THE EFFECT OF THE ATMOSPHERE ON BRICKS.
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.
Between 30 and 40 inches = Practically the whole of the south coast from Kent to Devonshire, the whole of Somerset, Wilts., and the west of England generally, with the exceptions about to be noticed.
Between 40 and 50 inches = A great part of Devon and Cornwall, the western half of Wales, with the exceptions presently to be given, a great part of Lancs., and Cumberland.
Between 50 and 75 inches = A small patch in the centre of Devon, a large strip in West Wales, and an enormous tract of country in Cumberland, Westmorland, with Lancs. and north-west Yorks.
Above 75 inches = The wettest parts of the country. A small part of Dartmoor, a region in Wales in the vicinity and to the south-east of Snowdon, and the Lake District.
With reference to statistics concerning rainfall, it should be borne in mind that those relating to special districts, especially to hilly parts of the country, are often very deceptive, and require careful local study. A slight difference in the physical features of a locality is often sufficient to lead to considerable variation—the proximity of a conical hill rising from the plain, the sudden convergence of the two sides of a valley, or, conversely, the widening of a valley into a flat stretch of land, all materially affect the local distribution of rain. A clump of trees situated in proximity to a house will frequently be the means of a downpour that would otherwise have passed over. With winding valleys great latitude must be allowed. Then, again, the geological structure of the locality is an important factor in determining the amount of moisture delivered at a given spot. Where we find a thick clay cropping out in the bottom of a valley, with more or less porous rocks rising on either side of it, we soon ascertain that the houses on the clay receive more moisture (or the latter is distributed over a longer period) than those edifices on the hill sides in the same district.
Our readers could no doubt give us plenty of instances where in a circumscribed area their bricks have behaved very erratically—the bricks of a house in one part of the district weathering well, and in another badly. That may often be due, not only to the actual distribution of the rain, but to the manner in which the rain or dew has fallen. If an inch of rain falls in the neighbourhood in one day, that would not tend to weather the bricks so vigorously as though the fall had been spread over, say, a week.
A very important aspect of the subject is that which deals with the “efflorescence” on bricks. This appears to be greatly misunderstood, being commonly assumed to be due to one set of circumstances rather than to the conspiracy of several. There are many kinds of efflorescence, and an explanation of one of them obviously will not apply to all. The “scum” that appears on the surface of bricks is, however, to some extent bound up in the composition of the rain in the particular locality where it occurs. Examined attentively, the commoner kinds of efflorescence are seen to be minute white and yellowish-white crystals. The substance of which these are formed has been drawn out of the brick, or the mortar, or both, and rain has been the principal agent in accomplishing this work, though its power in that respect must necessarily vary according to the chemical composition and structure of the brick or mortar, as compared with the nature of impurities in the rain. If some substance were present in the rain that could readily form an alliance with an ingredient of the brick, and the union was capable of crystallising out, the surface of the brick would naturally form a convenient spot for the crystallisation to take place. To prevent it, we ought to know the composition of the air at the spot where the house is to be erected, and also the chemical and physical structure of the brick to be employed. That is rather too much to expect from the manufacturer and architect; but there is a method—we will not say an infallible one—which may be adopted to get rid of that particular kind of scum. That method could not always be adopted, as will be seen. The bricks must be burned more thoroughly, and at a high temperature; that would lead in most cases to the active employment of practically all the ingredients of which the bricks are composed, and the impurities in the rain would, in consequence, stand less chance of successfully inducing some of them to break their allegiance. In practice, however, we believe it would be found that the high temperature requisite to bring about the result just stated would either tend to spoil the colour of the brick or partially melt it. The latter could be prevented with due care, but we are afraid the former could not be so easily dealt with, with the majority of brick-earths. And if the brick is to be permanently discoloured to prevent efflorescence, it is better to permit the latter to manifest itself. The life of the “scum” is very variable; sometimes, after having once appeared and disappeared, it will never come again. The passing shower may wash it off (though it is not always so easily removed), and it may come again and again for years. It behaves very erratically. The amount of the efflorescence may be such as, in course of time, to lead to the surface of the brick “bursting” and peeling off, or, on the other hand, it may be a mere film.
There is one thing in connexion with efflorescence which cannot be overlooked in regarding its practical effects in the building. In ever so many cases we find that the scum, or the major part of it, is only to be found in the neighbourhood of the mortar joints. That is a matter of direct observation, and we have taken some considerable trouble to verify it, as it has always been regarded as a point whereon to hinge a debate. We do not say that in all cases the efflorescence appears only in the position on the brick just indicated; but it unquestionably does so in too many instances to enable us to regard its occurrence as mere accident. Taking a large surface of brickwork just commencing to show efflorescence, we find that the vicinity of the mortar joints are the first places, in very many instances, where the nuisance begins to manifest itself. From thence it spreads over the surface of the brick until the whole is more or less discoloured.
It seems impossible to deny that the mortar is guilty, to some extent, in such cases. At the same time, we must confess that we have never seen the efflorescence spreading over the mortar. It would appear that something in the mortar enters into chemical alliance with certain ingredients of the brick, and that neither without the other could produce the phenomenon alluded to. The remedy suggesting itself most readily is to chemically analyse the efflorescence, the brick, and the mortar; supplementing the experiments with a micro-examination to see how far it is possible to locate the deleterious substances found to exist, so that they may be removed in the manufacture of the materials, if that is possible. But information on that head is of the scantiest description, and much more will have to be done before the question is definitely settled.
Another kind of “efflorescence” that often appears on bricks in damp situations is mere vegetable growth, which bears a superficial resemblance to the crystalline “scum” just described, though it can, of course, be easily differentiated on examination with a lens. The damp atmosphere is no doubt largely responsible for this, though ineffectual damp-courses are contributors. The remedy lies in having a less absorbent brick—one that will not afford ready foothold to the vegetation.
The influence of rain on the weathering of bricks may be considered from yet another standpoint. Where the brick is fairly porous, its durability is liable to be materially influenced through the agency of successive frosts. The water finds its way a short distance into the brick and saturates it. During frost the water is turned into ice at and near the surface of the brick. In forming, the ice exerts considerable expansive force, which forces asunder the particles (sand-grains and the like) of which the brick is composed—that is to say, near the surface of the brick. The accumulated effects of successive frosts in this way tends to weather the brick by breaking up its exposed surfaces. To be materially affected, however, the brick would have to be of very poor quality, and it will be seen that the presence of cracks would much facilitate the operation.
