THE CHEMISTRY OF BRICK-EARTHS (_Continued_).
In this chapter we shall fulfil our promise (_ante_ p. 58) to explain
in an elementary manner the precise meaning of ordinary commercial
chemical analyses of some typical earths used in brickmaking, etc. We
may commence by explaining a few terms used by the chemist.
An _atom_ is the smallest imaginable portion of matter, and all matter
is said to consist of atoms. A _molecule_ is the smallest conceivable
combination of atoms, and every compound substance is ultimately built
up of molecules. An _element_ is a substance that has hitherto defied
the efforts of the chemist to subdivide or split up. Over seventy of
these elementary substances are at present known, and their number is
being constantly added to. Again, by improvement in analytical methods,
a so-called element may be subdivided, and thus removed from the list.
The elements are classified into _metals_ and _non-metals_; and it is
convenient to give each of them a symbol to save trouble in writing,
and to render clearer to the reader the chemical nature of a compound
body. Thus, the symbol for the element aluminium is Al; for silicon Si;
for carbon C; for calcium Ca; for oxygen O; for iron Fe; for hydrogen
H; for chlorine Cl; and so on.
We are taught by chemistry that elements are capable of combining only
in definite proportions, and that each substance possesses a definite
proportion peculiar to itself. That proportion is called the _atomic
weight_ of the element; or, it is the relative weight of the atom of
each substance compared with that of the lightest substance known,
hydrogen.
Thus, the atomic weight of hydrogen being taken as 1, it is found that
an atom of chlorine is 35.5 times as heavy as that, so that the atomic
weight of chlorine is said to be 35.5. Now, in spite of the enormous
difference between the weight of the two elements just mentioned, they
combine in the same proportions by _volume_; and the union is known as
hydrochloric acid, or HCl.
But in certain cases elements do not combine in equal proportions;
for instance, an atom of oxygen will not combine with less than two
of hydrogen. Further, with this we find that the three volumes are
condensed into the space of two volumes--a very common phenomenon in
the chemical combination of gases. The union of hydrogen and oxygen
alluded to forms water, the chemical symbol of which is, consequently,
H2O.
_Chemical affinity_, or _chemical attraction_, is the force which is
exerted between molecules not of the same kind. Thus, in water, which,
as we have seen, is composed of hydrogen and oxygen, it is affinity
which unites these elements, but it is cohesion which binds together
two molecules of water. In compound bodies, cohesion and affinity
operate simultaneously; whilst in simple bodies, or elements, cohesion
alone has to be considered. To affinity are due all the phenomena of
combustion and of chemical combination and decomposition.
Certain gases, such as chlorine and nitrogen, and such substances
as sulphur, carbon, and silicon, with many others, form _acids_ in
conjunction with hydrogen, or hydrogen and oxygen. These combine
with greater or less facility with other elements which do not form
acids, and are termed _bases_. A combination of an acid and a base is
known as a _salt_. Salts the names of which end in _-ide_, such as
chloride, sulphide, etc., are combinations of a metal with a non-metal.
_Monoxide_ means an oxide containing one atom of oxygen; _dioxide_ one
containing two atoms; _protoxide_ means the first oxide, because it
is the first or lowest of the oxides of the given metal in amount of
oxygen present; the highest oxide is often known as _peroxide_. The
terminations _-ous_ and _-ic_ are frequently used for the lower and
higher oxides respectively. Examples:--
FeO, iron protoxide, or ferrous oxide.
Fe2O3, iron sesquioxide, or ferric oxide.
FeS2, iron disulphide.
Sb2S3, antimony trisulphide.
The following symbols may be indicated as referring to compounds
especially met with in brick-earths:--
CaO, lime, instead of calcium oxide.
Al2O3, alumina, instead of aluminium trioxide.
SiO2, silica, instead of silicon dioxide.
Na2O, soda, instead of sodium oxide.
K2O, potash, instead of potassium oxide.
MgO, magnesia, instead of magnesium oxide.
