this connection to quote
the figures calculated by Mr. David Stephenson, giving the power of
transport of different velocities of river currents:--
Ins. per Mile per
second. hour.
3 = 0.170 will just begin to work on fine clay.
6 = 0.340 will lift fine sand.
8 = 0.4545 sand as coarse as linseed.
12 = 0.6819 will sweep along fine gravel.
24 = 1.3638 will roll along rounded pebbles 1 inch in diameter.
36 = 2.045 will sweep along slippery angular stones of the
size of an egg.
These figures[2] have greater interest for us than in the connection
at present used, as will be noticed hereafter. We have seen that in
rounding the bends (Fig. 1) A, B, C, different portions of the stream
possess different velocities. We know it is charged with sediment and
stones all the time. The tendency, therefore, will be for the large
stones and coarse detritus to go round the outer side of the bend, to
bombard the banks near the points shown by the arrows, and to erode the
channel deepest in those situations; whilst a goodly proportion of the
fine muddy sediment will find its way to the quiet and shallow parts
near _x x x_, and in course of time become deposited there, whilst the
main course of the stream is eating its way and shifting its course
as indicated by the dotted lines _a a_. This action proceeds, it may
be, until the course of the river becomes straighter, as shown by the
dotted lines _b b_, when the whole of the loop B D is abandoned, its
former course there being evidenced by pools of water and irregular
heaps of gravel, sand and mud. The reader will now see that the whole
of the land marked _x x x_ has been formed of sediment brought down
by the river, and in the majority of cases such fine silt and sandy
mud or clay is specially suitable for brickmaking--many of our largest
brickmakers obtain their material from such a source. It should be
observed that the valley, as shown between the lines _v v_, may be two
or three miles in width, and it is often much more, so that the actual
amount of land made by the river at _x x x_ may be several thousands of
acres in extent.
Now as to the practical application of the foregoing observations. In
the first place, it will be seen that such deposits of brick-earth as
are made in this manner cannot be very thick, their total thickness
perhaps, resting on the bottom of the valley, not being more than 20
feet, and it is frequently much less. The next thing to be noticed is
that they must be very variable in character, a bed changing perhaps
every 100 feet or so horizontally, and more often every few feet.
Individual beds must of necessity be very irregularly developed under
the circumstances. The velocity of the stream being greater at certain
seasons of the year than at others, we frequently find some such
section as the following developed:--
[Illustration: Fig. 2.--Section of Fluviatile Brick-earth.]
_a_ = Mould and soil, of no use to the brickmaker.
_b_ = Sandy clay, with a large proportion of sand; useful for
moulding or incorporating with the “fat” clays below for
brickmaking.
_c_ = Gravel bed, lenticularly developed; suitable for mending
roads, paths, &c.
_d_ = Sandy clay; similar to _b_.
_e_ = Thin bed of marl, with a fair proportion of lime.
_f_ = Sands and small pebbles, irregularly stratified
(false-bedded).
_g_ = Laminated sandy clay.
_h_ = Stiff clay; can be mixed with _f_ and passed through the
pug mill.
_i_ = Sand; an irregular bed of very local occurrence.
_j_ = Gravel bed, with much sand.
The above is typical of deposits accumulated in river valleys; it
is different in character to deposits laid down in the sea (as will
presently be described); the section exhibits very different classes
of brick-earth also, and yields a totally different kind of brick to
that obtainable from brick-earths of marine origin. The importance of
the question of origin of a brick-earth, therefore, is just beginning
to dawn upon us. Many rivers are noted as having throughout a long
period of time wandered from one side of the valley to the other (by
the process depicted in Fig. 1) several times, in which cases the
brick-earth sections relating to them are liable to still greater
variation. The reader would perhaps be very much astonished to find how
much is known concerning peregrinations of that description in regard
to particular localities, by competent authorities--usually field
geologists.
We come to another important point in regard to river deposits. The
ceaseless flow of the river, and the abrading action of the large
stones rolled along at the bottom of its channel, tend to cut the
latter deeper and deeper, and we have excellent evidence that most
English rivers once flowed at a greater elevation in their valleys
than they now do. In consequence of this, the brickmaker may find his
pit somewhat higher than the neighbouring river, which at an earlier
stage of its existence made his brick-earths. To a certain extent,
small earth movements, as previously explained, are also undoubtedly
responsible for many of these brick-earths now being at a considerable
elevation above the surface of the river. This phenomenon is
illustrated in Fig. 3.
[Illustration: Fig. 3.--Section across a river valley, showing
formation of terraces of gravel and brick-earth.]