The style of a building, the manner of its construction, and especially the class of metals used for exterior decoration, all assist rain in its work. A projecting course will have its upper surface washed clean, whilst the underside remains very dirty—in cities, becoming quite black. The limit of this dark discolouration is often frayed out by the irregular action of the rain dripping from the projecting ledge, assisted by the wind. Where the projection is so designed that the rain is induced to drain to one point, and then to fall over on to the wall, an unsightly streak down the latter is the result. The free use of metal ornaments, railings, for supporting signs, for down-pipes, &c., is unfortunate in not a few instances. At the point of junction between the metallic substance and the brick into which it is inserted, or in the immediate neighbourhood above which it is fastened, the brickwork is sure to be discoloured. This may arise from the dripping of rain-water from the metal, or it may be from the decomposition of the latter, or from both. Iron rust leads to brown streaks, zinc-compo. to dirty red, and so on.
The action of the wind as affecting the durability of bricks is sufficiently important to warrant passing allusion. It drives rain and its deleterious acids farther into the brick than the moisture would soak in the ordinary way. It leads to wet walls interiorly, unless the latter are so constructed as to overcome the effects. On the other hand, a gentle breeze dries moisture on the face of the brickwork. In cities, wind indirectly assists rain and its impurities by blowing organic matter from the streets into niches and corners, where it lodges, and, decomposing, provides powerful acids capable of doing much work. Discolouration is the chief effect produced on the average brick through this medium. In certain countries, wind, by driving dust, sand, &c., acts as a species of sand blast.
Considerable diurnal variations in temperature are known to be peculiarly destructive to certain kinds of brick and terra-cotta work. Very porous bricks are not much affected, but the more compact kinds, and especially terra-cotta blocks, often suffer. These observations do not so much apply to our own country as to warmer climates; though we are not altogether without experience here. On being heated these materials expand; when made loosely, as in rubbers and the like, the effect of the expansion is not very manifest, because the motion is absorbed, so to speak, by the brick itself. On the other hand, increased compactness of the particles leads to a perceptible increase in the size of the bricks, and when the sun has gone down contraction takes place as the bricks are cooling. It often happens in hot climates that the brick or terra-cotta block is unable to part with its heat as rapidly as the surrounding air becomes cooler, although it tries hard to do so, and this leads to corners of the brick being broken off, the physical forces exerted during the struggle doing the damage.
A highly interesting case of the effects of temperature on terra-cotta was detailed by Mr. T. Mellard Reade, C.E., F.G.S., a few years ago.[12] He shews that the cumulative effect of small, but repeated changes of temperature is very striking, and describes the lengthening of a terra-cotta coping in that connexion. The coping in question, which was freely exposed to the direct rays of the sun, consisted of two courses of red Ruabon terra-cotta bricks set in cement upon a fence wall, built with common bricks in mortar, a brick and a half in thickness. The courses were level, but, in consequence of the inclination of the road, the coping stepped down at intervals, so that the undercourse of bricks of one length was just gripped and held in position by the top course of the next length of coping. It will be observed that that form of construction constituted, by liability to lifting, a more delicate test than ordinarily of any increase of length, that might take place in the coping. On subsequent examination of the coping, the end position of one length, abutting against the next length at the drop in the level, was found to be thrown up into an arch-shape bend of about 6 feet span; the coping bricks being lifted in the highest part one inch from their bed. There was a fracture at the crown of the arch, and another at the foot or springing, but for a distance of 30 feet the coping was practically one solid continuous bar. A careful examination shewed that the coping had “grown” about a quarter of an inch longer than when it was first set, and that this lengthening, as shewn by movement on the corbel bricks which occur at intervals, was evenly distributed along a length of 30 feet.
Mr. Mellard Reade tells us that this is by no means an isolated case. In the neighbourhood of Blundellsands inspection of brick copings shewed that it was quite a common feature, and he has noted several instances in which the end brickwork and piers have been badly fractured by the force of expansion. In a case where the coping was of blue Staffordshire bricks, the top course in cement and the under course in mortar, a change in length was clearly shewn by the coping being lifted off the wall at each of the two ramps which exist in its length, and the movement was readily measured on the corbel bricks as in the case previously detailed. In this case the lengthening was also a quarter of an inch, and was evenly distributed over a considerable length of coping.
Whilst speaking of changes of temperature in their effect on bricks, we may allude to the behaviour of the material in severe conflagrations. A general rule cannot be laid down, because it is customary now-a-days to use fire-bricks for ordinary building purposes which will withstand practically any heat to which they may be subjected. Leaving them out of the question, and referring to ordinary bricks, it may be said that those of an inferior class frequently become cracked all over during a fire, or, it may be, by the sudden cooling after the fire has been put out, or by the sudden lowering of the temperature in them by the continuous action of the fireman’s hose. All the same, the average brick withstands heat far better than any kind of granite, or similar igneous holo-crystalline rock; loosely compacted sandstones and limestones crumble up on the surface, or flake, or may be utterly destroyed when subjected to a conflagration that would not have the slightest effect on bricks.
CHAPTER XI.
THE MICRO-STRUCTURE OF BRICKS.
The reader may be tempted to enquire, What is the use of knowing the micro-structure of a brick? We have anticipated the question to some extent in dealing with the structure of brick-earths, but it may be well to enlarge upon it here. In the first place, the study of the minute structure enables the manufacturer to ascertain whether the brick is thoroughly and homogeneously burnt. It tells him whether the materials mixed together in the earlier stages of manufacture were thoroughly incorporated or not, whereby, if need be, he can improve that part of the process. In carefully examining what the average manufacturer would call a well-burnt brick, the microscope assists us in perceiving that it is often anything but well burnt, small local patches—“tears”—of semi-vitrified matter being observed, which should not exist, of course, in a perfectly homogeneous brick. And if the brick is not homogeneous, it suffers in respect of its strength as a whole, and in the majority of cases its colour is not uniform. To arrive at the cause of this lack of uniformity is to indicate the manner in which the manufacture of the brick may be improved, and the microscope often enables us to arrive at a satisfactory solution of the problem.
