The Science of Brickmaking

by George Frederick Harris

Air is a mixture of gases; dry air consists of at least four of them, namely, nitrogen, oxygen, carbonic acid, and argon. Of these, by far the most abundant is nitrogen, present to the extent of about 78 per cent., then oxygen, 20.96 per cent., argon about 1 per cent., and carbonic acid 0.04 per cent. Extremely minute quantities of ammonia and ozone, though practically always present, have been omitted from the preceding results of analysis of air.

We have been speaking of pure dry air; but the atmosphere is hardly ever of precisely the same chemical composition in two different places. By the seaside it has more ozone, and chloride of sodium is found in particular abundance. In cities, especially where large factories exist, nitric acid and sulphuric acid appear most conspicuously, and the proportion of ammonia becomes larger. In the air of streets and houses, the proportion of oxygen diminishes, whilst that of carbonic acid increases. Dr. Angus Smith has shown that very pure air should contain not less than 20.99 per cent. of oxygen, with 0.030 of carbonic acid; but he found impure air in Manchester to have only 20.21 of oxygen, whilst the proportion of carbonic acid in that city during fogs was ascertained to rise sometimes to 0.0679, and in the pit of a theatre to the very large amount of 0.2734. Although these may seem to be very small percentages, yet the total amount of carbonic acid in the atmosphere is enormous, and plays a conspicuous part in the decay of certain kinds of bricks.

Sulphuric acid is found in the air of large cities principally as a product of combustion, and is, of course, a distinct impurity. A portion of this acid is free, and a larger quantity is combined. Free sulphuric acid is very destructive to clay goods in the open; and it should be remembered that the relative abundance of this impurity depends on the precise locale in the city. A great deal has been said and written about the decomposition of the stone of which the Houses of Parliament are built. The air in the immediate vicinity must be highly charged with both sulphuric and nitric acid from the proximity of the busy factories on the opposite banks of the Thames in Lambeth. Had the Houses of Parliament been erected, say, in Kensington, where but few factories exist, it is conceivable that the stone would have behaved much better.

Air in itself, however, has no power to destroy bricks—the various gases, acids, chlorides, salts, solid carbon, inorganic and organic dust can do nothing by themselves. But the air is always laden with vapour, the most important of which is water vapour, which condenses into rain, hail, snow, and dew. When rain is formed, the drops of water take up minute quantities of air with its proportion of carbonic acid, sulphuric acid, or what not, and it is these acids, applied to the surface of bricks through the medium of rain and moisture generally, that are liable to do the damage if the nature and composition of the brick are favourable.

Let us assume that we have a brick composed of a goodly percentage of carbonate of lime. The carbonic acid in the rain reduces this to a bi-carbonate, which is soluble in water, and hence the surface of the brick decays, the rain water washing it away. Other things being equal, it follows that the same brick will decay most rapidly in a district where the rainfall is very great and where there is the largest proportion of these deleterious acids in the air.

Whilst speaking of the various acids which attack and destroy bricks, we must not forget those formed by the decomposition of organic matter on the surface of bricks which “vegetate.” The lichens, mosses, and so forth, growing from cracks in the wall, or spread over on to the brick from the mortar, yield, on decomposition, some of the most powerful acids in existence. A brick with a “crumbly” surface affords good foothold for these plants, and when they die they give rise to the so-called humus acids—crenic and apocrenic acid—which undoubtedly do an immense amount of damage. By keeping the surface of the brick moist, the plants permit the ordinary acids in rain to do more execution than they otherwise would. Taking two bricks, one which “vegetates” and one that does not, and exposing them in the same situation, it will be found that after a smart shower of rain the surface of the former has become thoroughly soaked, and the vegetation keeps it so, completely rotting it in time; whereas the surface of the latter, exposed to the same shower, may be quite dry within an hour or two after the rain has fallen.

Returning to the subject of rainfall, which exercises such material influence on the durability of bricks, we may give a few particulars concerning the distribution of rain in this country. Speaking generally, the east coast of England is the driest part of the country, the west coast having the greatest rainfall. The annual quantity at sea-level ranges from 60 to 80 inches on the west coasts of Ireland and Scotland, to about 20 inches on the east coast of England.[10] In some localities, however, the fall is much greater, amounting to 154 inches on the average of six years at Seathwaite, in Borrowdale, at the height of 422 feet above the sea.

The quantities which fall in particular showers are often very great, and this aspect of rainfall also has its interest for us. About London a fall exceeding an inch in 24 hours is comparatively rare, although on August 1, 1846, 3.12 inches were collected in St. Paul’s Churchyard in two hours and seventeen minutes.[11] On our west coasts this amount is often exceeded. On October 24, 1849, 4.37 inches were collected at Wastdale Head; June 30, 1881, 4.80 inches at Seathwaite; on April 13, 1878, 4.6 inches fell at Haverstock Hill, London; and a fall of 5.36 inches was recorded from Monmouthshire on the 14th July, 1875.

Taking averages of districts, we may give the following statistics, referring, of course, to annual rainfall:—

Less than 25 inches = Essex, Suffolk, Norfolk, Cambridgeshire, Huntingdonshire, Rutland, Middlesex, and parts of Surrey, Oxfordshire, Buckinghamshire, Bedfordshire, Northamptonshire, Leicestershire, Nottinghamshire, Lincolnshire, Yorkshire, and Durham. In other words, with the exception of parts of the North and East Ridings of Yorkshire and parts of Herts. and Bucks., which have a rainfall of from 25 to 30 inches, the eastern half of England, to the east of a line drawn from Sunderland to Reading, and then eastwards to the mouth of the Thames, has only a rainfall of 25 inches, or slightly less, per annum.