In this chapter we shall fulfil our promise (ante p. 58) to explain in an elementary manner the precise meaning of ordinary commercial chemical analyses of some typical earths used in brickmaking, etc. We may commence by explaining a few terms used by the chemist. An atom is the smallest imaginable portion of matter, and all matter is said to consist of atoms. A molecule is the smallest conceivable combination of atoms, and every compound substance is ultimately built up of molecules. An element is a substance that has hitherto defied the efforts of the chemist to subdivide or split up. Over seventy of these elementary substances are at present known, and their number is being constantly added to. Again, by improvement in analytical methods, a so-called element may be subdivided, and thus removed from the list. The elements are classified into metals and non-metals; and it is convenient to give each of them a symbol to save trouble in writing, and to render clearer to the reader the chemical nature of a compound body. Thus, the symbol for the element aluminium is Al; for silicon Si; for carbon C; for calcium Ca; for oxygen O; for iron Fe; for hydrogen H; for chlorine Cl; and so on.
We are taught by chemistry that elements are capable of combining only in definite proportions, and that each substance possesses a definite proportion peculiar to itself. That proportion is called the atomic weight of the element; or, it is the relative weight of the atom of each76 substance compared with that of the lightest substance known, hydrogen.
Thus, the atomic weight of hydrogen being taken as 1, it is found that an atom of chlorine is 35.5 times as heavy as that, so that the atomic weight of chlorine is said to be 35.5. Now, in spite of the enormous difference between the weight of the two elements just mentioned, they combine in the same proportions by volume; and the union is known as hydrochloric acid, or HCl.
But in certain cases elements do not combine in equal proportions; for instance, an atom of oxygen will not combine with less than two of hydrogen. Further, with this we find that the three volumes are condensed into the space of two volumes—a very common phenomenon in the chemical combination of gases. The union of hydrogen and oxygen alluded to forms water, the chemical symbol of which is, consequently, H2O.
Chemical affinity, or chemical attraction, is the force which is exerted between molecules not of the same kind. Thus, in water, which, as we have seen, is composed of hydrogen and oxygen, it is affinity which unites these elements, but it is cohesion which binds together two molecules of water. In compound bodies, cohesion and affinity operate simultaneously; whilst in simple bodies, or elements, cohesion alone has to be considered. To affinity are due all the phenomena of combustion and of chemical combination and decomposition.
Certain gases, such as chlorine and nitrogen, and such substances as sulphur, carbon, and silicon, with many others, form acids in conjunction with hydrogen, or hydrogen and oxygen. These combine with greater or less facility with other elements which do not form acids, and are termed bases. A combination of an acid and a base is known as a salt. Salts the names of which end in77 -ide, such as chloride, sulphide, etc., are combinations of a metal with a non-metal. Monoxide means an oxide containing one atom of oxygen; dioxide one containing two atoms; protoxide means the first oxide, because it is the first or lowest of the oxides of the given metal in amount of oxygen present; the highest oxide is often known as peroxide. The terminations -ous and -ic are frequently used for the lower and higher oxides respectively. Examples:—
FeO, iron protoxide, or ferrous oxide.
Fe_{2}O_{3}, iron sesquioxide, or ferric oxide.
FeS_{2}, iron disulphide.
Sb2S3, antimony trisulphide.
The following symbols may be indicated as referring to compounds especially met with in brick-earths:—
CaO, lime, instead of calcium oxide.
Al_{2}O_{3}, alumina, instead of aluminium trioxide.
SiO_{2}, silica, instead of silicon dioxide.
Na_{2}O, soda, instead of sodium oxide.
K_{2}O, potash, instead of potassium oxide.
MgO, magnesia, instead of magnesium oxide.
In analysing a body, the first step consists in determining the nature of the elementary substances contained therein. That may be accomplished in the dry way by means of the blowpipe and accessories, as explained in the last chapter. Such an examination, as previously remarked, is known as a qualitative analysis. Or, it may be accomplished in the wet way by ordinary chemical examination. The next step is to determine the amount of the constituents present, and that is known as a quantitative analysis. In making a qualitative analysis, the chemist is assisted by the knowledge that certain basic substances and certain acids produce peculiar78 phenomena in the presence of known substances or preparations termed reagents.
