Ordinary building bricks are made of clay or other earths subjected to several processes (which somewhat vary according to local practice, influenced by the nature of the material), formed to the required shape in moulds, and burnt.
It is beyond the province of these Notes to go into the chemistry of the subject, but it will be useful just to glance at the part played by each of these constituents, and the effect that it has upon earth considered as a material for brickmaking.
It may here be remarked that mere inspection or even chemical analysis of a clay gives very little information as to its suitableness for brickmaking. No test is satisfactory but that of actually trying the clay by making a few bricks with it.
In the absence of facilities for burning full-sized bricks, a fair indication of the quality of a clay for brickmaking may be arrived at by making it into a small brick about 3 inches long by l1/2 inch wide by 1 inch thick. This small brick may be burnt in a common house fire, being protected from contact with the fuel by placing it inside a shield made by roughly rolling a piece of sheet-iron round into a hollow cylinder of about 2 inches diameter.
Alumina is a principal constituent in nearly every kind of clay. It gives the material its plastic qualities, but it shrinks and cracks in drying, warps, and becomes very hard under the influence of heat.
Silica exists to a greater or less extent in all clay, in a state of chemical combination with the alumina, forming Silicate of alumina.
It is found also in nearly all clays in an uncombined state - as sand.
Silica is infusible alone, or in the presence of alumina only, except at very high temperatures.
If, however, the silica and alumina be in nearly equal proportions, the presence of a small quantity of oxide of iron will render them fusible at a comparatively low temperature.
Pure silicate of alumina is plastic, but shrinks when drying, and warps with heat.
The action of sand is to prevent cracking, shrinking, and warping, and to provide the silica necessary for a partial vitrification of the materials, which is generally desirable.
The larger the proportion of sand present, the more shapely and equable in texture will the brick be.
An excess of sand in clay renders the brick made from it too brittle.
The difference between the silica which is in a state of chemical combination and that which exists merely as sand is not shown in ordinary chemical analyses, and this is one reason why they are not so useful as they might be in determining the value of a clay for brickmaking. 1
Lime has a twofold effect upon the clay containing it.
It diminishes the contraction of the raw bricks in drying, and it acts as a flux in burning, causing the grains of silica to melt, and thus binding the particles of the brick together.
An excess of lime causes the brick to melt and lose its shape.
Again, whatever lime is present must be in a very finely divided state. Lumps of limestone are fatal to a clay for brickmaking. When a brick containing a lump of limestone is burnt, the carbonic acid is driven off, the lump is formed into "quicklime," and is liable to slake directly the brick is wetted or exposed to the weather. Pieces of quicklime not larger than pin-heads have been known to detach portions of a brick and to split it to pieces.
The presence of lime may be detected by heating the clay with a little dilute sulphuric acid. If there is lime present an effervescence will take place.
Bricks containing lumps of quicklime should be immersed for several hours before use, so as to kill the lime and prevent it from slaking after the bricks are made, or even built into the work.
1 Silica sometimes exists in clay in a soluble condition combined with lime; it is then injurious, as it may absorb moisture which has been known to destroy walls by making the bricks swell.
Iron Pyrites often occurs in clays, and should be carefully removed.
If not, the pyrites is partially decomposed in the kiln, will oxidise in the brick, crystallise, and split it to pieces.
Carbonaceous matter, when it exists naturally in clays to any considerable extent, is objectionable. "When not burnt completely out in the kiln, which is sometimes with the denser clays difficult, the bricks are of a different colour in the interior and exterior, and will not bear cutting for face work without spoiling the appearance of the brickwork.
"But, worse than this, such bricks when worked in the wall occasionally pass out soluble compounds like those absorbed from soot by the bricks of flues, and like those (when used again in new work) discolour plastering or stucco work."1
"Common salt is nearly always present in minute quantity in clays; but when these are taken from the sea-shore, or without or beneath the sea-washes, or from localities in and about the salt formations (Trias), they frequently, though in all other respects excellent clays, are unfit for burning into good bricks.
"Chloride of sodium 2 is not only a powerful flux when mixed even in very small proportion with clays, but it possesses the property of being volatilised by the heat of the brick kilns, and in that condition it carries with it in a volatile state various metallic compounds, as those of iron, which exist in all clays, and act as fluxes. The result is that bricks made of such clays warp, twist, and agglutinate together upon the surfaces long before they have been exposed to a sufficient and sufficiently prolonged heat to burn them to the core into good hard brick. Place bricks can be made of such clay, but nothing more, and these are nearly always bad, because never after free from hygro-metric moisture." 1
Oxide of Iron in clay influences the colour of the bricks to be produced (see p. 89). The tint resulting after burning depends upon the proportion of iron in the clay and the temperature to which it has been raised.
When in the presence of silica and alumina whose proportions are nearly equal, iron renders them fusible.