This type of disposition of fluviatile deposits is of common
occurrence. We will assume that the valley is carved out of clay (shown
by horizontal lines and dots). On both sides of it, and at the same
relative heights, are two masses (marked 1 and 2) of brick-earths and
gravels running along so as to form two distinct broad terraces. These
beds were laid down when the river, in flood, though occupying only
a small portion of the valley, was approximately of the height shown
by the dotted lines _a b_. Denudation has been hard at work, however,
since then, and only vestiges of these beds clinging to the sides of
the valley, as shown, remain. At a later period, and coming on towards
modern times, the broad expanse of beds (comparable in disposition
with those depicted in Fig. 2) some miles in width, marked 3, were laid
down, and we notice the river channel, as it now is, cutting its way
through them. Thus it comes to pass that brickyards may be situated
in terraces one above the other; and what is much more important, the
brick-earths may vary very widely in quality along these horizons,
those in 1 differing from 2 and both from 3. The brickyards may be
quite close to each other, and to the unscientific eye the earths
are of similar appearance, but they do not yield the same class of
brick, and no one seems to trouble to enquire the reason why. These
differences have resulted primarily from the materials having been
derived from other collecting grounds, other watersheds, than those
comprised within the basin of the river as at present constituted. They
are the inevitable accompaniment of the evolution of the river system,
and throw light on successive phases of the past history of the stream
and its tributaries. For us, as we have seen, they possess considerable
practical value of the first importance in selecting the site for a
brickyard.
Apart from differences of the character just described, serious
alterations sometimes take place on these brick-earths being traced
higher up the valley, and indeed an excellent brickmaking material may
become absolutely worthless in that respect, for the reasons about to
be explained. The reader will agree that neither stones nor sediment
can travel up a valley, and he will understand that no sediment can be
found in the valley earths other than that derived from the destruction
of rocks within the watershed of the river system, to which the valleys
belong, or did belong, at the time the earths were formed. We desire to
put the case in a very simple light, so as to be clearly comprehended.
Let us contemplate Fig. 4.
[Illustration: Fig. 4.--Map shewing river basin, with geological
formations depicted.]
Here we have represented a river basin, the limits (watershed) of which
are indicated by a sinuous dotted line. Three geological formations are
found therein; in the upper reaches of the main river is a series of
clays marked A; a large tract in the middle, B, is sandstone; and the
lower part, C, is occupied by limestone. Seeing that nothing but clay
crops out in the part A, it follows that the deposits of the river in
that region must be principally of an argillaceous character, to the
point _a_. On flowing over the sandstone B, the main stream, already
charged with clay particles, will be mixed with sand; the proportion
of sand increases as the first large tributary (_b_) to the east is
encountered, and is considerably augmented as the still more important
tributary (_b_) to the west enters it. The superficial deposits in
the valleys of the area B will likewise be very sandy and perhaps
gravelly at _b b_, but at _c c¹_ the sands and gravels will be mixed
with much clay. On passing over into the area C, much carbonate of
lime is added, though the larger proportion denuded from the rocks is
taken away, chemically, in solution. Nevertheless, nodules of “race”
(lime concretions), limestone pebbles, and perhaps chert and flint
gravel will come upon the scene at about the point marked _e_. At _d_
the deposits would principally consist of gravel and impure marls. To
sum up, the clays at _a_ would no doubt be too stiff of themselves to
make good bricks; similarly the beds at _b b_ would be nothing but
sand, though these might be made, with a little judicious treatment,
into a species of fire-brick; at _c_ we should find alternating loams
and clays suitable for turning out fair bricks; at _c¹_ the beds would
be more variable in character and more locally developed; they would
consist of thin beds of sand, clays, loams and gravels (principally
sandstone fragments), which as a whole might be made serviceable,
though difficult to deal with; nothing of much use to us would come
from point _d_, nor bordering the tributary running over C; there
would be too much lime present, though a trade might be started in
basic bricks should there be any demand for them in the neighbourhood;
this, however, would only pay under extremely favourable conditions.
At _e_ there may be a mixture of all the foregoing deposits, and
providing the beds above were easily weathered and thick beds of loam
were thus fairly well developed, good sites for brick-earth might be
found. The point _e_ might possess this advantage over the other sites
mentioned, viz., that marls would no doubt be present, and thus no
necessity should arise for grinding lime to be incorporated with the
brick-earth; the only danger would be that lumps of limestone might be
too numerous--especially if c were a hard limestone.
The general character of the deposits might be slightly modified by
mineral matter brought up in springs and thrown down at convenient
spots.