Clay loam

“ Thus it is an exception to the general rule to meet with a limestone as pure as ordinary white chalk, or with clay as aluminous as that used in Cornwall for porcelain, or with sand so entirely composed of siliceous grains as the white sand of Alum Bay in the Isle of Wight, or sandstone so pure [p.13] as the grit of Fontainebleau, used for pavement in France. More commonly we find sand and clay, or clay and marl, intermixed in the same mass. When the sand and clay are each in considerable quantity, the mixture is called loam. If there is much calcareous matter in clay it is called marl ”

“ Potters’ clay is a much less pure, and from that cause more fusible clay, which when burnt forms at a moderate heat a semi-fused, more or less hard mass, such as crockery and pottery ware. Brick-clay is a still more impure clay, or loam, containing considerable sand and usually iron oxid, and largely falls already within the limits of tillable soils or subsoils, rendered fusible by the presence of relatively considerable amounts of iron, magnesia and lime. ”

“ Whereas the sand and the loam will remain in fairly good condition when dry, the clay will have become "puddled," i. e., the particles will have run together and made a hard, compact mass. Thus it is found in practice that clay soils must be handled with far more care and intelligence than is required for the sand and loams, if the texture is to be kept perfect.
Experiment No. 4.—If, in the experiment above suggested, the clay soil is mixed with leaf-mould, or humus soil, from the woods, [118] it will be found to act very differently. ”

“ On the same chalk No. 1. is the Reading Oyster bed, No. 2. which though inconsiderable in thickness seems constantly to form the next stratum above the chalk, though organic remains have been noticed in it only at Reading. No. 3, at Newhaven, is the ash coloured sand of Woolwich in diminished thickness. Nos. 4, 5, and 6, appear to be an enlarged condition of the plastic clay bed No. 7 and 8 of Woolwich, and from 7 to 11 inclusive at Loam Pit Hill. ”

“ The true yellow clay is usually found in a thin stratum under the surface mould, and over a rock; harsh, tenacious, stony, strong loams, difficult to work, are not uncommon: but they are quite different from English clays.
Friable, sandy loams, dry but fertile, are very p. 161common, and they form the best soils in the kingdom for tillage and sheep. Tipperary and Roscommon abound particularly in them. The most fertile of all are the bullock pastures of Limerick, and the banks of the Shannon in Clare, called the Corcasses. These are a mellow, putrid, friable loam. ”

“ In warm climates, the incorporation of the sand with the clay is effected by frequent plowing and rains. On sand and gravel draw plenty of clay and loam, if it can be easily procured; thus it is easy to form a good friable, retentive loam, adapted to every variety of soil-culture. ”

“ Beneath these pebbles is a bed of sand identical with No. 12 at Loam Pit Hill. This sand lies on a bed of plastic clay which supports the water of the well in Mr. Conybeare's garden, and of all the wells on the plain of Blackheath at no great depth; it possesses the same peculiar dark red colour, with the plastic clay of Reading, Corfe Castle, and Paris, and has been used for pottery. [6] Beneath this clay the Woolwich shell beds and subjacent thick ash coloured sand are to be seen in several parts of the sloping terrace that surrounds the Blackheath plain. ”

“ It contains organic remains in very few places. Is it not improbable that this bed is contemporaneous with some of the upper strata of the plastic clay formation which we find at Loam Pit Hill, at Blackheath, and in the Isle of Wight, at which last place the beds belonging to this series are accumulated to a thickness far greater than has been yet noticed in any other spot, amounting at Alum Bay, according to Mr. ”

“ In some cases after the weathering of feldspar or limestones, the clay may remain just where it was formed, as a residual deposit; but, being so fine-grained, it is usually transported by rain water or by the wind and deposited somewhere else as a sedimentary bed. The quiet waters of a lake are favorable places for such deposits, and many clay beds represent former lake bottoms. Impure clays are often laid down on the flood plains of sluggish 208 streams. In fresh water the settling of the clay is a very slow process, requiring days, or when very fine, weeks, before the water wholly clears. ”

“ Lime and magnesia compounds may occur as silicates (varieties of felspar, mica, etc.) , but their most important occurrence is as chalk or limestone. Chalk is a constant constituent of malms [2] and of many marls, but the latter may contain limestone particles. Limestone occurs in many marls and to a smaller extent in other clays. In the boulder clays it frequently forms a large portion of the stony material. If the grains are very small (as in chalk) , the lime compounds [11] act as a flux, reducing the heat-resisting power of the clay and increasing the amount of vitrification ”

“ Brick clays are low grade clays and vary greatly in composition. The main requisites are that they mould easily and bake hard at relatively low temperatures with as little warping and cracking as possible. As most clays shrink both in the air drying and in the baking, sand is added when the clay is being mixed. The color is mostly due to the presence of iron impurities. If there are iron oxides and little or no lime, the brick bakes to a red color, but if there is an excess of lime over the iron oxides, it bakes to a cream or buff color, which on vitrifying turns green. ”

“ Yet the curious fact that the purest clays—the kaolins—are remarkably deficient in plasticity shows that molecular constitution is not, alone, sufficient. Others hold that the remarkably small size of clay particles enables them to pack together more closely than do particles of other [22] materials and to retain around them a film of water which acts partly as a lubricant, facilitating the change of shape of the mass when under pressure, and partly as an adhesive, causing the particles to adhere to each other when the pressure is removed. ”

“ In composition, ball clays appear to consist chiefly of a hydro-alumino-silicate corresponding to the formula H4Al2Si2O9, and in this they very closely resemble the china clays (kaolins) . The latter are, however, but slightly plastic whilst the ball clays are [65] amongst the most plastic clays known. The china clays are also much more refractory than the ball clays owing to the somewhat larger proportion of alkalies in the latter.
Pipe clays are an inferior quality of ball clay; they contain rather more iron and alkalies and considerably more silica. ”

