For household fire-places, whether open or closed, these may be
regarded as the material and the fuel of the future, and should be more
generally and better understood than they are.
The merits of fire-clay were fully appreciated and described nearly a
hundred years ago by that very remarkable man, Benjamin Thompson, Count
of Rumford. Any sound scientific exposition of the relative value of
fire-clay and iron as fire-place materials can be little more or less
than a repetition of what he struggled to teach at the beginning of the
present century.
It is impossible to fairly understand this subject unless we start with
a firm grasp of first principles. The business before us is to get as
much heat as possible from fuel burning in a certain fashion, and to do
this with the smallest possible emission of smoke.
Substances that are hotter than their surroundings communicate their
excess of temperature in three different ways; 1st, by _Conduction_;
2d, by _Convection_; 3d, by _Radiation_. All of these are operating in
every form of fire-place, but in very different proportions according
to certain variations of construction.
To demonstrate the conduction of heat, hold one end of a pin between
the finger and thumb, and the other end in the flame of a candle. The
experiment will terminate very speedily. Then take a piece of a lucifer
match of the same length as the pin, and hold that in the candle. This
may become red-hot and flaming without burning the fingers, as the pin
did at a much lower temperature. It matters not whether the pin be held
upwards, downwards, or sideways, the heat will travel throughout its
substance, and this sort of traveling is called “conduction,” and the
pin a “conductor” of heat. The conducting power of different substances
varies greatly, as the above experiment shows. Metals generally are
the best conductors, but they differ among themselves; silver is the
best of all, copper the next. Calling (for comparison sake) the
conductivity of silver 1000, that of copper is 736, gold 532, brass
236, iron 119, marble and other building stones 6 to 12, porcelain 5,
ordinary brick earth only 4, and fire-brick earth less than this. Thus
we may at once start upon our subject, with the practical fact that
iron conducts heat thirty times more readily than does fire-brick.
_Convection_ is different from conduction, inasmuch as it is effected
by the movements of the something which has been heated by contact
with something else. Water is a very bad conductor of heat, much worse
than fire-brick, and yet, as we all know, heat is freely transmitted
by it, as when we boil water in a kettle. If, however, we placed the
water in a fire-clay kettle, and applied the heat at the top we should
have to wait for our tea until to-morrow or the next day. When the
heat is applied below, the hot metal of the kettle heats the bottom
film of water by _direct contact_; this film expands, and thus, being
lighter, rises through the rest of the water, heating other portions
by contact as it meets them, and so on throughout. The heat is thus
conveyed, and the term “convection” is based on the view that each
particle is a carrier of heat as it proceeds. Air conveys heat in the
same manner; so may all gases and liquids, but no such convection is
possible in solids. The common notion that “heat ascends” is based on
the well-known facts of convection. It is the heated gas or liquid that
really ascends. No such preference is given to an upward direction,
when heat is conducted or radiated.
_Radiation_ is a flinging off of heat in all directions by the heated
body. Radiation from solids is mainly superficial, and it depends on
the nature of the heated surface. The rougher and the more porous the
surface of a given substance the better it radiates. Bright metals are
the worst radiators; lampblack the best, and fire-brick nearly equal to
it. To show the effect of surface, take three tin canisters of equal
size, one bright outside, the second scratched and roughened, the third
painted over with a thin coat of lampblack. Fill each with hot water of
the same temperature, and leave them equally exposed. Their rates of
radiation will then be measurable by their rates of cooling. The black
will cool the most rapidly, the rough canister next, and the bright one
the slowest.
Radiant heat may be reflected like light from bright surfaces, the
reflecting substance itself becoming heated in a proportion which
diminishes just as its reflecting powers increase. Good reflectors are
bad radiators and bad absorbers of heat, and the power of _absorbing_
heat, or becoming superficially hot when exposed to radiant heat, is
exactly proportionate to radiating efficiency.