From a chemical standpoint we know that a high percentage of iron in the average brick-earth is not conducive to the production of a good brick. In the same manner by “rule of thumb” we learn that a high percentage of lime prevents the manufacture of the raw material into a fire-brick, unless, indeed, we are making basic bricks. The chemist tells us also of the respective values of potash and soda. Too much iron will cause the brick to “run”; salt has a similar effect; but beyond this the chemist cannot go, except that in the broad sense he explains what unions take place to produce such results.
The microscope, on the other hand, enables one to see exactly what has taken place; the deleterious constituents are detected at their work, and careful chemical investigation teaches us what to add to the brick-earth to neutralise the effects observed; for it is only from its effects that the artificial constitution of the brick-earth can be properly regulated.
The same instrument is extremely useful in all questions concerning the relations subsisting between a brick and the glaze upon it, the cause and prevention of the cracking of the latter, and its general quality from a physical aspect. And, speaking of cracks, we may again draw attention to the influence these have on the strength and durability of the brick: many of these minute fissures cannot be seen by the naked eye. In a similar way can the microscope be made use of in the manufacture of terra-cotta and faïence. The cracking of glazes is one of the most troublesome features the high-class brick and tile manufacturer has to deal with. If the character of the surface of the brick is not suitable for “taking” the glaze, the maker knows in a moment; the trouble is where the glaze takes readily and then, some time after the operation is finished, it becomes covered with “spider-web” cracks, unsightly and considerably detracting from the value of the brick. The cause of the cracking is commonly attributed to the composition of the glaze, and the manner in which the latter is allowed to cool, and no doubt a great deal is due on both those heads. At the same time, we know of many instances where the same glaze being used under similar conditions on two different surfaces of bricks made from one and the same brick-earth, the glaze cracks in the one case, and hardly ever in the other. The direction of the cracks points to their origin, and the character of the surface is brought in guilty. And yet the average manufacturer would not detect any difference in the quality of the surface—he could not, without a good lens or low power objective, perceive the slightest discrepancy.
The ordinary glaze behaves very much like Canada balsam with reference to surfaces on which it is laid, and something akin to what petrologists call “perlitic” cracks is produced in the glaze. We can make these cracks, and imitate the structure artificially, by suitably distributing the Canada balsam over the surface of a piece of ground glass, and in other ways. That direct relationship exists between the cracks and the grain of the surface on which the preparation is laid, is certain, for we may vary the distribution of the cracks by varying the grain of the surface. An intelligent appreciation of the disposition of cracks in glazes should be the means of preventing them altogether, and not only with bricks, but with faïence and vitrified work generally, the study may be best carried on by aid of the microscope.
The microscope, also, may be made use of in identifying bricks in case of dispute, though its applications in this respect are not so important as in dealing with building stones.
Questions of durability may frequently be decided on appeal to that instrument. Take a case in which a brick is known to contain a rather high percentage of lime: if the lime were in a combined state, the quality of the brick would not be materially affected; but assuming it were not so employed, it is possible that in a short space of time the brick would be thoroughly decomposed by atmospheric agencies. The microscope tells us at a glance the state in which that and other ingredients exist, in a well-burnt brick. We draw the line at bricks intended for the “jerry” builder; they may well be left to take care of themselves; we allude only to high-class productions in which science may be some aid to the manufacturer.
And now as to the microscope—for we do not use an ordinary one in such investigations. The best kinds of microscope are those used by petrologists in the study of the minute structure of rocks and minerals. The reader will find these fully described in works specially devoted to the subject,[13] but we may say a few words thereon.
A common form of “Student’s” petrological microscope, as manufactured by Swift of London, may be described as follows:—
Eye Pieces and Objectives.—These need not be expensive, clear definition being the principal object to aim at; the objectives should be of low power, 2-inch, 1-inch and ½-inch objectives being plenty for the purpose. Unless the reader desires to follow the subject from a purely petrological point of view, to study the development of trichites, globulites, skeleton crystals, etc., in vitrified bricks, in such places as these latter have cooled from igneous fusion, there is no occasion to resort to higher powers. We are far from saying that the brickmaker of the present day would not derive any advantage from studying this subject in its higher aspects, for the origin of crystallization appeals strongly to the imaginative mind, and is one of the most remarkable problems that Nature offers for our investigation. But in an elementary treatise of this kind we cannot go into the matter; and, as previously remarked, low power objectives are sufficient for our present purpose. The eye-pieces should be fitted with cross-wires, the use of which will presently be explained.
The Stage.—In the instrument we are now describing this is circular with a hole in the middle, and is so arranged as to revolve horizontally on a collar about an axis, the centre of which comes exactly underneath the centre of the objective. In other words, a straight line drawn through the eye-piece down the centre of the barrel of the microscope, and passing through the objective passes through that axis. To assist in more accurately centreing than is otherwise possible (depending on the lenses) with this cheap form of instrument, a collar with adjustable screws is ordinarily affixed to the lower part of the barrel of the microscope. The stage, with suitable clips to hold the object to be examined, is graduated so that on its being revolved it is easy to ascertain the number of degrees, at any period of the revolution, through which it has been turned. Thus, it will be observed that the object revolves with the stage. A pointer is placed in a suitable position on the frame of the microscope to facilitate the observation.
The Polariscope.—This is an indispensable adjunct, for determinative purposes it is often necessary to observe the object in polarised light. Briefly, the polariscope consists of two parts—the analyser, placed in the barrel of the microscope above the objective, and the polariser, arranged underneath the revolving stage. The analyser is so fitted that it may be shot in and out of the barrel in order that the polariser alone may be used, or the latter may be removed, leaving only the analyser in position, or both may be removed to enable the object to be examined in ordinary light, either reflected or transmitted. The lower nicol[14] is made to revolve, and the collar in which it is fixed is broadly graduated and furnished with a pointer.
Reflector.—An ordinary reversible and adjustable reflector is arranged beneath all.