“ Of all the concretions these are perhaps the commonest, being found in the clays of all types and in many regions. They are made of lime and precipitated around some nucleus of foreign matter. The shapes vary widely, usually discs, flattened ovals or even rings, in most all cases however flattened. This is indicative of the water moving though the clay more freely in some layers than others. Often clay stones occur so abundantly that two or more have grown together making fantastic shapes, sometimes resembling animals, and all sorts of fancied but unrelated objects. ”

“ So long as the adventitious materials consist chiefly of silica and [127] chalk and the mixture is sufficiently plastic to make strong bricks, it may be used satisfactorily in spite of its low content of clay, but if the so-called 'brick clay' contains limestone, either in large grains or nodules, it will be liable to burst the bricks or to produce unsightly 'blow-holes' on their surfaces. If too much sand or other non-plastic material is present, the resulting bricks will be too weak to be satisfactory. ”

“ Some clays, however, such as fireclays, contain very little potash or soda, while they are rich in alumina; and it is a fair inference that hydrated aluminous silicates, such as kaolin, are well represented in these rocks. There are, in fact, a few clays which contain about 45% of alumina, that is to say, more than in pure kaolin. It is probable that these are related to bauxite and certain kinds of laterite.
A few of the most important clay rocks, such as china-clay, brick-clay, red-clay and shale, may be briefly described here.
China-clay is white, friable and earthy. ”

“ If the transporting stream flows through valleys whose sides are formed of limestone, chalk, sandstone or other materials, these will become mixed with the clay, and to so great an extent has the mixing occurred that very few clays occur in a state even approximating to purity. The majority of clays are contaminated with iron oxide, lime compounds and free silica in such a fine state of division that it is impossible to purify them completely without [50] destroying the nature of the clay. ”

“ It is commonly supposed that all clays are plastic, but some of the purest china clays are almost devoid of this property and some of the most impure earths used for brickmaking possess it in a striking degree. Shales, on the one hand—whilst clearly a variety of clay—are hard and rock-like, requiring to be reduced to powder and very thoroughly mixed with water before they become plastic; many impure surface deposits, on the other hand, are so highly plastic as to necessitate the [3] addition of other (sandy) materials before they can be used for the manufacture of bricks and tiles. ”

“ As to the deposits of Pennsylvania and the West, there appears to be considerable difference of opinion, but the existence of clay for making every kind of ware, from drain-pipes to porcelain vases, is beyond all doubt. A partial analysis of Georgia kaolin showed that in the leading ingredients of silica and alumina it approached very nearly the standard given in the table. The whole question appears to be one of analysis, preparation, and experiment, so that when the manufacturers buy clay for a special purpose, they can depend absolutely upon what they obtain. ”

“ E. H. L. Schwarz (35) has suggested that many clays are composed of small globular masses of plates so arranged as to form an open network (fig. 4) which is sufficiently strong not to be destroyed by pressure. In the presence of water and much rubbing the plates are separated and are made to lie flat on each other, thereby giving a plastic and impermeable mass. If this is really the case it would explain the porosity and large surface of some clays and might account for their adsorptive power.
Fig. 4. Illustrating the structure of a 'clay crumb.' (After Schwarz.) ”

“ If the plasticity of some clays is really due to the colloidal nature of their particles, it is obvious that they must have been formed by a process of coagulation or precipitation at a distance from the site of the minerals from which they have been derived. According to the 'colloid theory,' felspar and other alumino-silicates are decomposed by 'weathering,' the chief effect of which is the formation (by hydrolysis) of a colloidal solution of 'clay.' This apparently clear solution flows along in the form of a small streamlet, joins other streamlets and continues its journey. ”

“ If, however, the temperature is raised to about 500° C. a decomposition of the material commences and water [154] is evolved. This water—which is commonly termed 'combined water'—is apparently an essential part of the clay-molecule and when once it has been removed the most important characteristics of the clay are destroyed and cannot be restored. The reactions which occur when clay is heated are complex and are rendered still more difficult to study by the apparent polymerization of the alumina formed. ”

“ The bed which lies immediately below the limestone and greenstone is (as I have already mentioned) a soft, rather sandy slate-clay. Its colour is bluish-brown and greyish white, and some of the strata contain egg-shaped nodules highly impregnated with clay iron, in closing the impressions of marine remains. It is very shivery and easy of decomposition, passing into a tenacious blue clay. On the south of the Severn, along the bottom of Wenlock-edge, it may very distinctly be seen supporting the limestone, and, like this latter, rising west by north at an angle of about 8° ”

“ Ever since the publication of Seger's memorable [148] papers (7) , and to a small extent before that time, it has been generally understood that china clay or kaolin represented the true essential constituent of clays, but several investigators have been so imbued with the idea that all true clay substance must have a crystalline form that they have frequently used the term 'kaolinite' to include the 'amorphous' substance in plastic clays. ”

“ The manufacturer of china-ware and porcelain requires china clay or kaolin which, in addition to the foregoing characteristics, shall be highly refractory. It must, therefore, be free from more than about 2 per cent. of lime, magnesia, soda, potash, titanic acid and other fluxes.
It is a mistake to suppose that all white clays of slight plasticity are china clays or kaolins. Some pipe clays have these characteristics, but they contain so large a proportion of impurities as to be useless for the purposes for which china clay is employed and are consequently of small value. ”

“ Clays which are porous can be dried more readily and with less risk of cracking than those which are more dense. For this reason, some clayworkers mix non-plastic material such as sand or burned clay with their raw material.
The impermeability of plastic clay to water is a characteristic which is important for many purposes. ”