MORE ABOUT THE CLAY
Apparatus required.
_Clay, about 6 lbs.; a little dried, powdered clay; sand, about 6 lbs.
Six glass jars or cylinders [2]. Six beakers [1]. Six egg-cups [1].
Six funnels and stands [2]. Six perforated glass or tin disks [2].
Six glass tubes [2]. Two tubulated bottles fitted with corks. Some
seeds. Six small jars about 2 in. x 1 in. [2]. Bricks. The apparatus
in Fig. 9. Pestle and mortar._
We have seen in the last chapter that clay will float in water and only
slowly settles down. Is this because clay is lighter than water?
Probably not, because a lump of clay seems very heavy. Further, if we
put a small ball of clay into water it at once sinks to the bottom.
Only when we rub the clay between our fingers or work it with a
stick--in other words, when we break the ball into very tiny
pieces--can we get it to float again. We therefore conclude that the
clay floated in our jars (p. 6) for so long not because it was lighter
than water, but because the pieces were so small.
Clay is exceedingly useful because of its stickiness. Dig up some
clay, if there is any in your garden, or procure some from a brick
works. You can mould it into any shape you like, and the purer the
clay the {10} better it acts. Enormous quantities of clay are used for
making bricks. Make some model bricks about an inch long and half an
inch in width and depth, also make a small basin of about the same
size, then set them aside for a week in a warm, dry place. They still
keep their shape; even if a crack has appeared the pieces stick
together and do not crumble to a powder.
If you now measure with a ruler any of the bricks that have not
cracked, you will find that they have shrunk a little and are no longer
quite an inch long. This fact is well known to brickmakers; the moulds
in which they make the bricks are larger than the brick is wanted to
be. But what would happen if instead of a piece of clay one inch long
you had a whole field of clay? Would that shrink also, and, if so,
what would the field look like? We can answer this question in two
ways; we may make a model of a field and let it dry, and we can pay a
visit to a clay meadow after some hot, dry weather in summer. The
model can be made by kneading clay up under water and then rolling it
out on some cardboard or wood as if it were a piece of pastry. Cut it
into a square and draw lines on the cardboard right at the edges of the
clay. Then put it into a dry warm place and leave for some days. Fig.
4 is a picture of such a model after a week's drying. The clay has
shrunk away from the marks, but it has also shrunk all over and has
cracked. If you get an opportunity of walking over a clay field during
a dry summer, you will find similar but much larger cracks, some of
which may be two or three inches wide, or even more. Sometimes the
cracking is so bad that the roots of plants or of trees are torn by it,
and even buildings, in some instances, have suffered through their
foundations shrinking away. {11} We can now understand why some of our
model bricks cracked. The cracks were caused by the shrinkage just as
happens with our model field. As soon as the clay becomes wet it
swells again. A very pretty experiment can be made to show this. Fill
a glass tube or an egg-cup with dry powdered clay, scrape the surface
level with a ruler, and then stand in a glass jar full of rain water so
that the whole is completely covered. After a short time the clay
begins to swell and forces its way out of the egg-cup as shown in Fig.
5, falling over the side and making quite a little shower. In exactly
the same way the ground swells after heavy rain and rises a little,
then it falls again and cracks when it becomes dry. Darwin records
some careful measurements in a book called _Earthworms and Vegetable
Mould_--"a large flat stone laid on the surface of a field sank 3.33
millimetres[1] whilst the weather was dry between May 9th and June
13th, {12} and rose 1.91 millimetres between September 7th and 19th of
the same year, much rain having fallen during the latter part of this
time. During frosts and thaws the movements were twice as great."
[Illustration: Fig. 4. Clay was plastered over a square piece of board
and completely covered it. After drying for a week the clay had shrunk
and cracked]
You must have found out by now how very slippery clay becomes as soon
as it is wet enough. It is not easy to walk over a clay field in wet
weather, and if the clay forms part of the slope of a hill it may be so
slippery that it becomes dangerous. Sometimes after very heavy rains
soil resting on clay on the side of a hill has begun to slide downwards
and moves some distance before it stops. Fortunately these land slips
as they are called, are not common in England, but they do occur. Fig.
6 shows one in the Isle of Wight, and another is described by Gilbert
White in _The Natural History of Selborne_.