Fire-clay is a good absorber of radiant heat, _i.e._, it becomes
readily heated when near to hot coals or flames, without requiring
actual contact with them. It is an equally good radiator.
Let us now apply these facts to fire-clay in fireplaces, beginning
with ordinary open grates used for the warming of apartments; first
supposing that we have an ordinary old-fashioned grate all made of
iron—front, sides, and back, as well as bars, and next that we have
another of similar form and position, but all the fire-box and the back
and cheeks of the grate made of fire-clay.
It is evident that the fire-clay not in actual contact with the
coals, but near to them, will absorb more heat than the iron, and
thus become hotter. Even at the same temperature it will radiate much
more heat than iron, but being so much hotter this advantage will be
proportionately increased. An open fireplace lined throughout with
fire-clay thus throws into the room a considerable amount of its own
radiation in addition to that thrown out from the coal.
But what becomes of this portion of the heat when the fireplace is all
of metal? It is carried up the chimney by convection, for the metal,
while it parts with less heat by radiation, gives up more to the air
by direct contact. Therefore, if we must burn our coals inside the
chimney, we lose less by burning them in a fire-clay box than in a
metal box.
Count Rumford demonstrates this, and described the best form of open
firegrate that can be placed in an ordinary English hole-in-the-wall
fireplace. The first thing to be done, according to his instructions,
is to brick up your large square fireplace recess, so that the back of
it shall come forward to about 4 inches from the front inside face of
the chimney, thus contracting the _throat_ of the chimney, just behind
the mantel, to this small depth (Rumford’s device for sweeping need not
be here described). The sides or “covings” of this shallowed recess
are now to be sloped inwards so that each one shall horizontally be at
an angle of 135 deg. to the plane of this new back, and meet it at a
distance of six or more inches apart, according to the size of grate
required. The covings will thus spread out at right angles with each
other, and leave an annular opening to be lined with fire-brick, and
run straight up to the chimney. The fire-bars and grate-bottom to be
simply let into this as far forward as possible.
By this simple arrangement we get a fire-grate with a narrow flat back
and out-sloping sides; all these three walls are of fire-brick; the
back radiates perpendicularly across the room; and the sloping sides
radiate outwards, instead of merely across the fire from one to the
other, as when they are square to the walls.
At Rumford’s time our ordinary fireplaces were square recesses; now
we have adopted something like his suggestion in the sloping sides
of our register grates, and we bring our fireplaces forward. We have
gone backwards in material, by using iron, but this, after all, may
be merely due to the ironmongery interest overpowering that of the
bricklayers. The preponderance of this interest in the South Kensington
Exhibition may account for the fact that Rumford’s simple device was
not to be seen in action there. It could not pay anybody to exhibit
such a thing, as nobody can patent it, and nobody can sell it. I have
seen the Rumford arrangement carried out in office fireplaces with
remarkable success. To apply it anywhere requires only an intelligent
bricklayer, a few bricks, and some iron bars.
Although nobody exhibited this, a very near approach to it was
described in an admirable lecture delivered at South Kensington, by
Mr. Fletcher, of Warrington. In one respect Mr. Fletcher goes further
than Count Rumford in the application of fire-clay. He makes the
bottom of the fire-box of a slab of fire-clay instead of ordinary iron
fire-bars. This demands a little more trouble and care in lighting
the fire, owing to the absence of bottom-draught, but when the fire is
well started the advantages of this further encasing in fire-clay are
considerable. They depend upon another effect of the superior radiant
and absorbent properties of fire-clay that I will now explain.
So far, I have only described the beneficial effect of its radiation
on the room to be heated, but it performs a further duty inside the
fireplace itself. Being a bad conductor, it does not readily carry
away the heat of the burning coal that rests upon it, and being also
an excellent absorber, it soon becomes very hot—_i.e._, superficially
hot, or hot where its heat is effective. This action may be seen in
a common register stove with fire-clay back and iron sides. When
the fire is brisk the back is visibly red-hot, while the sides are
still dull. If, after such a fire has burnt itself out, we carefully
examine the ashes, there will be found more fine dust in contact with
the fire-brick than with the iron—_i.e._, evidence of more complete
combustion there; and one of the advantages justly claimed by Mr.
