DRYING AND BURNING.
Of the merely mechanical aspects of the operations of drying and
burning bricks, we shall say little or nothing. But there are just a
few points of a more or less scientific nature that offer themselves at
this juncture to which we desire to allude.
The brickmaker hardly needs to be told that if he places his bricks
in the sun to dry, they, or a large percentage of them, will crack,
and become practically worthless from a commercial standpoint. To dry
a brick properly in the open air is a lengthy operation--too lengthy
for many manufacturers, who, in consequence, have had recourse to
artificial drying. Many a brickyard has had to be abandoned from the
inability of the worker to produce bricks that did not crack at some
period of the operation, either in the drying, or burning, or both. And
several manufacturers have their particular methods of “doctoring” the
raw earths to prevent cracking. These are invariably “trade secrets;”
though usually of a very open and transparent character, however, to
the student of the subject.
It is most curious to learn the different reasons for adding this or
that ingredient to the earths to prevent the brick from cracking. One
who in a district has found that the addition of a little sand is
beneficial, imparts that information by degrees, either personally or
through his workmen, and in time it is laid down as a general axiom
that “sand will prevent cracking.” Another has discovered that clay
should be mixed in small quantity to produce the desired result, so
he and his neighbours do that, and pity the ignorance of the “sand
mixers.” A third feels quite certain that crushed brick, or brick dust,
is a good thing; while a fourth will add a little lime. Now, each of
these ingredients is useful in its way; everything depends upon the
class of brick-earth to be dealt with. It may happen that what will,
in a measure, prevent cracking, will be a bad thing in the burning,
and the art of the brickmaker is to know what to do under the varied
conditions.
As a general rule, where care is exercised in the drying, the cracks
arise from the brick-earth being too wet or plastic in the first
place, and it cannot be too well understood that, _cæteris paribus_,
the wetter the earth the more liable it is to crack during drying. The
contraction, even when the unburnt brick is shielded, and in the open
air, often proves too much for the material. Then we have that class of
brick-earth composed of too much clay, and that would be improved by
the addition of sand--just how much depends on the particular earth;
and there is no better method of ascertaining the quantity required
than by subjecting the materials to direct practical experiment in
the kiln. Where no sand is available, it frequently happens that
brick-dust will answer the purpose, though this may be at the expense
of homogeneity in the long run. In the semi-dry process of manufacture
the initial causes of cracking are not present, the block having to
contract so little that it may be taken from the press and stacked in
the kiln for burning. Unless the brick-earth be carefully prepared,
however, the surfaces of the hard blocks produced by that process are
liable to develop minute cracks. And here it may be stated that unless
the clay, with brick dust or other foreign substance, be thoroughly
incorporated prior to being sent under the press, and the whole ground
very fine, it is impossible to prevent cracking during some part of the
process.
Apart from the fierce and variable drying action of the open air, we
have a fruitful source of cracks in the indentations made by stamping
the makers’ name or trade-mark upon the blocks. With bricks burnt
very hard this does not so much matter, but on the commoner kind of
materials one may often perceive minute, hair-like cracks radiating
from the indentations. We presume that in this age of advertising it is
impossible to convince many makers of that fact, yet if full justice is
to be done to the material, it will be better not to make any sharp or
deep marks on the brick.
The commoner kinds of brick-earth, as we have seen, mostly possess
gross particles, grit, pebbles, &c.; these act as so many centres from
which cracks radiate either during the drying or burning, and apart
from their influence in a chemical sense, they are apt to seriously
weaken the brick.
It is truly marvellous to see how little attention many large makers
pay to the initial drying; often the long rows of drying blocks are
left unprotected except for a rude kind of roof placed over them. The
passing shower of rain drives in underneath, and wets the exposed
surfaces, causing the clay to swell. These surfaces, being moister than
the remaining portions of the brick, contract at a different rate,
the centre occasionally being drier than the outside. The unequal
contraction produces minute cracks even in most excellent earths.