There is a great difference between a chemical compound and a simple mixture of elements; and it is not always easy (e.g., some alloys) to say whether a substance is in the one state or the other. This distinction is well exemplified by the air we breathe. The chemist finds by analysis that the air is nearly constant in composition, containing essentially in 100 parts 76.8 by weight of nitrogen (including about 1 per cent. of the recently-discovered element, argon), and 23.2 of oxygen. Small proportions of water vapour, carbon dioxide, etc., may be ignored for our present purposes. In view of this comparatively uniform composition, the question at first arises as to whether the air is, or is not, a chemical compound? The answer is in the negative, for, amongst other things, it can be shown that the ratio of 76.8 to 23.2 is not that of the atomic weights of the two elements present, viz., 14 : 16, nor of any simple multiples of these.
We will now quote a few analyses of well-known earths, and explain each in turn:
Chemical Composition of China-clays.8
  Kaolin. Kaolin average. Sandy Kaolin.
Silica 46.32   44.60 66.68
Alumina 39.74   44.30 26.08
Iron oxide .27   .20 1.26
Lime .36} 1.60 .84
Magnesia .44} trace
Water 12.67   8.74 5.14
79 The kaolin alluded to in the first column is a remarkably pure material, perfectly white, and contains an enormous quantity of water. It refers to one of the finest washed china-clays in the market, and is extensively used in porcelain manufacture. It is quoted here principally to give an idea of what a really pure clay is like chemically. We notice that, in spite of its relative purity, it contains .27 per cent. of iron oxide. This could have been well done without, from the manufacturer’s standpoint, but is of course a very minute proportion. Small as it is, it must exert a slight amount of colouring influence. The lime and magnesia are present in slightly larger proportions, and a little more of either would be advantageous rather than otherwise, as assisting to flux the material. This is an earth with which practically anything may be done by judicious blending and careful preparation.
With reference to the second column, the figures do not refer to any particular clay, but they have been compiled to show the average composition of kaolins as used in the market. It will be observed that the silica and alumina are present in approximately equal proportions, which is a characteristic of fairly good china-clays. The iron oxide remains as before, but there is a larger proportion of lime and magnesia—as much as can be permitted except in a second-rate clay.
The evidence of the third column shows that the sand in the china-clay is to a large extent quartzose, and this is at the expense of the alumina. Such a material would be suitable for making a species of white fire-brick, and it might do for the commoner kinds of china-ware. The earth is really of the nature of a loam—a sandy clay. There is too much iron in it for the production80 of perfectly white goods. The proportion of lime might be increased to advantage.
Chemical Composition of Fire-clays from Newcastle-on-Tyne.9
  1 2 3 4 5 6 7
Silica 51.10 47.55 48.55 51.11 71.28 83.29 69.25
Alumina 31.35 29.50 30.25 30.40 17.75 8.10 17.90
Iron oxide 4.63 9.13 4.06 4.91 }2.43 1.88 2.97
Lime 1.46 1.34 1.66 1.76 }1.30
Magnesia 1.54 .71 1.91 trace 2.30 2.99
Water, etc. 10.47 12.01 10.67 12.29 6.94 3.64 7.58
The reader will see at a glance that the range of variation permissible in fire-clays is very wide. These earths are all found close together, and are utilised for similar purposes, though often blended to produce desired results. It will be noticed that one of them (No. 6) contains as much as 83.29 of silica, whilst another has no more than 47.55 per cent. The range with reference to alumina is very wide also, from 8.10 percent. (No. 6) to 31.35. The refractory character of any sample of fire-clay is determined by the proportions in which the silica and alumina are contained, and by the absence of lime, iron, and other easily fluxible substances. The proportion of iron discovered in sample No. 2 is certainly much in excess of the requirements of the material, as a fire-clay, and this no doubt is tempered by admixture, unless utilised for inferior goods. The iron oxide in the other samples is about sufficient for general purposes. The amount of lime present in all the samples constitutes a good feature; much lime cannot on any account be allowed in earths for fire-clay goods. With so much iron present, and81 the fair proportions of magnesia (except in sample No. 4) these clays may be regarded as typical, with the exception of No. 6. They have been utilised for many years in the manufacture of fire-bricks and the like.
Chemical Composition of Fire-clays, from Welsh localities.