Fletcher is, that with his solid fire-clay bottom there will be no
unburnt cinders—nothing left but the incombustible mineral ash of the
coal. Economy and abatement of smoke are the necessary concomitants of
such complete combustion.
A valuable “wrinkle” was communicated by Mr. Fletcher. The powdered
fire-clay that is ordinarily sold is not easily applied on account of
its tendency to crumble and peel off the back and sides of the stove
after the first heating. In order to overcome this, and obtain a fine
compact lining, Mr. Fletcher recommends the mixing of the fireclay
powder with a solution of water-glass (silicate of soda) instead of
simple water. It acts by forming a small quantity of glassy silicate of
alumina, which binds the whole of the clay together by its fusion when
heated.
Londoners, and, in fact, Englishmen generally, have hitherto regarded
anthracite as a museum mineral and a curiosity, rather than an everyday
coal-scuttle commodity. If it is to be the fuel of the future, it is
very desirable that we should all know something about its merits and
demerits, as well as the possibilities of supply.
Anthracite is a natural coke. From its position in the earth, and its
relations to bituminous coal, as well as from its composition, we are
justified in regarding it as a coal that was originally bituminous,
but which has been altered by heat, acting under great pressure. In
the great coal-field of South Wales, to which we must look for our
main supply of anthracite, we are able to trace the action of heat
in producing a whole series of different classes of coal in a single
seam, which at one part is highly bituminous—soft, flaming coal,
like the Wallsend, then it becomes harder and less bituminous, then
semi-bituminous “steam coal,” then less and less flaming, until at
last we have the hard, shiny form of purely carbonaceous coal, that
may be handled without soiling the fingers, and which burns without
flame, like coke or charcoal. This change proceeds as the seam extends
from the east towards the west. In some places the coal at the base of
a hill may be anthracite, while that on the outcrop above it may be
bituminous.
An artificial anthracite may be made by heating coal in a closed vessel
of sufficient strength to resist the expansion of the gases that are
formed. It differs from coke in being compact, is not porous, and
therefore, of course, much denser, a given weight occupying less space.
That we Englishmen should be about the last of all the coal-using
peoples to apply anthracite to domestic purposes is a very curious
fact, but so it is. In America it is the ordinary fuel, and this is
the case in all other countries where it is obtainable at the price
of bituminous coal. Our perversity in this respect shows out the more
strikingly when we go a little further into the subject by comparing
the two classes of coal in reference to our methods of using them,
and when we consider the fact that our South Wales anthracite is far
superior to the American.
Our open fires only do their small fraction of useful work by
radiation. Their convection is all up the chimney. Such being the case,
and we being theoretically regarded as rational beings, it might be
supposed that for our national and especially radiating fireplaces we
should have selected a coal of especial radiating efficiency, but,
instead of this, we do the opposite. The flaming coal is just that
which flings the most heat up the chimney, and the least into the
room, and, as though we were all struggling to destroy as speedily as
possible the supposed physical basis of our prosperity, we select that
coal which in our particular fire-places burns the most wastefully. If
we had closed iron stoves with long stove-pipes in the room, giving
to the air the heat they had obtained by the convective action of the
flame and smoke, there might be some reason for using the flaming coal,
as the flame would thereby do useful work, but, as it is, we stubbornly
persist in using only the radiated heat, and at the same time select
just the coal which supplies the smallest quantity of what we require.
No scientific dissertation is necessary to prove the superior radiating
power of an anthracite fire to anybody who has ever stood in the front
of one. This is most strikingly demonstrated by those grates that stand
well forward, and are kept automatically filled with the radiant-carbon.
