facilitate the escape of the moisture in drying and in the early stages of burning, as well as enable the product to withstand sudden changes of temperature. If sand is added for this purpose, it may act as a flux at high temperatures, and this action will be the more intense the finer its grain.1

Large particles of grog are undesirable, especially if they are angular in form, because in burning the clay shrinks around them, and the sharp edges, serving as a wedge, open cracks in the clay, which may expand to an injurious degree. Large pebbles will do the same, and at many of the common brickyards in the State, the writer has seen numbers of bricks split open during the burning because of some large quartz pebble left in the clay, as the result of improper screening of the tempering sand. For common brick, the type of sand used does not make much differen, as long as it is clean, but if sand is to be added to fire brick mixtures, it should be coarse, clean quartz sand. Burned clay grog is more desirable than sand for high-grade wares, since it does not affect the fusibility of the clay, or swell with an increase of temperature as sand does, but precaution should be taken to burn the clay to its limit of shrinkage before using it.

FUSIBILITY.

The changes occurring in the early stages of burning have already been referred to on pp. 93-96, and in the table of tests there given it was seen that the clay had become steel-hard. The temperature at which this occurs varies with the character of the material, impure, easily fusible clays becoming so at a low temperature, such as cone 05, while others, such as kaolins, will not become steel-hard before cone 5 or possibly 8.2

The attainment of a steel-hard condition represents the beginning of fusion, not of the whole mass, but of some of the more fusible elements in the clay, the result of this preliminary soften

1

1 See Chapter on Fire Clays and Fire-Brick Industry.

2

The cones referred to are small pyramids of definite chemical composition and a theoretic fixed fusion point. Their exact nature and method of use are explained on p. 101.

ing being to stick the grains together. This is termed incipient fusion, but the softening has not been sufficient to prevent identification of the coarser grains in the clay. With a further variable increase in the temperature, depending in amount on the clay, and ranging from 27.7° C. (50° F.) to 111.1° C. (200° F.) or sometimes even more, an additional amount of shrinkage occurs, and most of the particles become sufficiently soft to allow them to settle into a compact impervious mass, thus closing up all the pores in the clay. This condition is termed vitrification, and a piece of vitrified clay when broken shows a very smooth fracture and sometimes a slight luster, since all the particles except the coarse quartz grains have been welded into a dense solid mass. This condition, since it represents one of the closest compactness of the clay particles, also represents the maximum of shrinkage. If the heat is raised still further the clay softens so that it can no longer hold its shape and flows or gets viscous.

We can, therefore, recognize three stages in the burning of a clay,1 viz.:

Incipient fusion.

Vitrification.

Viscosity.

It is sometimes difficult to recognize precisely the exact attainment of these three conditions, for the clay may soften so slowly that the change from one to the other is very gradual.

The difference in temperature between the points of incipient fusion and viscosity varies with the composition of the clay. In many calcareous clays these points are within 27.7° C. (50° F.) of each other, while in refractory clays they may be 377.7° C. (700° F.) to 444-4° C. (800° F.) apart. The glass-pot clays, which are refractory, but still burn dense at a comparatively low temperature, approach the last mentioned condition quite closely.

It is of considerable practicable importance to have the points of incipient fusion and viscosity well separated, because in the manufacture of many kinds of clay products the ware must be vitrified or rendered impervious. If, therefore, the temperature interval between the points of vitrification and viscosity is great,

'H. A. Wheeler, Vitrified Paving Brick, p. 12, 1895, Indianapolis.

it will be safer to bring the ware up to a condition of vitrification, without the risk of reaching the temperature of viscosity and melting all the wares in the kiln, because it is impossible to control the kiln temperature within a range of a few degrees. In many clays the point of vitrification seems to be midway between that of incipient fusion and viscosity, but in others it is not.

Temperature of fusion. The temperature at which a clay fuses depends on: 1) the amount of fluxing impurities; 2) the condition of the fluxes; 3) the size of the grains, and 4) the condition of the kiln atmosphere, whether oxydizing or reducing.

1. Other things being equal, the temperature of fusion of a clay will fall with an increase in the percentage of total fluxes. If we compare the analyses of a brick clay and a fire clay we shall find that the analysis of the former shows perhaps 12 or 15 per cent. of fluxing or fusible ingredients, while that of the latter may show only 2 or 3 per cent., and that their fusion points are perhaps 1093° C. (2000° F.) and 1644° C. (3000° F.) respectively. All fluxing impurities do not, however, act with equal energy, some being more active than others.

2. The condition of chemical combination may also affect the result. Thus lime, for example, will induce a fluxing action in clay at a lower temperature if present in the form of carbonate of lime than as silicate of lime.

3. The size of the mineral grains in a clay undoubtedly exerts more effect than some investigators have been willing to admit.1 Other things being equal, a fine-grained clay will fuse at a lower temperature than a coarse-grained one,2 for the reason that when the particles of a clay begin to fuse or flux with each other, this action begins on the surface of the grains and works inward towards the centre. If, therefore, the easily fusible grains are of small size, they fuse more rapidly, and are more effective in their fluxing action than if the grains were large. Since some of the mineral grains in the clay are more refractory than others, the clay in the earlier stages of fusion can be regarded as a mixture

2

'H. O. Hofman, Trans. Amer. Inst. Min. Engrs. XXVIII, p. 440, 1898.

See Chapter XVI, The Fire Clays and Fire-Brick Industry; also paper by H. Ries, Trans. Amer. Inst. Min. Engrs., Feb., 1903.

of fused particles, with a skeleton of unfused ones. If the proportion of the former to the latter is very small there will be a strong hardening of the clay with little shrinkage, and the burned clay will still be porous. With an increase of temperature, and the fusion of more particles, the pores fill up more and more, and the shrinkage goes on until, at the point of vitrification, the spaces are completely filled. Above this point there is no longer a sufficiently strong skeleton to hold the mass together, and the clay begins to flow. The conditions which influence the difference in temperature between vitrification and viscosity still remain to be satisfactorily explained, but it probably depends on the relative amounts of fluxes and nonfluxes and the size of grain of the latter.

4. Finally, it is found that the same clay will fuse at a lower temperature, if in burning it is deprived of oxygen, than it will if burned in an atmosphere containing plenty of the latter.1

Classification of clays based on fusibility.-The fact that different clays fuse at different temperatures makes it possible to divide them into several different groups, the divisions being based on the degree of refractoriness of the material. Such a grouping however is more or less arbitrary, since no sharp natural lines can be drawn between the different groups, and it is to be expected that no grouping proposed will meet with universal approval. The following classification has been adopted in this report:

1. Highly refractory clays, those whose fusing point is above cone 33. Only the best of the so-called No. I fire clays belong to this class.

2. Refractory clays, those whose fusion point ranges from cone 31-33 inclusive. This group includes some of the New Jersey No. 1, as well as some No. 2 fire clays.

3. Semirefractory clays, those whose fusion point lies between cone 27 and 30 inclusive.

4. Clays of low refractoriness, those whose fusion point lies between cone 20 and 26 inclusive.