Turning to a smaller matter, the hand-barrow coming from the drying
stacks to the kiln is unprotected, which often means that a good brick
is spoilt. Of course, we are not alluding, in this connexion, to what
takes place during clamp stacking; the brick produced by such a process
must take its chance. The method of stacking in the kiln or clamp is
very often responsible for damage to the bricks. A common method is to
build them sloping outwards, and all sorts of strains and stresses are
thus set up, which have their effect in producing lines of weakness, if
not of actual visible cracks.
The “London stock,” if not a thing of beauty, is usually strong, and
that in spite of the “breeze” which forms so many points from whence
cracks radiate. We must not forget, however, that a really good London
stock is, above all things, thoroughly burnt, and that is a set-off
against the numerous and often wide cracks.
We will assume that the brick has been either naturally or artificially
dried, that no cracks have made their appearance, and that it is
properly stacked in the kiln ready for burning. Now comes a most
important part of the process. It is possible that any microscopic
cracks will be closed by fusion or agglutination; but it more
frequently happens that in unskilled hands the kiln is responsible for
many cracked and “starred” bricks. To know exactly how to introduce
the heat so gradually that the bricks shall not be impaired, is an art
begotten only of considerable experience. Even when dealing with one
particular kind of brick-earth, the maker must be careful to notice
the relative moistness of his charge, and vary the mode of procedure
accordingly. Suppose the brick to be as “dry as a bone” before being
put in the kiln, we shall notice a considerable amount of moisture
coming out of it as soon as the fires are alight; and if the heat
is applied too suddenly, the bricks are not improved--they contract
unevenly and too quickly, and warp. When well alight, care should
be taken to keep the temperature as uniform as possible, and when
sufficiently burnt it must be lowered by almost imperceptible degrees.
Above all things, there should not be too great a disparity between
the temperature in the kiln and the outside air when unloading. Except
to those who had minutely studied this matter, such a precaution might
seem superfluous; it may be that no damage caused will be visible to
the naked eye, but the microscope frequently shows flaws due apparently
to this cause. The manufacturer may test this for himself by heating
a good, sound medium burnt brick to the temperature usually found in
his kiln when unloading, and suddenly plunging it in snow. It is not,
perhaps, that any one of these things is especially dangerous to the
brick, but it is the combined effect of all of them trending in the
same direction. We desire to be clearly understood on this point.
The cracks produced may not seriously impair the strength of the
brick; they may be merely superficial, and they mostly are. But they
materially assist the agents of denudation in “scaling” the brick, and
weathering it unevenly. To this we shall return later on.
Let us now say something concerning the superficial changes produced in
bricks by burning. The most important of all is the change of colour,
upon which the sale of the brick depends in ninety-nine cases out of
a hundred. We said a few words on this subject when dealing with the
behaviour of individual minerals in the kiln. The production of an
uniform tint is the main point aimed at; and it may be at once remarked
that unless the brick-earth employed is very homogeneous, or has been
most carefully prepared and thoroughly incorporated, the production of
an uniform colour is impossible. In regard to the tint to be produced,
it should be remembered that the temperature employed in burning is a
most potent factor. It is frequently laid down that such and such a
temperature will form a red brick, and another and higher temperature,
a blue one. That is a most absurd notion. In a general sense the
principle could be correctly applied to a limited district, and with
one class of brick-earth; but it cannot be made to apply all round.
There is nothing like experience in regard to a point like this. In
a general way, of course, a pink, red, or blue tint may be produced
from one brick-earth depending upon the temperature employed; but the
bulk of brick-earths would melt and the whole kiln-full be ruined
in any attempt to attain such a temperature as is used in burning a
sound “Staffordshire blue.” Quite a large number of bricks made in the
Southern half of England, may be described as having been dried in
the kiln only--they cannot be said to be burnt, except that the heat
employed was enough to turn them red, or to make them piebald; the
particles are not agglutinated by fusion, and, indeed, there is often
no trace of the constituents having been melted. On the other hand, we
have red bricks in which the constituents are distinctly agglutinated
by fusion, and the whole burnt thoroughly. The brick-earth of which
these latter are made, would barely turn tint--would certainly not
become red--at so low a temperature as that employed in producing the
red in the non-agglutinated bricks alluded to.
