ABSORPTION.
The advantage of knowing the relative absorptive capacity of bricks has
been stated in these pages in divers connexions. The means of arriving
at the total capacity for absorption of water, as generally practised
by experimenters, are very incomplete and founded on an erroneous
principle. It is admitted by all that absorption is one of the very
best tests as to the quality of a brick, but such tests are meaningless
unless they imitate one or other or several of the influences to which
the brick would be subjected on being used in the building, or other
structure.
A common method is to weigh the brick when dry and then to immerse
it in water for periods varying from one to three days, subsequently
re-weighing it, the difference in weight between the dry and wet states
being termed the brick’s “absorptive capacity.”
Mr. Heinrich Ries remarks[16] that the absorption is determined by
weighing the thoroughly dry samples, immersing in clean water from
48 to 72 hours, then wiping dry and weighing again. Vitrified bricks
should not show a gain in weight of over 2 per cent. There are cases
where bricks of apparently good quality shew a greater absorption than
this, but they have great toughness and refractory qualities. Bricks
made from fire-clays which will not vitrify so easily will, naturally,
show higher absorption.
Again, Mr. E. S. Fickes, of Steubenville, Ohio, has recently made[17] a
large series of valuable tests of both paving and building bricks, in
which he shews the connexion between the power of absorption and the
strength of the materials experimented with. Mr. Fickes’ more important
conclusions are:--
1. The strength of the building brick, both transverse and crushing,
varies in tolerably close inverse ratio with the quantity of water
absorbed in twenty-four hours. The strongest bricks absorb the least
water.
2. Good building bricks absorb from 6 to 12 per cent. in 24 hours, and
with no greater absorption than 12 per cent. will ordinarily show from
7,000 to 10,000 or more pounds per square inch of ultimate crushing
strength.
3. Poor building bricks will absorb one-seventh to one-fourth of their
weight of water in 24 hours, and average a little more than one-half
the transverse and crushing strength of good bricks.
4. An immersed brick is nearly saturated in the first hour of
immersion, and in the remaining 23 hours the absorption is only
five-tenths to eight-tenths of 1 per cent. of its weight, as a rule.
These experiments are of much interest and are probably approximately
correct; but we venture to think that if the absorption experiments had
been carried out in a different manner, the results would have been
still more valuable.
Long before the publication of the results of the last mentioned series
of experiments, the present writer had discovered the close connexion
which subsists between the relative absorptive capacity of bricks and
their strength; a slight correction must be applied for specific
gravity. We are not prepared to enter into this subject at any length,
but it may be observed that we should not have arrived at such close
results had we experimented in the same way as the American authors
just quoted (or others, for the matter of that).
When you completely immerse a brick in water you prevent the escape of
air to a very large extent from the pores in the interior of the brick.
An old-fashioned way of overcoming this difficulty, was to place the
brick in the receiver of an air-pump and exhaust the air, subsequently
immersing the brick. This latter method certainly possessed the merit
of enabling the experimenter to arrive at total absorption very
rapidly, but it did not imitate natural processes any more than does
the thorough immersion of the brick in water.
A writer in the _Builder_ of May 25th, 1895, p. 397, experimented as
follows:--The bricks were placed in water in a large vessel, on edge,
supported where necessary by flat blocks, to bring the uppermost face
of each brick about ¼-inch above the surface of the water. Experience
had shewn that by completely immersing a brick, the air did not get
an opportunity of escaping from its pores with the same facility as
when one surface was left out of water. This disability, it was found,
materially impaired the results of the rate of absorption (rate, as
well as total tests, being carried out). By arranging the experiments
in the manner described, there can be no doubt that each brick absorbed
the maximum quantity of water possible; at any rate, there was no
water-pressure from above to retard the expulsion of the air.
The tests in the last-mentioned case extended over one week, the
relative absorption being taken at intervals of 1 second, 1 minute, 30
minutes, 1 day, and at the end of the week. It was found that English
vitrified bricks absorbed from 1.16 to about 1.85 per cent. in one
week; white glazed and good red and blue facing bricks from 5.31 to
10.34 per cent. in one week; wire cut facers and rubbers, with white
gaults, imbibed as much as from 12.93 to 20.50 per cent. of their dry
weight in one week. The rate of percolation suggested many interesting
problems, not the least important being the effect of chemical
decomposition in prolonged immersions, whereby after being quiescent
for a few days (after taking in the water for a few hours), absorption
“burst out” again and continued to the end of the week. One thing is
very apparent from this, namely, that for the lower grade brick even an
immersion for one week is not sufficient for practical purposes. The
writer remarks, “some of the red bricks from Bracknell, being placed
in the vicinity of the white gault bricks (in the water), discoloured
the latter to such an extent as to disfigure them. It was not merely a
surface colouration; it extended to at least ¼-in. into the interior.
The red colouring matter was iron, but there was not enough of it by
weight dissolved to materially interfere with the experiments. This
very clearly shews, however, the folly of erecting a building coursed
with white and red bricks, when both are very absorbent and the red has
so little hold of the iron of which it is partly composed--unsightly
stains are bound to appear.”
This question of the solubility of certain ingredients of bricks, has
not received the attention it deserves; and closely connected with
that is gradual decomposition, whereby the brick becomes more and more
porous--a potent factor in its ultimate destruction.