  1 2 3
Silica 50.35 56.90 54.80
Alumina 23.50 24.90 27.60
Iron oxide 10.40 2.83 2.56
Soda 1.55 3.00 2.00
Magnesia 1.45 1.07 1.00
Water, etc. 11.85 11.60 11.80
The first thing that will strike the reader on looking at these results on Welsh materials, is their uniform composition as compared with the clays from Newcastle. Yet there is as much as 10.40 per cent. of iron in sample No. 1, which cannot be a first-rate clay. Its proportion of silica to alumina is, however, excellent, and, as in sample No. 3, the amount of soda and magnesia is not excessive. The soda in sample No. 2 (which acts somewhat like lime in the kiln) taken together with the magnesia and iron in the same material, is too much for a first-class clay, and would have to be suitably modified before good results could be obtained. On the whole, it is possible that sample No. 3 would yield the best results from the chemical standpoint.
We should not forget that remarkable substance of which the well-known Dinas bricks are made. The proportion of silica present ranges from about 96 to 99 per cent., the remainder consisting principally of alumina, though traces of iron, lime, and magnesia frequently occur. There is not, of course, sufficient natural flux82 for this “clay,” so a small proportion (2.5 to 3 per cent.) of lime is added, which produces the desired effect. In other words, if we can obtain a pure siliceous sand, with hardly any lime, iron or magnesia in it, we have the material of which the better kinds of fire-bricks are made. Such sandy earths are not uncommon in the South of England, but strange to relate, they are not used for the purpose indicated.
The earths from which the superior Stourbridge bricks are made, are approximately of the following chemical composition:—Silica, 64.10; alumina, 23.15; iron oxide, 1.85; magnesia, .95; water and loss, 10.00 per cent. It will be observed that the proportion of iron and magnesia here is very small, whilst lime is altogether absent. It is a most excellent earth for the purposes for which it is used, and the chemical results may be taken as a standard for that class of material. Another Stourbridge earth yields as much as 4.14 per cent. of iron, however, whilst its proportion of silica is lower, 51.80, and alumina higher, 30.40, which serves to remind us of the variability of even good earths used in the manufacture of fire-clay goods.
Let us now turn to the consideration of pottery clays, of which the following results may be taken as typical:—
Chemical Composition of Pottery Clays.
  1 2 3
Silica 46.38 49.44 58.07
Alumina 38.04 34.26 27.38
Iron oxide 1.04 7.74 3.30
Lime 1.20 1.48 .50
Magnesia trace 1.94 trace
Water 13.57 5.14 10.30
83 Some of the chief qualifications, from a chemical point of view, of earths suitable for making pottery, is the proportion and potentiality of the colouring matters present. Where the pottery is to be glazed, that is not so important; but with ordinary unglazed ware, colour and uniformity are two highly essential desiderata. We know that the temperature employed will modify the tint, but under similar conditions the clays alluded to in the above table will give, approximately, the following results. Sample No. 1 is typical of an excellent blue pottery clay, which burns white. It contains more alumina than is commonly met with in such materials, in which respect it differs markedly also from the fire-clays just described. The proportion of oxide of iron is very small, not sufficient to perceptibly colour the finished product, though, no doubt, on careful examination it would be seen not to be perfectly white. The latitude of the term “white” is pretty considerable with clayworkers, as the reader is probably aware.
The pottery clay (also used for bricks) referred to in the middle column, is brown in colour; it is an ordinary kind, used primarily for black and common red ware. The proportion of iron is high, and considerable quantities of both lime and magnesia exist. As might naturally be expected of such material, it will not bear exposure to great heat, though that might be regarded as a qualification in some brick and pottery yards.
The proportion of silica is high in sample No. 3, which appertains to a common yellow clay, with, possibly, some siliceous sand in it. The amount of alumina is correspondingly low, but the iron oxide is not excessive—for a common pottery clay. It is used for the manufacture of coarse ware, and burns yellow.
The chemical composition of earths used for terra-cotta84 and bricks of that substance is so variable, that without going into each case specifically it would be impossible to convey an adequate idea. It may be stated generally that it is not one whit less important to consider the composition of the raw earths for ordinary brickmaking, than in respect of that for high-class bricks and pottery.
An excellent earth, from the neighbourhood of Ruabon, is of the following composition:—
Chemical Composition of Ruabon Clay.
Silica 63.00
Alumina 20.10
Sesquioxide of iron 4.84
Protoxide of iron 1.51
Potash 2.37
Soda 3.10
Combined water 3.54
Moisture 1.54
The proportion of silica in this is higher than in many clays used for brick- or terra-cotta making, but the alkalis, potash and soda, are in strong force, so that any refractoriness on the part of the silica is soon sub............