In _analysing_ a body, the first step consists in determining the
nature of the elementary substances contained therein. That may be
accomplished in the _dry way_ by means of the blowpipe and accessories,
as explained in the last chapter. Such an examination, as previously
remarked, is known as a _qualitative_ analysis. Or, it may be
accomplished in the _wet way_ by ordinary chemical examination. The
next step is to determine the amount of the constituents present, and
that is known as a _quantitative_ analysis. In making a qualitative
analysis, the chemist is assisted by the knowledge that certain basic
substances and certain acids produce peculiar phenomena in the
presence of known substances or preparations termed _reagents_.
There is a great difference between a _chemical compound_ and a simple
_mixture_ of elements; and it is not always easy (_e.g._, some alloys)
to say whether a substance is in the one state or the other. This
distinction is well exemplified by the air we breathe. The chemist
finds by analysis that the air is nearly constant in composition,
containing essentially in 100 parts 76.8 by weight of nitrogen
(including about 1 per cent. of the recently-discovered element,
argon), and 23.2 of oxygen. Small proportions of water vapour, carbon
dioxide, etc., may be ignored for our present purposes. In view of this
comparatively uniform composition, the question at first arises as to
whether the air is, or is not, a chemical compound? The answer is in
the negative, for, amongst other things, it can be shown that the ratio
of 76.8 to 23.2 is not that of the atomic weights of the two elements
present, viz., 14 : 16, nor of any simple multiples of these.
We will now quote a few analyses of well-known earths, and explain each
in turn:
_Chemical Composition of China-clays._[8]
-----------+---------+---------------+-------------
| Kaolin. |Kaolin average.|Sandy Kaolin.
+---------+---------------+-------------
Silica | 46.32 | 44.60 | 66.68
Alumina | 39.74 | 44.30 | 26.08
Iron oxide | .27 | .20 | 1.26
Lime | .36 } | 1.60 | .84
Magnesia | .44 } | | trace
Water | 12.67 | 8.74 | 5.14
-----------+---------+---------------+-------------
The kaolin alluded to in the first column is a remarkably pure
material, perfectly white, and contains an enormous quantity of
water. It refers to one of the finest washed china-clays in the
market, and is extensively used in porcelain manufacture. It is
quoted here principally to give an idea of what a really pure clay is
like chemically. We notice that, in spite of its relative purity, it
contains .27 per cent. of iron oxide. This could have been well done
without, from the manufacturer’s standpoint, but is of course a very
minute proportion. Small as it is, it must exert a slight amount of
colouring influence. The lime and magnesia are present in slightly
larger proportions, and a little more of either would be advantageous
rather than otherwise, as assisting to flux the material. This is an
earth with which practically anything may be done by judicious blending
and careful preparation.
With reference to the second column, the figures do not refer to any
particular clay, but they have been compiled to show the average
composition of kaolins as used in the market. It will be observed that
the silica and alumina are present in approximately equal proportions,
which is a characteristic of fairly good china-clays. The iron oxide
remains as before, but there is a larger proportion of lime and
magnesia--as much as can be permitted except in a second-rate clay.
The evidence of the third column shows that the sand in the china-clay
is to a large extent quartzose, and this is at the expense of the
alumina. Such a material would be suitable for making a species of
white fire-brick, and it might do for the commoner kinds of china-ware.
The earth is really of the nature of a loam--a sandy clay. There is
too much iron in it for the production of perfectly white goods. The
proportion of lime might be increased to advantage.