Accessories.—For the more accurate determination of minerals, a quartz wedge, a quartz plate, etc., are used by the petrologist, but the description of these is beyond the scope of the present work. For examination in reflected light it is highly desirable to have a “bull’s-eye” condenser.
An ordinary microscope with a revolving stage may be readily converted to petrological purposes, though it is better to have a special instrument.
* * * * *
The object to be examined may be in the form of (a) a fragment of the brick, or (b) a very thin slice of the same.
The fragment may be securely clipped and held in position on the stage, the “bull’s-eye” condenser being brought into use to throw a strong light on the part immediately under the objective. The polarising apparatus is no use for this, and may be thrown out of gear. A very low power should be employed. The observation may be directed towards ascertaining how far the fragments composing the brick are agglutinated, and their size may be noted. Anything like a discolouration should be specially observed, and a minute description jotted down. In bricks that have not been burnt very hard, and in those that have merely been baked, we shall often be able to detect particles of mineral matter which further investigation, after the manner presently to be described, shows are opaque. Different forms of iron, iron pyrite, fragments of clay that have merely been dried in the process of baking, and minute pieces of chalk (now converted into lime) are amongst the most prominent opaque substances met with in common bricks. These may generally be differentiated and determined at sight, and bricks thus composed are never of good quality, though the ingredients have been ground very fine, and there may be nothing superficially to find fault with. Their bad qualities are usually brought out in the weathering. A great deal may, therefore, be learned from a careful examination of fragments in this manner.
In regard to the examination of very thin slices, that is in the majority of instances the most instructive, and, if we may say so, the most interesting method of investigation, though it must always go hand in hand with the other. The slice of the brick is so thin that the bulk of the constituents is rendered transparent, or semi-transparent. The preparation of such slices[15] is not difficult, but demands some experience; those who have neither the time nor patience to make them will find it convenient to send the fragments of brick to Damon, of Weymouth, or some other first-class dealer in geological and mineralogical specimens. The price charged, per slide, is usually 1s. 6d. At the same time, the student will find it eminently to his advantage to prepare the slices himself. In the process he will learn much that escapes attention when the work is done by another.
The thin slice mounted on a slip of glass is placed on the stage of the microscope and firmly clipped, as with the fragment. The reflector is brought into position, and a beam of light thrown through the slice—the thin section is now being examined in transmitted light. At first it will be convenient to study it with the polariser and analyser thrown out of position. A certain proportion of the constituents is found to be opaque, and should be examined in reflected light, as above described. The remainder are more or less transparent, and some of the grains will, possibly, be coloured. We notice the way in which the whole of the fragments are bound together—say, by some opaque mineral such as iron—or whether they seem to be partially or wholly fused together. In the case of a vitrified brick, the latter phenomenon is most usual, and we shall find that although crystalline fragments have been melted, or partially fused, there is commonly a centre or nucleus of each fragment in its original condition remaining, which passes through insensible gradations from the crystalline to the non-crystalline, or amorphous state. This latter circumstance may be ascertained by using the polariscope. Ignoring the opaque matter adverted to, we shall then see that what was transparent in ordinary light appears, for the most part, to be opaque in polarised light. Those portions which still let the light through are truly crystalline, and by revolving the stage we notice that they frequently change tint, becoming alternately light and dark. In that brick where the particles are agglutinated by igneous fusion, we shall observe the light decreasing in intensity from the crystalline portion (forming the nucleus, as it were, of each particle) outwards, and where the crystal fragment has been melted, so as to become fused to its neighbour, the periphery, or rather what was originally the boundary of the fragment, is quite dark. Polarised light cannot pass through non-crystalline matter, and in being melted that portion of the crystal fragment had passed from the crystalline to the non-crystalline stage. It is very easy, therefore, to determine how far the fragments composing a vitrified brick have been melted down and fused together; but to observe the phenomena under the most favourable conditions, the brick must be thoroughly well-burnt, and the section taken, by preference, from near the outside surface of the brick.
In some instances, partial fusion is so well exemplified (especially in bricks from fairly pure china clay), and the brick, after being burnt, has been permitted to cool so slowly, that devitrification has set in, when we are presented with aggregates of crystallites closely resembling the “felspathic matter” of petrologists. That is a circumstance which the maker should note well, for he has burnt the brick to the best advantage, and it is not then so brittle as it might have been had more “glass” made its appearance in the section. Prolonged heat, just above the agglutinating point, has accomplished this, and the microscope here clearly shows the advantage of allowing the kiln to cool slowly, and to permit the lapse of several days in the operation.
CHAPTER XII.
THE MICRO-STRUCTURE OF BRICKS (Continued).
Turning now to the actual appearance of minerals commonly found in bricks as they are examined under the microscope, we may remind the reader, that the physical aspect of the majority of them has already been described in those chapters dealing with the “Mineral Constitution of Brick Earths” and “Minerals: their behaviour in the Kiln,” and the particulars that follow may be read in conjunction with what was there said.
It will be convenient now to describe the appearance of certain well-known minerals, as they are seen (A) in reflected light and (B) in thin sections in transmitted light, whilst the latter will be subdivided into 1 denoting the phenomena observed in ordinary light, and 2 in polarised light. To save repetition, the letters and figures will be used to denote the methods of examination as indicated.
Quartz.—Present in nearly all rubber-bricks, and in the vast majority of common stocks, as well as in vitrified goods and fire-bricks. In the last mentioned, the grains are usually partially agglutinated, and are extremely minute.
A. As more or less rounded, or sub-angular fragments, white and crystalline, like clear window glass.
B. 1—Clear white, often broken up by thin hair-like lines running in various directions, and rows and patches of minute specks, which, as previously remarked, have been shown to contain fluid, &c. 2—On revolving the stage of the microscope, the crystals are usually seen to present beautiful, clear transparent colours, which in characteristic sections are very vivid—red, blue, yellow, &c.
Flint.—Found in the same class of bricks as quartz.
A. Bluish horn colour; irregular fragments and splinters.
B. 1—Translucent; often melted more thoroughly than quartz in hard burnt bricks; colourless. 2—Opaque unless in some such form as chalcedony, when an extremely minute granular aspect results, becoming slightly transparent. Melted portions always opaque.