Let us now see _why_ anthracite is a better radiator than bituminous
coal. This is due to its chemical composition. Of all the substances
that we have upon the earth carbon in its ordinary black form is the
best radiator. Anthracite contains from 90 to 94 per cent of pure
carbon, bituminous coal from 70 to 85, and much of this being combined
with hydrogen burns away as flame. On a rough average we may say that
the fixed or solid carbon capable of burning with a smokeless flameless
glow, amounts to 65 per cent in ordinary British bituminous coal,
against an average of 92 per cent in British anthracite. The advantages
of anthracite as a fuel for open radiating grates are nearly in the
proportion of these figures. Besides this it contains about half the
quantity of ash. Thus we see that from a purely selfish point of view,
and quite irrespective of our duty to our fellow-citizens as regards
polluting the atmosphere, anthracite is preferable to ordinary coal on
economical grounds, supposing we can obtain it at the same price as
bituminous coal, which is now the case.
Another great advantage of anthracite is its cleanliness, It may be
picked up in the fingers without soiling them, and it is similarly
cleanly throughout the house. It produces no “blacks,” no grimy dust,
and if it were generally in use throughout London one half of the
house-cleaning would be saved. White curtains, blinds, etc., might hang
quite four times as long, and then come down not half so dirty as now.
The saving in soap alone, without counting labor, would at once return
a handsome percentage on the capital outlay required for reconstructing
all our fireplaces.
Let us now look on the other side, and ask what are the disadvantages
of anthracite, and why is it not at once adopted by everybody? There is
really only one disadvantage, viz., the greater difficulty of starting
an anthracite fire. Practically this is considerable, seeing that
laziness is universal and ever ready to find excuses when an innovation
is proposed that stands in its way. To light an anthracite fire in an
ordinary fireplace the bellows are required unless a specially suitable
draught or fire-lighter is used. Some recommend that an admixture of
bituminous coal should be used to start it, but this is a feeble device
calculated to lead to total failure, seeing that the sole originator
and sustainer of our ordinary use of bituminous coal is domestic
ignorance and indolence, and if both kinds of coal are kept in a house
a common English servant will stubbornly use the easy-lighting kind,
and solemnly assert that the other cannot be used at all. The only way
to deal with this obstacle, the human impediment, is to say, “This you
must use, or go.” This is strictly just, as a simple enforcement of
duty.
At the same time some help should be supplied in the way of artificial
modes of creating a draught in starting an anthracite fire. This may
be done by temporarily closing the front of the fire by a “blower,”
or better still by selecting one of the grates specially devised for
burning anthracite, of which so many now are made. Another and rather
important matter is to obtain the anthracite in suitable condition. It
is a very hard coal, too hard to be broken by the means usually at hand
in ordinary houses. For domestic purposes it should always be delivered
broken up of suitable size, from that of an egg to a cocoa-nut. For
furnaces, of course, large lumps are preferable.
Then, again, anthracite must not be stirred and poked about; once
fairly started it burns steadily and brightly, demanding only a steady
feeding. The best of the special grates are more or less automatic
in the matter of feeding, and thus the trouble of lighting is fully
compensated by the absence of any further trouble.
As regards the supply. This for London and the greater part of England
will doubtless be derived from the great coal-field of South Wales.
The total quantity of available coal in this region after deducting
the waste in getting, was estimated by the Government Commissioners
at 32,456 millions of tons. It is very difficult or impossible to
correctly estimate the proportion of anthracite in this, but supposing
it to be one tenth of true anthracite it gives us 3245 millions of
tons, or about enough for the domestic supply of the whole country
during 100 years, assuming that it shall be used less wastefully than
we are now using bituminous coal, which would certainly be the case.
But, including the imperfect anthracite, the quantity must be far
larger than this, and we have to add the other sources of anthracite.
We need not, therefore, have any present fear of insufficient supply;
probably before the 100 years are ended we shall find other sources
of anthracite, or even have become sufficiently civilized to abolish
altogether our present dirty devices, and to adopt rational methods of
warming and ventilating our houses. When we do this any sort of coal
may be used.