_Chemical Composition of Fire-clays from Newcastle-on-Tyne._[9]
------------+-------+-------+-------+-------+-------+-------+--------
| 1 | 2 | 3 | 4 | 5 | 6 | 7
+-------+-------+-------+-------+-------+-------+--------
Silica | 51.10 | 47.55 | 48.55 | 51.11 | 71.28 | 83.29 | 69.25
Alumina | 31.35 | 29.50 | 30.25 | 30.40 | 17.75 | 8.10 | 17.90
Iron oxide | 4.63 | 9.13 | 4.06 | 4.91 |} 2.43 | 1.88 | 2.97
Lime | 1.46 | 1.34 | 1.66 | 1.76 |} | |} 1.30
Magnesia | 1.54 | .71 | 1.91 | trace | 2.30 | 2.99 |}
Water, etc. | 10.47 | 12.01 | 10.67 | 12.29 | 6.94 | 3.64 | 7.58
------------+-------+-------+-------+-------+-------+-------+--------
The reader will see at a glance that the range of variation permissible
in fire-clays is very wide. These earths are all found close together,
and are utilised for similar purposes, though often blended to produce
desired results. It will be noticed that one of them (No. 6) contains
as much as 83.29 of silica, whilst another has no more than 47.55 per
cent. The range with reference to alumina is very wide also, from 8.10
percent. (No. 6) to 31.35. The refractory character of any sample of
fire-clay is determined by the proportions in which the silica and
alumina are contained, and by the absence of lime, iron, and other
easily fluxible substances. The proportion of iron discovered in sample
No. 2 is certainly much in excess of the requirements of the material,
as a fire-clay, and this no doubt is tempered by admixture, unless
utilised for inferior goods. The iron oxide in the other samples is
about sufficient for general purposes. The amount of lime present in
all the samples constitutes a good feature; much lime cannot on any
account be allowed in earths for fire-clay goods. With so much iron
present, and the fair proportions of magnesia (except in sample No.
4) these clays may be regarded as typical, with the exception of
No. 6. They have been utilised for many years in the manufacture of
fire-bricks and the like.
_Chemical Composition of Fire-clays, from Welsh localities._
------------+-------+-------+-------
| 1 | 2 | 3
+-------+-------+-------
Silica | 50.35 | 56.90 | 54.80
Alumina | 23.50 | 24.90 | 27.60
Iron oxide | 10.40 | 2.83 | 2.56
Soda | 1.55 | 3.00 | 2.00
Magnesia | 1.45 | 1.07 | 1.00
Water, etc. | 11.85 | 11.60 | 11.80
------------+-------+-------+-------
The first thing that will strike the reader on looking at these results
on Welsh materials, is their uniform composition as compared with the
clays from Newcastle. Yet there is as much as 10.40 per cent. of iron
in sample No. 1, which cannot be a first-rate clay. Its proportion of
silica to alumina is, however, excellent, and, as in sample No. 3, the
amount of soda and magnesia is not excessive. The soda in sample No.
2 (which acts somewhat like lime in the kiln) taken together with the
magnesia and iron in the same material, is too much for a first-class
clay, and would have to be suitably modified before good results could
be obtained. On the whole, it is possible that sample No. 3 would yield
the best results from the chemical standpoint.
We should not forget that remarkable substance of which the well-known
Dinas bricks are made. The proportion of silica present ranges from
about 96 to 99 per cent., the remainder consisting principally of
alumina, though traces of iron, lime, and magnesia frequently occur.
There is not, of course, sufficient natural flux for this “clay,” so a
small proportion (2.5 to 3 per cent.) of lime is added, which produces
the desired effect. In other words, if we can obtain a pure siliceous
sand, with hardly any lime, iron or magnesia in it, we have the
material of which the better kinds of fire-bricks are made. Such sandy
earths are not uncommon in the South of England, but strange to relate,
they are not used for the purpose indicated.
The earths from which the superior Stourbridge bricks are made, are
approximately of the following chemical composition:--Silica, 64.10;
alumina, 23.15; iron oxide, 1.85; magnesia, .95; water and loss, 10.00
per cent. It will be observed that the proportion of iron and magnesia
here is very small, whilst lime is altogether absent. It is a most
excellent earth for the purposes for which it is used, and the chemical
results may be taken as a standard for that class of material. Another
Stourbridge earth yields as much as 4.14 per cent. of iron, however,
whilst its proportion of silica is lower, 51.80, and alumina higher,
30.40, which serves to remind us of the variability of even good earths
used in the manufacture of fire-clay goods.