Felspar.—The alteration which the different kinds of felspar have undergone in a hard burnt brick, when present, render it almost impossible to recognise them specifically.
A. Milk white, or more rarely light pink; the mineral, even when red in the raw earths, becomes white on the application of moderate heat, as in the burning of common bricks. It is often closely fractured, and but rarely powdered.
B. The characteristic parallel lines of the triclinic varieties may often be observed, especially in rubber bricks; but great heat, such as leads to partial peripheral fusion, frequently obliterates them to a large extent, and in a well-burnt brick it is quite impossible in the majority of cases to determine whether the felspars present are triclinic or monoclinic. More particularly is this the case when the mineral has been more or less decomposed prior to its having been burnt. The bulk of the fragments of the mineral can only be alluded to in the general term “felspars,” and in ordinary light these are opaque or “fleecy,” whilst in polarised light minute portions may be found to be slightly birefringent. In a decomposed state it forms a prominent constituent of brick-earths in the first place, and that is precisely the material which most readily agglutinates in presence of a suitable flux. Crystallites are not uncommon in the melted peripheries, as may be seen in a hard-burnt brick in ordinary light.
Mica.—In minute flakes, shining, or glistening, and commonly black, silvery or bronze-coloured.
A. Detected at once by its thin shining scales, which frequently have not suffered much in the kiln except near the outside of the brick.
B. 1—The darker micas are usually citron coloured or light brown, and unless cut parallel to the cleavage of the mineral, exhibit a number of closely-set parallel lines, the fragments being much “frayed out” and “ragged” at the edges. 2—Using one nicol only, the mineral changes from dark to light on the revolution of the stage, and is said (in common with other minerals exhibiting a similar property) to be dichroic. With both nicols in position but little further difference is noted, except that in changing tint the whole is darker. Vivid colours are not observed except in yellows and browns. Muscovite mica is often quite white and transparent.
Iron.—Common except in white bricks made from the purest china-clays.
A. Brown or reddish-brown specks; sometimes as blue black films in fire-bricks; dull and frequently powdery in common bricks. Surrounding, film-like, grains of mineral matter of which the brick is composed. A grain of quartz, for instance, is frequently seen enveloped by a film of red iron. Other metallic iron is more lustrous and whiter than magnetite when seen in reflected light, but such unaltered particles of the mineral could only occur in a brick that had not been subjected to great heat.
B. Opaque either in 1 or 2.
Iron Pyrite only occurs as such in bricks that have not been thoroughly burnt, or in common “baked” bricks. Higher temperatures lead to the separation of the iron from the sulphur and the general incorporation of both in the agglutination of the brick during partial fusion.
A. Brassy yellow particles.
B. Opaque both in 1 and 2.
Calcite.—Not found in burnt bricks, nor indeed in any except those that have been sun-dried, or have been subjected to very little heat. Small pellets of lime are of common occurrence in poorly-burnt bricks. In reflected light such pellets are generally of a dirty white tint; opaque in transmitted light.
Dolomite.—Practically the same observations apply as to calcite, crystals of dolomite not being found except in sun-dried bricks and the like. Under the action of much heat the mineral, like calcite, is reduced to lime.
Selenite.—This is not rare in the commoner class of bricks, though the application of much heat reduces it to the state of powder. In reflected light it is found to be present as extremely minute specks or “tears” of whitish powdery plaster. Opaque, of course, in transmitted light.
The description of the micro-appearance of many other minerals which occur but rarely in bricks does not fall within the scope of the present elementary treatise; for practical purposes they may be ignored.
CHAPTER XIII.
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.
CHAPTER XIV.
STRENGTH OF BRICKS.
A very great deal is known concerning the strength of bricks. In addition to the innumerable experiments carried out by public bodies, we have the results of painstaking investigation by professors in universities and colleges, and the results carried out for and published by brickmakers themselves. Yet another large series of results have been published from time to time by professional journals, and it is, indeed, to these that we must look (at any rate in Britain) for anything like detailed work. The “Minutes of Proceedings of the Institution of Civil Engineers,” the “Transactions of the Royal Institute of British Architects,” the “Proceedings” of several allied provincial architectural societies, the “Builder,” the “British Clayworker,” builders’ “Price Books,” and several engineering “Handbooks,” have all contributed to our knowledge in regard to the strength of bricks. Of works consecrated entirely to the subject there are none—applied to British materials; but we have that excellent text-book by Professor Unwin, F.R.S., “The Testing of Materials of Construction,” and the important work by Mr. David Kirkaldy, both of the greatest possible value as being the results, largely, of original work. The experiments of recent years have been made almost exclusively by Mr. David Kirkaldy at his works in Southwark; by Professor W. C. Unwin at the Central Institution of the City and Guilds of London Institute; and by the Yorkshire College, Leeds.
With such a wealth of information a whole treatise might profitably be written, but it will be understood that in a small work like the present we can only give a comparatively few results, prefaced by observations to impart a general idea.
With the strength of brickwork, it is different, and it would seem rather remarkable, at first sight, that architects and engineers, who are every day using thousands of bricks, should have been at little pains to ascertain the “safe load” which this or that brick pier or wall would carry. Experience is, of course, of great value in all work of that description; but there is always the lurking suspicion that the engineer is making his piers too big, and that the architect is by no means running the thing close. The real reason why so little has been done to test the strength of brickwork is the difficulty in getting machines of such capacity as would crush sufficiently large masses. Small piers have been built from time to time, and bricks embedded in putty for mortar have served their purpose, but practically nothing of a really serious nature was carried out in Britain until a few months ago. The science committee of the Institute of Architects, well knowing the advantage of information as to the strength of brickwork, have partially carried out a most elaborate series of experiments, the first fruits of which have already been published, but it would be out of place to allude to them here. When the remaining brickwork shall have been built long enough at the experimental station, the final experiments will be made, and the results will, we have no doubt, be the most important contribution to our knowledge concerning the strength of brickwork that has ever been published in the kingdom.
