Alkalies alone seem to exert little or no coloring influence on the burned clay, although in some instances potash seems to deepen the color of a ferruginous clay in burning.

Titanium.

Titanium is an element which is found in several minerals, some of which are more common in clays than is usually imagined, although they appear rare because they are seldom found in large quantities. Among the titanium-bearing minerals, the commonest is rutile, which is an oxide of titanium (TiO2), containing 60 per cent. of metallic titanium and 40 per cent. of oxygen. So far as known, it is never found in clays in sufficiently large grains to be visible to the naked eye, although a microscopic examination may often show its presence in the form of little needles or grains. Its frequent occurrence is, no doubt, due to the fact that it is quite resistant to weathering.

It may be asked, why, if the material is so widespread, the presence of titanium is so rarely shown in an analysis of clay. This is because its determination by chemical methods is attended with more or less difficulty and is rarely carried out. In the ordinary process of chemical analysis it is included with the alumina. Very few state geological surveys, in investigating their clay resources, have made special determination of this mineral, but in an investigation of clays of New Jersey, made in 1877, Prof. Cook found that in 21 clays examined (p. 276),1 it ranged from 1.06 to 1.93 per cent. In a series of Pennsylvania fire clays2 the percentage of titanium oxide ranged from 0.87 per cent. to 4.62 per cent. The clay at Hackensack contained 0.85 per cent., and analyses of a number of fire bricks. from this State gave percentages ranging from 0.85 per cent. up to 4.30 per cent., so that the probable effect of such a widespread substance is well worth investigation.

Unfortunately, but little experimental work seems to have been done along this line, although some years ago, Messrs.

1

Report on the Clays of New Jersey, 1878, Cook and Smock.

2 Second Geological Survey of Pennsylvania, Rept. MM, pp. 261 et seq.

Seger and Cramer, of Berlin, made experiments to determine the effect of this substance in clay. They took two samples of Zettlitz kaolin (which contained 98.5 per cent. of kaolinite or clay substance), and to these were added 6.5 per cent. and 13.3 per cent. of titanium oxide, respectively; both were then heated to a temperature above the fusing point of iron, the result being that the first softened considerably on heating and showed a blue fracture, while the second fused to a deep-blue enamel. A second series of mixtures, consisting each of one hundred parts of kaolin, with 5 per cent. and 10 per cent. of silica, respectively, showed no signs of fusion, and burned simply to a hard white body, thus indicating that the titanium acts as a flux at a lower temperature than quartz.

As these experiments were not sufficiently extensive to be applicable to New Jersey materials, which contain small but persistent quantities of titanium, it was thought desirable to make some mixtures of a white-burning refractory clay, with varying percentages of titanium. The clay employed was a white-burning sedimentary clay from Columbia, South Carolina, the fusing point of which is above that of cone 34. The titanium was added in the form of rutile, which had been very finely ground in a ball mill, most of it being fine enough to remain in suspension for several days.

Series of mixtures for tests on effect of Titanium oxide.

These mixtures were then formed into small cones and tested in the Deville furnace, the results of these tests being shown graphically by the curve in Fig. 25. In this figure the vertical line at the left represents the cone number of the Seger series,1 and the horizontal line at the bottom the per cent. of titanium

'See Fusibility, Chapter IV.

oxide. No. VII, at the extreme left, represents the fusion point of the clay alone, while I, II, etc., indicate, respectively, the fusion points of the clay and titanium mixtures. From this it will be seen that even one-half per cent. of titanium oxide lowered the fusing point of the clay half a cone, while 5 per cent. lowered it two cones. All the mixtures, when heated to cone 27, were apparently vitrified, and showed a deep-blue fracture. This coloration was, however, destroyed by the presence of a few per cent. of silica. At lower temperatures (cone 8), a mixture containing 5 per cent. of titanium oxide burned yellow.

Fig. 25. Curve showing effect of Titanium oxide on fusibility of clay.