It is not always an easy matter in burning a red brick to obtain two
kilns full of the same tint, even in the same yard. When the employment
of pyrometers becomes more general, that will be considerably
simplified; but it is a difficult matter to get a reliable instrument,
none of the forms hitherto invented being altogether suitable. That by
Professor Roberts-Austen is as good as any. Many manufacturers, we are
sorry to say, place colour before everything else; they even sacrifice
durability to attain a certain tint. And there is much excuse for
them so long as they find a ready sale for the material. When colours
are made from artificially introduced mineral matter (which is not so
often the case as some appear to think) the mineral introduced is,
most commonly, iron; though it will be understood, from what we have
previously said, that it must be used very sparingly.
The ultimate tint assumed by the brick cannot always be judged
beforehand from the colour of the brick-earth. In brickmakers’
language, a red clay is one that produces a red brick, a blue clay
a blue brick, and so on. For the most part, colour depends on the
proportion of hydrated oxide of iron in the clay; if iron is present
in an earth that contains no lime, or similar mineral substance, the
colour produced in the brick at a moderate red heat will be red, and at
the same temperature, with the same brick-earth, the more iron present
the deeper the tint. In an ordinary brick-earth, when more than 10 per
cent. of iron is present, the clay is apt to burn bluish, however, and,
in certain cases, almost black. With a smaller proportion of iron, and
the application of intense heat, the same tint may result, and the
brick become vitrified. A brown colour may frequently be obtained when
the brick-earth has from 2.75 to 4 per cent. of magnesia, or a similar
proportion may be artificially added to the earth.
To obtain a white brick, so that it shall also be of excellent quality,
the pure white clays of Devon and Cornwall are the best, though the
so-called “white” is, in the majority of cases, a light cream colour,
unless, of course, the brick is glazed. In the neighbourhood of
London, a whitish brick results from a mixture of chalk (carbonate
of lime) with clay or loam, and is known as a “malm.” In parts of
Yorkshire, white pressed bricks are manufactured from common red clay
mixed with magnesian lime (made from magnesian limestone) in a slacked
condition. The latter ingredient, on introduction, immediately absorbs
about 40 per cent. of the moisture present in the clay.
Yellow bricks can easily be manufactured from the more impure kaolins;
also from certain clays in Cambridgeshire, Huntingdonshire, Kent, &c.
(gault bricks); “malms” are mostly yellow, though called white.
Laboratory experiments, many years old, show that with white clay as
a basis the following tints may be obtained. Phosphates of lime of
various kinds = very light blue bricks. The phosphates, mixed with a
quarter by weight of alum = brighter blue bricks. A mixture of white
vitriol (sulphate of lime) three-quarters, with borax one-quarter =
light dirty green. Sulphur and tin oxide in equal proportions = yellow.
These experiments are interesting, but the ingredients would, as a
rule, be too expensive for ordinary brick manufacture. They are more
applicable for the production of ornamental tiles.
A time-honoured method of producing black bricks is to make any
ordinary bricks red-hot and to dip them in a cauldron of boiling
coal-tar for a few seconds. It is essential that the brick should be
very hot, or the black staining will rub off. A good test that the
operation has been successful is, that the surface shall be dull black,
not shining. And there are many other ways of obtaining different
tints, the description of which would be beyond the scope of the
present work.
Unless a brick is extremely well burnt it is not uniform in colour
throughout. A considerable proportion of a “draw” is often ruined in
regard to tint by the adoption of an unsuitable form of kiln. Where
the brick is actually burned (as distinguished from being baked),
the contact of the flame from the fires is almost sure to lead to
uncertainty in that respect along the flues. Impurities in the coal,
such as iron pyrite, are the chief delinquents, and there is sure to be
a certain amount of “flash.” In that, as well as in the baking method,
bricks are liable to be discoloured by the bringing out of impurities
which they themselves contain.