But we must give our attention solely to the strength of bricks. To begin with, we must deprecate the idea that experiments as at present carried out give anything like the actual strength of bricks—the results are generally either too high or too low. Neither are the results comparative, except to a limited extent. One kind of brick has a “frog” on one side, another is recessed on both sides, a third is stamped with the maker’s name, or some device by way of trade mark, a fourth is as flat on all sides as may be, a fifth is pressed, a sixth is hand made, and a seventh wire-cut, and there are many other varieties of make. With such different kinds it is next to impossible to arrive at comparative data that shall be of much use for working purposes. Again, the whole brick may be subject to the experiment, or only the half-brick. The faces placed between the dies of the crushing machine may not be flat, and they are most frequently irregular. If the dies are applied to such bricks it is evident that corners will be broken off before the brick has really suffered much, and that to get the best result the faces must either be made perfectly true and parallel to each other, or some other method adopted to put matters right. That commonly employed is to place some yielding substance between the faces and the surface of the dies. Sometimes thin sheets of lead or pine wood are inserted. Professor Unwin has the faces of the brick made smooth and parallel by means of plaster of Paris, and the brick is then crushed between two pieces of millboard or between the iron pressure-plates, one plate having an arrangement to allow for any slight want of parallelism between the two surfaces of the brick applied to the plates.
Now it will be obvious, what with the difference in the shape and the various modes of experimenting, that the results are by no means comparative unless the precise facts are given; and when they are, it is but rarely that you can find more than half-a-dozen or so kinds of bricks of each category that offer all the elements necessary for comparison. So that, with all the wealth of information, we are by no means laden with much that is of actual comparative value, and if the experiments and their results are not comparative, of what use are they? So long as experimenters are each allowed a different method of research, and so long as makers will have partial or whole “frogs,” will stamp their names or initials, or will produce plain bricks only, so long will it be impossible to arrive at the best results that are really attainable. What we want is a government testing station as they have in Germany; or, at least, the mode of experimenting should be under some central control. The experimenter, further, should select the samples to be crushed, and should be at liberty to publish all results obtained. At present, if the brickmaker does not like the results arrived at, he, of course, does not publish them. And, if he has had a number of experiments carried out from time to time, he will, usually, quote only the highest results on his bricks. That is perfectly natural, and would be understood as “business.” All brickmakers may not do that, and a few may publish every or average results (we do not mean of one set of experiments, on say six bricks) of different experiments, but we fancy they are very rare. Therefore, in a matter so important to the architect and the engineer, and indeed to the general public, from the point of view of safety, we maintain that the whole thing should be carried out under some central control, as on the continent.
And now to proceed with the description of results on a few typical bricks. Glancing at table I, we may say that the strength of bricks as a whole is often quoted as here given, and has done duty for many years as the average strength of bricks. These bricks were crushed in a Clayton machine, and all were bedded upon a thickness of felt and laid upon an iron faced plate, and the experiments were conducted by the Metropolitan Board of Works.
Strength of Bricks.—I.
| Description. | Pressure in tons to | |
|---|---|---|
| Crack. | Crush. | |
| Four white bricks, each | 16.25 | 41.00 |
| Three „ „ „ | 17.05 | 41.05 |
| Red bricks, ordinary | 13.00 | 26.25 |
| Red bricks, not well burned | 13.75 | 25.05 |
| Best Paviours | 14.00 | 23.00 |
| Grey Stocks, London | 12.00 | 14.00 |
Turning to the second table, compiled for the most part from brickmakers’ circulars, and from the original results obtained for the late Building Exhibition, at the Agricultural Hall, all the experiments, we believe, having been carried out by Mr. David Kirkaldy, it will be noted that great variation in strength is apparent, following the different kinds of bricks. The highest result, 1064.2 tons per square foot, was obtained on a blue Staffordshire brick, though that is very closely run by bricks made from slate débris (1056.2 tons) from South Wales. The lowest result, 139.5 tons per square foot, was from a Worcester brick.
Strength of Bricks.—II.
| Locality. | Description. | Dimensions, Inches. | Mean stress of six samples in tons per square ft | |
|---|---|---|---|---|
| Cracked | Crushed | |||
| West Bromwich | Blue | 2.74, 9.03 × 4.36 | 548.6 | 1064.2 |
| „ „ | Blue (another make) | 2.80, 8.75 × 4.12 | 260.7 | 651.0 |
| „ „ | White glazed, “Terra Metallic,” recessed both sides | 3.10, 8.80 × 4.22 3.16, 8.70 × 4.34 | } 225.0 | 273.7 |
| „ „ | Blue vitrified | 2.55, 9.03 × 4.30 | 245.1 | 654.9 |
| Worcester | “Pressed,” recessed top and bottom | 3.20, 9.14 × 4.50 | 65.0 | 139.5 |
| „ | “Builders.” recessed top and bottom | 3.20, 9.30 × 4.50 | 56.1 | 155.5 |
| Saltley, Birmingham | Red, recessed one side | 3.20, 8.90 × 4.35 3.25, 8.95 × 4.40 | } 138.7 | 180.5 |
| Rowley Regis, Staffs. | Blue vitrified no recess | 2.85, 8.75 × 4.20 | 385.6 | 722.7 |
| Leicester | Red, recessed both sides | 2.65, 8.90 × 4.25 2.75, 9.10 × 4.36 | } 105.9 | 150.6 |
| Napton-on-the-Hill, Rugby | Light brown, wire cut | 2.85, 8.92 × 4.20 2.90, 9.10 × 4.25 | } 131.6 | 303.9 |
| Ruabon | Red, no recess | 3.10, 8.75 × 4.28 3.15, 8.73 × 4.29 | } 439.2 | 676.8 |
| „ | Blue, no recess | 3.02, 8.99 × 4.37 3.01, 8.95 × 4.36 | } 358.9 | 561.2 |
| Glogue, Whitland, S. Wales | Slate débris | 2.33, 8.70 × 4.25 | 556.4 | 1056.2 |
| Ravenhead, St. Helens, Lancs. | Red, brown wire cut | 2.90, 9.00 × 4.20 2.90, 8.90 × 4.27 | } 215.8 | 354.7 |
| Earith, St. Ives, Hunts. | Yellow, wire cut | 2.50, 8.70 × 4.10 2.50, 8.80 × 4.20 | } 135.9 | 178.8 |
| Gillingham, Dorset | Red, wire cut | 2.60, 8.90 × 4.30 2.60, 8.90 × 4.25 | } 159.5 | 261.7 |
| Newton Abbot, Devon | Vitrified “granite” | 2.80, 8.90 × 4.35 2.80, 9.10 × 4.55 | } — | 445.2 |
Table III. is by Professor Unwin,[18] and records the strength of several well-known bricks. Professor Unwin’s mode of experimenting we have already alluded to.