Let us now turn to the consideration of pottery clays, of which the
following results may be taken as typical:--
_Chemical Composition of Pottery Clays._
------------+-------+-------+--------
| 1 | 2 | 3
+-------+-------+--------
Silica | 46.38 | 49.44 | 58.07
Alumina | 38.04 | 34.26 | 27.38
Iron oxide | 1.04 | 7.74 | 3.30
Lime | 1.20 | 1.48 | .50
Magnesia | trace | 1.94 | trace
Water | 13.57 | 5.14 | 10.30
------------+-------+-------+--------
Some of the chief qualifications, from a chemical point of view, of
earths suitable for making pottery, is the proportion and potentiality
of the colouring matters present. Where the pottery is to be glazed,
that is not so important; but with ordinary unglazed ware, colour
and uniformity are two highly essential desiderata. We know that the
temperature employed will modify the tint, but under similar conditions
the clays alluded to in the above table will give, approximately, the
following results. Sample No. 1 is typical of an excellent blue pottery
clay, which burns white. It contains more alumina than is commonly
met with in such materials, in which respect it differs markedly also
from the fire-clays just described. The proportion of oxide of iron
is very small, not sufficient to perceptibly colour the finished
product, though, no doubt, on careful examination it would be seen
not to be perfectly white. The latitude of the term “white” is pretty
considerable with clayworkers, as the reader is probably aware.
The pottery clay (also used for bricks) referred to in the middle
column, is brown in colour; it is an ordinary kind, used primarily
for black and common red ware. The proportion of iron is high, and
considerable quantities of both lime and magnesia exist. As might
naturally be expected of such material, it will not bear exposure to
great heat, though that might be regarded as a qualification in some
brick and pottery yards.
The proportion of silica is high in sample No. 3, which appertains to
a common yellow clay, with, possibly, some siliceous sand in it. The
amount of alumina is correspondingly low, but the iron oxide is not
excessive--for a common pottery clay. It is used for the manufacture of
coarse ware, and burns yellow.
The chemical composition of earths used for terra-cotta and bricks
of that substance is so variable, that without going into each case
specifically it would be impossible to convey an adequate idea. It may
be stated generally that it is not one whit less important to consider
the composition of the raw earths for ordinary brickmaking, than in
respect of that for high-class bricks and pottery.
An excellent earth, from the neighbourhood of Ruabon, is of the
following composition:--
_Chemical Composition of Ruabon Clay._
Silica 63.00
Alumina 20.10
Sesquioxide of iron 4.84
Protoxide of iron 1.51
Potash 2.37
Soda 3.10
Combined water 3.54
Moisture 1.54
The proportion of silica in this is higher than in many clays used for
brick- or terra-cotta making, but the alkalis, potash and soda, are
in strong force, so that any refractoriness on the part of the silica
is soon subdued in the kiln. The iron, also, is in abundance. The
principal colouring ingredient is the sesquioxide, and we can quite
understand the manufacturer when he informs us that, in spite of the
rich tint of the goods produced, nothing is artificially mixed with
this clay to produce such a result. We may call attention to the method
of expressing the chemical analysis in this case, which might be copied
to advantage. In the first place, the combined and the uncombined
iron are separately shown, or rather the degree of combination is
indicated; and secondly, the proportion of water chemically combined
is differentiated from that which has simply soaked into the clay,
though expelled, following a well-known practice of chemists, prior to
commencing the analysis proper. It is of very little use giving the
amount of water, unless the proportions are divided in this manner. In
the result given above we learn that there is very little chance of the
clay shrinking, as it only contains moisture to the extent of 1.54 per
cent.; but if that had been added to the water combined, we should have
had a result of 5.08 per cent., which is not nearly so clear in its
meaning. We may add that the Ruabon earth referred to is utilised also
in the manufacture of tesselated and encaustic tiles.
In regard to the composition of earths employed in the manufacture of
the commoner kinds of bricks, we may give the following examples:--
_Chemical Composition of Common Brick-earths._
-------------------+-------+-------+------+---------+----------+------
| |Alumina| | | | Water
|Silica.| and | Lime.|Magnesia.|Manganese.| and
| | Iron. | | | | Loss.
