If

crack in burning, and may also show high shrinkage. kaolinite is the only mineral present containing chemically combined water, the percentage of the latter will be approximately one-third that of the percentage of alumina, but if the clay contains much limonite or hydrous silica the percentage of chemically combined water may be much higher.1

5. Excess of silica. A large excess of silica indicates a sandy clay. If present in the analysis of a fire clay, it indicates low refractoriness.

6. The quantity of organic matter. If this is determined separately, and it is present to the extent of several per cent., it would require slow burning if the clay was dense.

7. The presence of several per cent. of both lime (CaO) and carbon dioxide (CO2) in the clay indicates that it is quite cal

careous.

These are the main points determinable from the ultimate analysis.

Analyses of several different types of clay:

2. Ball clay, Edgar, Florida.

3. Fire clay, Woodbridge, N. J., W. B. Dixon Est.

4. A buff-burning clay, Sayreville, N. J.

1 See analysis of yellow clay from Tilton's yard, Toms River (Loc. 206),

Chapter XIX, Ocean County.

5. A red-burning brick clay, Sayreville, N. J. 6. A calcareous clay, Canandaigua, N. Y.

Rational analysis.'-This method has for its object the determination of the percentage of the different mineral compounds present, such as quartz, feldspar, kaolinite, etc., and gives us a much better conception of the true character of the material. Most kaolins and other high-grade clays consist chiefly of kaolinite, quartz and feldspar, the kaolinite forming the finest particles of the mass, while the balance is quartz, feldspar and perhaps some mica. The finest particles are known as the clay substance, which may be looked upon as having the properties of kaolinite. Now, as each of these three compounds of the kaolin clay substance, quartz and feldspar-have characteristic properties, the kaolin will vary in its behavior according as one or the other of these constituents predominates or tends to increase.

As to the characters of these three, quartz is nearly infusible, nonplastic, has very little shrinkage, and is of low tensile strength; feldspar is easily fusible, and alone has little plasticity; kaolinite is plastic and quite refractory, but shrinks considerably in burning. The mica, if extremely fine, may serve as a flux, and even alone is not refractory. It is less plastic than the kaolinite, and, when the percentage of it does not exceed 1 or 2 per cent., it can be neglected. To illustrate the value of a rational analysis, we can take the following example: Porcelain is made from a mixture of kaolin, quartz and feldspar. Suppose that a manufacturer of porcelain is using à kaolin of the following rational composition:

Clay substance,
Quartz,

Feldspar,

67.82%

30.93%

1.25%

If now to 100 parts of this there are added 50 parts of feldspar, it would give a mixture whose composition is:

1

The method is described in the Manual of Ceramic Calculations, issued by the American Ceramic Society. See also Langenbeck, Chemistry of Pottery, 1895, p. 8.

If, however, it became necessary to substitute for the one in use a new kaolin, which had a composition of:

Clay substance,

Quartz,

Feldspar,

66.33%

15.61%

18.91%

and added the same quantity of it as we did of the old kaolin, it would change the rational analysis of the body to the following proportions:

Clay substance,

Quartz,

Feldspar,

44.22%

10.41%

45.98%

Such an increase of feldspar, as shown by this formula, would greatly increase the fusibility and shrinkage of the mixture; but, knowing the rational composition of the new clay, it would be easy, by making a simple calculation, to ascertain how much quartz and feldspar should be added to bring the mixture back to its normal composition.

The rational composition of a clay can be determined from an ultimate analysis, but the process of analysis and calculation becomes much more complex. The rational analysis is furthermore useful only in connection with mixtures of high-grade clays, in which the variation of the ingredients can only be within comparatively narrow limits. For ordinary purposes the ultimate analysis is of greater value.

Clays may agree closely in their ultimate analysis, and still differ widely in their rational composition.

MINERAL COMPOUNDS IN CLAY AND THEIR CHEMICAL EFFECTS.

All the constituents of clay influence its behavior in one way or another, their effect being often noticeable when only small amounts are present. Their influence can perhaps be best discussed individually.

Silica,1

This is present in clay in two different forms, viz., uncombined as silica or quartz, and in silicates, of which there are several. Of these one of the most important is the mineral kaolinite, which is found in all clays and is termed the clay base or clay substance. The other silicates include feldspar, mica, glauconite, hornblende, garnet, etc. These two modes of occurrence of silica, however, are not always distinguished in the ultimate analysis of a clay, but when this is done, they are commonly designated as "Free" and "Combined silica," the former referring to all silica except that contained in the kaolinite, which is indicated by the latter term. This is an unfortunate custom, for the silica in silicates is properly speaking combined silica, just as much as that contained in the kaolinite. A better practice is to use the term sand to include quartz and silicate minerals, other than kaolinite, which are not decomposable by sulphuric acid. In the majority of analyses, however, the silica from both groups of minerals is expressed collectively as total silica.

The percentage of both quartz and total silica found in clays varies between wide limits, as can be seen from the following examples. Wheeler gives a minimum2 of 5 per cent. in the flint clays, and the sand percentage as 20 per cent. to 43 per cent. in the St. Louis fire clays, and 20 per cent. to 50 per cent. in the loess clays. Twenty-seven samples of Alabama clays analyzed by the writer contained from 5 per cent. to 50 per cent. of insoluble residue mostly quartz. In seventy North Carolina clays1 the insoluble sand ranged from 15.15 per cent. to 70.43 per cent.

3

The following table gives the variation of total silica in several classes of clays, the results being obtained from several hundred analyses:

1

See also description of the minerals quartz, feldspar, kaolinite, and mica above.

With the exception of kaolinite, all of the silica-bearing minerals mentioned above are of rather sandy or silty character, and, therefore, their effect on the plasticity and shrinkage will be similar to that of quartz. In burning the clay, however, the general tendency of all is to affect the shrinkage and also the fusibility of the clay, but their behavior is in the latter respect more individual.

Sand (quartz and silicates) is an important antishrinkage agent, which greatly diminishes the air shrinkage, plasticity and tensile strength of clay, its effect in this respect increasing with the coarseness of the material; clays containing a high percentage of very finely divided sand (silt) may absorb considerable water in mixing, but show a low air shrinkage. The brickmaker recognizes the value of the effects mentioned above and adds sand or loam to his clay, and the potter brings about similar results in his mixture by the use of ground flint.

In considering the effects of sand in the burning of clays, it must be first stated that the quartz and silicates fuse at different temperatures, and each changes its form but little up to its fusion point. A very sandy clay will, therefore, have a low fire shrinkage as long as none of the sand grains fuse, but when fusion. begins a shrinkage of the mass occurs. We should, therefore, expect a low fire shrinkage to continue to a higher temperature in a clay whose sand grains are refractory.

Of the different minerals to be included under sand, the glauconite is the most easily fusible, followed by hornblende and garnet, mica (if very fine grained), feldspar and quartz. The glauconite would, therefore, other things being equal, act as an antishrinkage agent only at low temperatures. Variation in the size of the grain may affect these results, but this point is discussed under Fusibility (Chapter IV).