Strength of Bricks.—III.
| Description. | Dimensions. Inches. | Cracked, at tons per sq. ft. | Crushed at tons per sq. ft. | Colour. | Remarks. |
|---|---|---|---|---|---|
| London stock | 4.6 × 4.1 × 2.4 | 128 | 177 | Yellow | Half brick |
| „ „ | 4.6 × 4.0 × 2.45 | 133 | 181 | „ | „ |
| „ „ | 9.2 × 4.1 × 2.8 | — | 129 | „ | |
| „ „ | 8.9 × 4.2 × 2.3 | — | 113 | „ | |
| „ „ | 8.9 × 4.25 × 2.5 | — | 103 | „ | |
| Aylesford, common | 8.9 × 4.4 × 2.7 | 48 | 183 | Pink | |
| „ „ | 8.9 × 4.4 × 2.7 | 111 | 228 | „ | |
| „ pressed | 9.1 × 4.3 × 2.7 | 71 | 141 | Red | Deep frog |
| Rugby, common | 9.5 × 4.2 × 2.9 | 158 | 190 | „ | {Between} |
| „ „ | 9.0 × 4.2 × 3.0 | — | 120 | „ | {pine bds.} |
| Lodge Colliery, Notts | 9.0 × 4.2 × 3.4 | 127 | 159 | „ | |
| „ „ | 9.0 × 4.2 × 3.25 | 55 | 122 | „ | |
| Digby Colliery, Notts | 9.3 × 4.1 × 3.25 | 248 | [353] | „ | Not crushed |
| „ „ | 4.6 × 4.2 × 3.2 | 414 | 414 | „ | Half brick |
| Ruabon, pressed | 8.8 × 4.3 × 2.7 | 361 | [361] | „ | Not crushed |
| Grantham, wire cut | 9.2 × 4.4 × 3.2 | — | 83 | „ | |
| Leicester, „ „ | 4.4 × 4.1 × 2.6 | 251 | 337 | Pale red | Half brick |
| „ „ „ | 4.3 × 4.1 × 2.6 | 109 | 308 | „ | „ |
| „ „ „ | 9.06 × 4.2 × 2.8 | 115 | 229 | „ | |
| Cranleigh, pressed | 4.7 × 4.6 × 2.5 | 149 | 181 | „ | Half brick frog. |
| „ „ | 4.6 × 4.6 × 2.5 | 165 | 237 | „ | „ „ „ |
| Candy, pressed | 8.8 × 4.3 × 2.8 | 80 | 381 | — | |
| Gault, wire cut | 8.7 × 4.1 × 3.0 | 111 | 173 | White | |
| „ „ | 4.4 × 4.2 × 2.5 | 119 | 145 | „ | Half brick |
| „ „ | 8.7 × 4.1 × 2.9 | — | 169 | „ | |
| Staffordshire blue, common | 4.5 × 4.3 × 3.0 | 216 | 464 | Blue | „ |
| „ „ „ | 4.3 × 4.2 × 3.0 | 152 | 386 | „ | „ |
| „ „ „ | 8.9 × 4.3 × 3.1 | 240 | [353] | „ | Not crushed |
| Staffordshire blue, pressed | 9.0 × 4.3 × 3.1 | — | 275 | „ | |
| Glazed brick | 8.8 × 4.4 × 3.3 | 69 | 166 | — | Frog. |
| „ „ | 8.9 × 4.4 × 2.9 | 166 | 174 | — |
Table No. III. is specially instructive as indicating the relative strength of several well-known bricks, the experiments being carried out solely for scientific purposes. Yet the figures must not be taken too seriously. Glancing at those relating to “London Stocks,” we find the strength varied from 103 tons per square foot to 181 tons. But more recent experiments made by Professor Unwin[19] on some London Stocks from Sittingbourne, in Kent, shewed that with four samples one crushed at 60.76 tons per square foot and another gave out 94.6 tons, the mean strength of the four yielding 84.27 tons per square foot. With such heterogeneous materials as London Stocks, we ought not to be surprised at these results, but they form a striking commentary on the value of general statements concerning the strength of bricks of varied character going by the same name in the market.
When we consider the strength of homogeneous bricks, and especially where these latter are made of thick marine clays, or where the relative proportions of earths employed are carefully attended to in the raw material, the results appear to be more generally applicable—as far as they go.
With ordinary Gault bricks we find a range in strength from 145 tons to 173 tons per square foot; but Professor Unwin,[20] in his more recent experiments, finds that of four Gault bricks, one reached as high as 197.6 tons per square foot, and he gives 182.2 tons as the average strength.
To shew the absurdity of alluding to the strength of “blue Staffordshire” bricks, without also giving the precise locale of the samples dealt with, the reader is requested to refer to Table III., where the figures indicate a range from 275 tons to 464 tons per square foot, and to compare them with the results on Staffordshire bricks as stated in Table II., where we find a range from 651 tons to 1,064.2 tons per square foot. Of what value can a single formula be which gives the strength of Staffordshire bricks as a whole as based on such widely divergent figures as these? Professor Unwin, in his recent series of experiments alluded to, finds that with four Staffordshire blue bricks, the weakest gave a result of 564.8 tons per square foot, and the strongest 788 tons; the mean of the four being 701.1 tons per square foot.
The results on the Leicester “reds” are no more encouraging; the figures in the foregoing tables are 150.6 tons, 229 tons, 308 tons, and 337 tons per square foot. Similarly, Professor Unwin has more recently found that the Leicester “reds” from Elliston, near Leicester, bear a crushing strain varying from 311.4 tons to 591.4 tons per square foot in four samples.