+-------+-------+------+---------+----------+------
Reddish-brown | | | | | |
brick clay | 52.6 | 30.8 | 3.4 | 2.8 | 1.4 | 9.0
Red-brick clay | 50.4 | 24.0 | 2.7 | 1.3 | --- | 21.6
Common brick-earth | 33.0 | 11.2 | 39.8 | 6.0 | --- | 10.0
Sandy-clay (loam) | 60.2 | 24.0 | 2.4 | 1.6 | --- | 11.8
-------------------+-------+-------+------+---------+----------+------
Reviewing these results, it will be noted that the brown colouring
imparted to the brick in the first-mentioned example is due, to a
large extent, to the presence of manganese, a rather uncommon feature
in brick-earths, except where these have resulted from the denudation
of iron-producing rocks rich in manganese. It will be noticed also
that the proportion of water is not high for a common earth, and it
must be a fairly easy material to deal with. There seem to be some
possibilities in it that might, in competent hands, lead to higher
things. The amount of lime and magnesia is, however, a rather serious
one for a first-class clay.
In regard to the “red-brick” clay, an essential feature is the
comparative absence of lime, and it would, no doubt, make “rubbers”
of an ordinary kind. Unfortunately, in the results given, the iron is
not separated from the alumina, but clearly the latter is very small
in amount, and the results refer to a sandy material. The proportion
of water is disastrous for the employment of this earth by unskilful
hands. In drying, the greatest care would have to be exercised to
prevent undue shrinking, and, in any case, the earth would have to be
very thoroughly incorporated to make a really serviceable brick. It
is with earths of this character that the majority of brickmakers _in
embryo_ come to grief; they know not how to handle them successfully,
and twisting, warping, cracking, and “bursting” follow as a natural
consequence. It is a common and treacherous material, that could only
be made to succeed by perseverance and wide experience.
The “common brick-earth,” as will be seen, contains an abnormal
quantity of lime, and doubtless refers to a marl, though not much
alumina is shown. Malm bricks could be made from it, and the product
would have to be burned at a low temperature. For bricks useful to the
“jerry-builder” this earth could be strongly recommended. It was, no
doubt, mainly derived from limestone rocks; and, judging from the high
proportion of magnesia, probably from within a watershed composed to
some extent of magnesian limestone.
The “sandy-clay” or loam is of a very common type, and produces
light-red bricks. There is much in common between this and the
“red-brick clay” previously referred to.
The practice resorted to in various parts of the world of making bricks
from slate _débris_, although not hitherto adopted to any large extent
in this country, merits some description in this place. Slates may be
regarded as a highly compressed clay, the original structure of which
has been materially modified by the great pressure exerted during their
manufacture in Nature’s laboratory. To all intents and purposes they
are silicates of alumina, _plus_ iron, lime, magnesia, and so on, and
have, practically, the same range of variation as have ordinary clays.
But during their manufacture, and subsequently, certain adventitious
mineral matter has been frequently introduced, as may be gathered from
the following results:--
_Chemical Composition of Slates._
-------------------------+-------+-------+-------+-------
| 1 | 2 | 3 | 4
+-------+-------+-------+-------
Silica | 60.50 | 60.15 | 48.00 | 50.88
Alumina | 19.70 | 24.20 | 26.00 | 14.12
Iron (protoxide) | 7.83 | 5.83 | --- | 9.96
„ (sesquioxide) | --- | 1.82 | --- | ---
„ | --- | --- | 14.00 | ---
Lime | 1.12 | --- | 4.00 | 8.72
Magnesia | 2.20 | --- | 8.00 | 8.67
Potash | 3.18 | --- | --- | .88
Soda | 2.20 | --- | --- | ---
Alkalis (not determined) | --- | 4.28 | --- | ---
Carbon dioxide | --- | --- | --- | 6.47
Water, &c. | 3.30 | 3.72 | --- | ---
-------------------------+-------+-------+-------+-------
_Analysis No. 1_ refers to a blue Welsh roofing slate of Cambrian
age. It is quite certain that the large proportion of alkalis present
would render this material unsuitable for brickmaking, except for the
commonest kinds of bricks. The iron, again, is very large in quantity,
whilst the amount of alumina is low. We could not recommend this slate
for good bricks under any consideration.