[Illustration: Fig. 5. Clay swelling up when placed in water and
overflowing from the egg-cup into which it was put]
[Illustration: Fig. 6. Landslip in the Isle of Wight]
Another thing that you will have noticed is that anything made of clay
holds water. A simple way of testing this is to put a round piece of
tin perforated {14} with holes into a funnel, press some clay on to it
and on to the sides of the funnel (Fig. 7), and then pour on rain
water. The water does not run through. Pools of water may lie like
this on a clay field for a very long time in winter before they
disappear, as you will know very well if you live in a clay country.
So when a lake or a reservoir is being made it sometimes happens that
the sides are lined with clay to keep the water in.
[Illustration: Fig. 7. A thin layer of clay _a_ entirely prevents the
water running through]
If water cannot get through can air? This is very easily discovered:
plug a glass tube with clay and see if you can draw or blow air
through. You cannot. Clay can be used like putty to stop up holes or
cracks, and so long as it keeps moist it will neither let air nor water
{15} through. Take two bottles like those in Fig. 8, stop up the
bottom tubes, and fill with water. Then put a funnel through each cork
and fit the cork in tightly, covering with clay if there is any sign of
a leak. Put a perforated tin disk into each funnel, cover one well
with clay and the other with sand. Open the bottom tubes. No water
runs out from the first bottle because no air can leak in through the
clay, but it runs out very quickly from the second because the sand
lets air through. These properties of clay and sand are very important
for plants. Sow some seeds in a little jar {16} full of clay kept
moist to prevent it cracking, and at the same time sow a few in some
moist sand. The seeds soon germinate in the sand but not in the clay.
It is known that seeds will not germinate unless they have air and
water and are warm enough. They had water in both jars, and they were
in both cases warm, but they got no air through the clay and therefore
could not sprout. Pure clay would not be good for plants to grow in.
Air came through the sand, however, and gave the seeds all they wanted
for germination.
[Illustration: Fig. 8. Sand allows air to pass through it, and so
water runs out of the bottle. Clay does not let air pass, and the
water is therefore kept in, even though the tube is open.]
This also explains something else that you may have noticed. If you
tried baking one of your model bricks in the fire you probably found
that the brick exploded and shattered to pieces: the water still left
in the brick changed to steam when it was heated, but the steam could
not escape through the clay, and so it burst the clay. In a brick
works the heat is very gradually applied and the steam only slowly
forms, so that it has time to leak away, then when it has all gone the
brick can be heated strongly. You should try this with one of your
model bricks; leave it in a hot place near the stove or on the radiator
for a week or more and then see if you can bake it without mishap.
Let us now compare a piece of clay with a brick. The differences are
so great that you would hardly think the brick could have been made
from clay. The brick is neither soft nor sticky, and it has not the
smooth surface of a piece of clay, but is full of little holes or
pores, which look as if they were formed in letting the steam out. A
brick lets air through; some air gets into our houses through the
bricks even when the windows are shut. Water will get through bricks
more easily than it does through clay. After heavy rain you {17} can
often find that water has soaked through a brick wall and made the wall
paper quite damp. A pretty experiment can be made with the piece of
apparatus shown in Fig. 9: bore in a brick a hole about an inch deep
and a quarter of an inch wide, put into the hole the piece of bent
glass tubing, and fix it in with some clay or putty, then pour some
water blackened with ink into the tube, marking its position with a
label. Stand the brick in a vessel so full of water that the brick is
entirely covered. Water soaks into the brick and presses the air out:
the air tries to escape through the tube and forces up the black liquid.
[Illustration: Fig. 9. A brick standing in water. The air in the
brick is driven inwards by the water and forces the liquid up the tube
in order to escape]
One more experiment may be tried. Can a brick be changed back into
clay? Grind up the brick and it forms a gritty powder. Moisten it,
work it with your fingers how you please, but it still remains a gritty
powder and never takes on the greasy, sticky feeling of {18} pure clay.
Indeed no one has ever succeeded in making clay out of bricks. All
these experiments show that clay is completely altered when it is
burnt. We also found that soil is completely altered by burning, and
if you look back at your notes you will see that the changes are very
much alike, so much so that we can safely put down some of the changes
in the burnt soil--the red colour, the hard grittiness, and the absence
of stickiness--to the clay. Let us now examine a piece of dry, but
unburnt, clay. It is very hard and does not crumble, it is neither
sticky nor slippery. Directly, however, we add some water it changes
back to what it was before. Drying therefore alters clay only for the
time being, whilst baking changes it permanently.
[1] A little more than one-eighth of an inch.
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