From the foregoing it will appear to the reader that average results are of very little value to the architect or engineer, unless—(1) the brickyard is mentioned from which the bricks experimented with came; (2) the particular class of brick from that yard; (3) the method of experimenting, as to whether any substance was placed between the dies of the press and the brick to be crushed, and if so, what; (4) if recessed or initialled; (5) whether machine or hand made, and (6) as to whether the surfaces of the bricks were concave, convex, or flat.
Results on bricks not localised are not of much value, and it is absolutely useless for working purposes to give in one figure the strength of “London Stocks,” “Staffordshire blues,” “Leicestershire reds,” and the like. In a general way, of course, it will be admitted that the “Staffordshire blue” is a stronger brick than the “London Stock,” and so forth; but that is as much as can be permitted—it is of no practical use to give relative figures in general terms.
It frequently happens that the capacity of the machine used for testing the strength of bricks is not enough for those bricks having a very high resistance to crushing. In the recent experiments by Professor Unwin, more than once alluded to in this article, it was found necessary to experiment with half-bricks only, and he ascertained that bricks tested as half-bricks shew about 25 per cent. less resistance per square foot than when tested as whole bricks.
Further observations on strength are made under the next heading in connexion with other forms of testing the value and physical properties of bricks.
CHAPTER XV.
ABRASION, SPECIFIC GRAVITY.
Abrasion.—In this country it is not customary to test bricks and stone by means of the abrasion process, though many English materials have been dealt with in this manner on the continent.
Abrasion tests are of special value in regard to paving bricks, and this mode of experiment is largely carried out in the United States. As Mr. H. Ries remarks,[21] the abrasion test approximates closely the conditions under which the paving brick is used, and is, therefore, an important one. The usual method of conducting this test is to put the bricks in an ordinary “foundry rattler,” filling it about one-third full. It is then rotated at the rate of about 30 revolutions per minute, and about 1,000 turns are sufficient. The bricks are weighed before and after to determine loss by abrasion.
A more recent modification is to line the “rattler” with the bricks to be tested and then put in loose scrap iron. This is claimed to give more accurate results, and avoids loss by chipping due to the bricks knocking against each other, as in the previous method, although that has been somewhat obviated by Professor Orton, jun., by the introduction of a few billets of wood into the rattler.
The abrasion test may also be made by putting the weighed bricks on a grinding table covered with sand and water, and noting the weight before and after grinding. This last method seems to us to be decidedly the best, provided the bricks be weighted, that the weight is constant, that the feed of sand and water is uniform, and that the bricks to be tested are placed equidistant from the centre of the turning table. If this last point be not attended to, it will be obvious that in course of the revolutions the sand will tend to accumulate towards the centre of the table, and the bricks placed in that vicinity would receive more than their fair share of abrasion, as compared with those bricks situated near the edge of the table. Conversely, those bricks near the periphery would be subjected to greater grinding action, from the circumstance that the table would move faster underneath them than under those bricks nearer the centre of the table.
The bricks should certainly be weighted in such abrasion tests, and it seems desirable that the weights should be so adjusted that the weight of the brick is also taken into account. It is obvious that the abrading action of, say, street traffic, will be the same on a brick, no matter what the latter weighs, depending on the area of surface exposed to traffic. And if we experiment with one brick, weighing say 7 lbs., and another weighing 14 lbs., the greater weight of the latter, will (cæteris paribus), by the abrasion tests as usually adopted, give a much higher result than would the lighter brick. On the other hand, if the 7 lbs. brick be weighted another 7 lbs., then the results would be strictly comparable, provided always that the area exposed to abrasion in each case be the same, and that the other conditions we have laid down are strictly observed.
Knowing as we do that the rough and ready method of “rattling” cannot possibly give truly comparative results, we do not intend to enlarge much on the results of the American tests; but the following are suggestive as shewing the connexion between the tests for absorption, rattling, and strength combined.
Some valuable and interesting tests were recently made by the Ohio Geological Survey, to determine the relative merits of fire-clays and shales for the manufacture of paving bricks, as well as the influence, if any, of the method of manufacture adopted. Twenty-two varieties of shale bricks, or bricks the largest constituent of which is shale, were grouped together: fifteen varieties of fire-clay brick; four varieties composed of shale and fire-clay mixed in equal proportion; and three varieties made from Ohio River sedimentary clays. The averages of these four classes of results were as follow:—
Tests of Fire-clay and Shales.—Paving Bricks.
| Absorption. | Rattling. | Crushing. | ||
|---|---|---|---|---|
| Square Inches. | Cubic Inches. | |||
| Shales | 1.17 | 17.61 | 7,307 | 1,764 |
| Fire-clay | 1.62 | 17.32 | 6,876 | 1,678 |
| Mixture | 1.44 | 18.72 | 5,788 | 1,400 |
| River Clay | 1.36 | 19.02 | 4,605 | 1,176 |
From a series of tests recently made by Mr. Fickes,[22] the following factors were educed:—
1. A brick which stands the “rattling” test well, has ample crushing strength and rarely chips under less than 5,000 lbs. per square inch, or crushes under less than 10,000 lbs. The crushing strength tends to vary with the resistance to abrasion, however, but more slowly and irregularly.
2. The transverse strength also tends to vary with the resistance to abrasion, but more slowly and irregularly.
3. The toughest bricks usually absorb the least water.
Specific Gravity.—The practical value of knowing the specific gravity of a brick has, perhaps, been a little over-rated by writers on the subject. At the same time we do not deny that there is some use in ascertaining this property. Foremost, we have to mention its value in conjunction with absorption in arriving at a rough and ready means of gauging the strength of a brick, without having actual recourse to the crushing machine. It appears to us, however, that the specific gravity of bricks is rarely quoted in a proper manner, and until there is one uniform method, the results will always be at a discount. We allude to the fact that some experimenters take the specific gravity of a porous brick, without stating whether the amount of water absorbed, during the process, was taken into account in arriving at the specific gravity or not. Theoretically, of course, the substance to be dealt with is non-porous, and experimenters, worthy the name, either render the brick waterproof, or, ascertaining the amount of water the brick has absorbed, take that into consideration in calculating results.
The writer is in the habit of quoting the specific gravity in two ways, viz.: (a) the true specific gravity, and (b) the specific gravity of the particles. In an elementary treatise like the present, however, it is not desirable to enlarge on this subject.
THE END.