_Analysis No. 2_ is of a dark-blue slate from Llangynog, in North
Wales. The amount of iron present is high, but from the low content
of alkalis this material, under proper treatment, should make fairly
good bricks. The ferruginous constituent is too powerful, however, for
fire-bricks to be made of this slate.
_Analysis No. 3_, of a purple slate from Nantlle, shows a remarkable
diminution in silica and a corresponding increase in iron. Lime and
magnesia being present to such an enormous extent, taken in conjunction
with the iron, would render this slate absolutely useless for
brickmaking. There is not a redeeming feature about it.
_Analysis No. 4_, which refers to a green Westmorland slate, has a low
percentage of alumina and very large quantities of iron, lime, and
magnesia. Only bricks of an exceedingly inferior quality could result
from such material.
Summing up the general characteristics of these slates from the
chemical aspect, one would say that none of them are very suitable for
high-class bricks. No. 2 is the best. Several minor differences will
be observed between the results quoted and those referring to ordinary
brick-earths--in particular, the distribution of the alkalis. A general
impression is abroad that any purple slate will do for brickmaking, and
manufacturers do not yet seem to have realised that the chemical nature
of slates is as variable as of brick-earths. That may account for the
difficulties experienced in many cases in turning out a satisfactory
material. The microscope is of much use in this connexion, however,
and the practical effects of chemical analyses are not always as bad
as they seem at first sight.
The remainder of this chapter will be devoted to the consideration of
rarer kinds of brick-earth and other raw earths used principally in the
manufacture of bricks for special purposes, or as pointing to certain
anomalies. As an example of what some manufacturers can do, we may
quote the chemical composition of a peculiar brick-earth employed in
Zurich, in Switzerland:--
_Chemical Composition of Brick-earth, Zurich._
----------------------------------------+------
|Yellow| Blue
|Clay. | Clay.
+------+------
Carbonate of Lime |23.68 |27.80
„ „ Magnesia | -- | 5.7
Other carbon dioxide | 2.85 | 1.55
Silica |42.39 |38.25
Alumina |18.16 |12.44
Iron oxide | 3.66 | .73
Lime (as silicate) | -- | 1.85
Magnesia | -- | .15
Potash | 2.14 | 1.54
Soda | 1.27 | 3.05
Moisture (at 100° C.) | 1.27 | 1.37
Water, &c., chemically combined | 3.85 | 4.72
---------------------------------+------+------
Here we have two clays with the carbonates of lime and magnesia
present, in one case of over 35 per cent., and in the other of over
26 per cent. Professor Lunge, of Zurich, states that the bricks made
from them, if burned at the ordinary heat, say a moderate red heat, are
_red_, and do not keep in the air, but crumble away very soon, as the
quicklime slackens on combining with the moisture. When burned at a
bright red heat, about 200° C. above the former, however, they become
nearly white. The lime is then present as a ferri-alumina-calcic
silicate, which causes the red colour of the iron oxide to disappear,
and, at the same time, entirely prevents any action of the moisture,
quicklime being no longer present. We have no hesitation whatever in
saying that most British makers would look down upon raw earths such
as these from Zurich, and yet many millions of really good bricks
have been made from them during the past twenty years, and they are
especially noted for their durability. The crux of the case is the
temperature at which the earths are burned, as the reader has perceived.