The effect produced by replacing silica by titanium oxide in a mixture of silica and kaolin is mentioned on another page.1

Water in Clay.

Under this head are included two kinds of water. 1. Mechanically combined water or moisture. 2. Chemically combined water.

Mechanically combined water.-The mechanically combined. water is that which is held in the pores of the clay by capillary

action, and fills all the spaces between the clay grains. When these are. all small, the clay may absorb and retain a large quantity, because each interspace acts like a capillary tube. If the space exceeds a certain size, they will no longer hold the moisture by capillary action, and the water, if poured on the clay, would fast drain away. The fine-grained clays and sands, for these reasons, show high powers of absorption and retention, while coarse sandy clays or sands represent a condition of minimum absorption. This same phenomenon shows itself in the amount of water required for tempering a clay. Thus, a very coarse sandy clay mixture from near Herbertsville required only 15.9 per cent. of water, while a very fat one from Woodbury took 45 per cent. of water. It is not the highly aluminous ones, however, that always absorb the most water. The total quantity found in different clays varies exceedingly. In some air-dried clays it may be as low as 0.5 per cent., while in those freshly taken from the bank it may reach 30 to 40 per cent., without the clay being very soft.

Water held mechanically in a clay will pass off partly by evaporation in air, but can all be driven off by heating the clay to 100° C. (212° F.). The evaporation of the mechanical water is accompanied by a shrinkage of the mass, which ceases, however, when the particles have all come in contact, and before all the moisture is driven off, because some remains in the pores of the clay. This last portion is driven off during the early stages of burning, and this part of the burning process is referred to as water-smoking or steaming. The shrinkage that takes place when the mechanical water is driven off varies, and ranges from I per cent. or less in very sandy clays up to 10 per cent. or 12 per cent. in very plastic ones.

Since most clays having a high absorption shrink a large amount in drying, there is often danger of their cracking, especially if rapidly dried, owing to the rapid escape of the water vapor. Mechanical water may hurt the clay in other ways. Thus, if the material contains any mineral compounds which are soluble in water, the latter, when added to the clay, will dissolve a portion of them at least. During the drying of the brick, the water rises to the surface to evaporate, and brings

out the compounds in solution, leaving them behind when it vaporizes. It may also help the fire gases to act on certain elements of the clay, a point explained under burning.

Chemically combined water.-Chemically combined water, as its name indicates, is that which exists in the clay in chemical combination with other elements, and which, in most cases, can be driven out only at a temperature ranging from 400° C. (752° F.) to 600° C (1112° F.).1 This combined water may be driven from several minerals, such as kaolinite which contains nearly 14 per cent., white mica or muscovite with 4 per cent. to 51⁄2 per cent., and limonite with 14.5 per cent. Unless a clay contains considerable limonite or hydrous silica, the percentage of combined water is commonly about one-third the percentage of alumina found in the clay. In pure or nearly pure kaolin, there is nearly 14 per cent., and other clays contain varying amounts, ranging from this down to 3 per cent. or 4 per cent., the latter being the quantity found in some very sandy clays. The loss of its combined water is accompanied by a slight, but variable shrinkage in the clay, which reaches its maximum some time after all the volatile matters have been driven off.

Organic Matter.

Under this head are included all fragments of vegetable origin, large and small, found in clay, which have been washed into the water where the clay was being deposited, and settled down with it. In many cases the material has settled down in layers, and these, on account of their coarseness and foreign character, destroy the cohesiveness between the clay particles, causing the clay to split along these planes when taken from the bank.

Indeed, the gray or black color given by organic matter to many clays is so strong as to obscure the presence of other coloring agents, such as iron oxide, so that two clays both colored black, may burn nearly white and red, respectively. The black

1 See Bourry, Treatise on Ceramic Industries, p. 103, also W. M. Kennedy Transactions American Ceramic Society, Vol. IV, p. 146, and, further, experiments under Fire Shrinkage (Chap. IV).