Under the heading of “magnesia,” we have said a few words regarding
basic bricks. In this country they have been made primarily from
magnesian limestone, the chemical composition of which is shown in the
following results of analyses:--
_Chemical Composition of Magnesian Limestones._
----------------------+------+-------+------+-------
| 1 | 2 | 3 | 4
----------------------+------+-------+------+-------
Silica | 3.6 | 2.53 | .8 | --
Carbonate of lime | 51.1 | 54.19 | 57.5 | 55.7
„ „ magnesia | 40.2 | 41.37 | 39.4 | 41.6
Iron, alumina | 1.8 | .30 | .7 | .4
Water, &c. | 3.3 | 1.61 | 1.6 | 2.3
----------------------+------+-------+------+-------
_Analysis No. 1_ refers to the well-known magnesian limestone of
Bolsover.
_Analysis No. 2_ to that from Huddlestone.
_Analysis No. 3_ to that from Roach Abbey.
_Analysis No. 4_ to that from Park Nook.
These results were obtained by Professors Daniell and Wheatstone in
connexion with an enquiry many years ago as to the kind of stone
suitable for the erection of the Houses of Parliament.
Regarding them generally, it may be said that they are remarkable as
not containing much acid, practically the whole substance of the rocks
(except No. 1) being made of the carbonates of lime and magnesia. In
manufacturing bricks of such materials as these, it will be seen that
the ordinary methods of brickmaking would not suffice. On heating
magnesian limestone, the carbonic acid is driven off, leaving the base
behind; it is estimated that the loss of the acid, _plus_ moisture
dried out, leads to its reduction in weight of from 40 to 45 per cent.,
and the shrinkage is from 25 to 35 per cent. If water were mixed with
this material, after calcination, strong chemical reactions would
result, and of such a nature as to render a coherent mass of the kind
required for making bricks impossible. Seeing that water cannot be
employed, crude petroleum oil, coal oil, resin oil, &c., have been
employed, all of them with more or less satisfactory results. The
petroleum, &c., is mixed with the lime, and when the whole is burned
the oil passes off, leaving bricks of solid lime. In manufacture it is
highly essential to see that the lime is well burned, and it must be
fresh, and not have been exposed to a damp atmosphere. An improvement
has been effected by mixing from 5 to 7½ per cent. of burned clay,
which makes the lime harder after burning. An admixture of from 3 to 5
per cent. of iron oxide also consolidates the lime, though it increases
shrinkage. The bricks are commonly made, in the first instance, under
hydraulic pressure.
The diatomaceous earth known as Kieselguhr, which is used in the
manufacture of fire-bricks for chemical works and the like, and which,
for the most part, is of German origin, has the following chemical
composition:--
_Chemical Composition of Kieselguhr._
Silica 83.8
Lime .8
Magnesia .7
Alumina 1.0
Oxide of Iron 2.1
Organic matter 4.5
Water, &c. 7.1
The reader will perceive that this earth is composed very largely of
silica, though there is enough iron, &c., to flux it, at any rate,
without material addition. The product is extremely light, and when
properly made, Kieselguhr bricks are the lightest known. They are
usually of a light yellow tint, with iron spots. The silica is not in a
crystalline form, the bulk of the material being composed of the hard
parts of microscopic plants known as diatoms; it is more like flint.
An earth of a similar character is found in the Isle of Skye, as
previously mentioned, though that burns into a redder colour.
An infusorial earth from Tuscany is composed of silica 55, magnesia 15,
water 14, alumina 12, lime 3, and iron 1 per cent. That also is made
into very light bricks. The general principle underlying the method
of utilising those earths of organic origin is similar to that of the
Dinas bricks, though they do not always require artificial fluxing.
At Saarbrücken, in the Rhenish Province of Germany, a material known as
“iron brick” is manufactured. It is made by mixing equal proportions of
finely-ground red clay-slate with fine clay, and adding 5 per cent. of
iron ore. This mixture is then treated with a 25 per cent. solution of
sulphate of iron, together with a certain quantity of finely divided
iron ore. It is then moulded and baked in a special manner. We do not
intend to describe the chemical composition of the various volcanic
ashes, trass, and other volcanic ejectamenta used for brickmaking on
the Continent in several localities. The materials of which glass-sand
bricks, slag-bricks, &c., are made have no special interest in
connexion with our present subject, their composition naturally varying
according to the particular kinds of “refuse” employed.