Reference was made to the attempt by Hoffman and Desmond to test the refractoriness of clay by toning up or down as the case required. The only reason that they failed, aside from the fact that they were not taking note of the ultimate composition of the clays, was the unequal degree of homogeneity of mixture of the fusing components. Their method would have failed even had the clays and flux been ground and mixed as thoroughly as is possible by any physical means so far devised. If they had desired to be extravagant of time and fuel they could have caused their mixtures to fuse at the arbitrarily chosen temperatures, or even lower. They, however, were not seeking to determine the true melting point of their mixture but rather its refractoriness. If they had been seeking the true melting point they could have resorted to the customary method of noting the point at which the fused mass solidified.
Refractoriness of a clay is its ability to withstand heat treatment. The relation between refractoriness and true melting point of a clay is as difficult to trace as the relation between refractoriness and ultimate chemical composition—if, indeed, it is not more difficult. This is due principably to the character of the mineral aggregate contained in the clay.
In the case of shales, the same is true to a very much more marked degree. In the shales the rate and final attainment of fusion is af fected so largely by the character of the . mineral aggregates that we find clays which are serviceable for paving brick manufacture differing very greatly in physical properties. It is for this reason in large part that coarse-grained clays vitrify more closely and form stronger bricks. In fact, the writer does not know of a single factory in which paving brick is manufactured from fine-grained clays, although in the labora tory several fine-grained clays have given promising results. If there is a preponderance of stable, not easily fusible minerals present, there is no reason, so far as the pyro-chemical properties are concerned, why fine-grained clays cannot be used in the manufacture of paving brick.
Volatile Matter—Chemically combined water, carbonic acid gas, car bon, etc., do not of themselves, on expulsion, cause thermo-physical and chemical reactions to take place between the stable bases, acids and sili cate compounds left behind, but their expulsion does involve changes in, physical and, in some senses, chemical conditions that provoke thermal reactions between the remaining substances. For example, in terra-cotta lumber, sawdust is added, so that when it burns out, the mass will be left extremely porous, i. e., not dense, as it would otherwise have been. The sawdust in this instance has been effective in opening the structure of the ware and preventing the particles of clay from coming within fluxing distance of one another as they otherwise would.
What is true in the case of the sawdust in terra-cotta lumber is true of combustible organic matter in clays. It is obvious, however, that the influence of carbon in this connection depends to a very large degree on the. size of the carbon particles.
The effect of the expulsion of CO, from such compounds as ferrous carbonate, calcium carbonate, etc., on the thermo-chemical behavior of clays, is another familiar phenomenon, the importance of which is not recognized in the attempt to interpret the results of an ultimate chem ical analysis. If two equal portions of the same clay are taken, and to the one a quantity of red iron oxide (Fe203), while to the other an equivalent quantity of powdered ferrous carbonate (FeCO3) is added, and the two mixtures burned under the same thermal conditions, it will be found that the mixture containing the ferrous carbonate will begin to fuse earlier, exhibit a more erratic rate of decrease in specific gravity as the intensity of the heat increases, and may or may not, depending upon conditions other than those here considered, cause an earlier ulti mate fusion. The same is true to a greater or less extent in the relative fluxing effect of the oxides and carbonates of other bases. The same phenomena are also notable in the comparative fluxing effect of such hydrous and anhydrous silicate compounds as raw and calcined kaolin.
Meade' and have shown that mineral mixtures containing alkalies lose when burned as high as 20 per cent of the total alkalies present. Such a loss is bound to affect the fusibility of the mass very considerably. Now we know that the alkalies are less volatile when combined with some constitutents than with others. The amount of alkali volatilized, and hence the effect on the fusibility of the clay, is dependent, therefore, quite largely upon the manner of its combination.
Structure of Ware—Intimacy of contact of the clay grains with one another is probably affected more largely by the manner in which the mass is formed into ware than by any other factor within the power of man to control, save .the grinding of the clay. In dry-pressed bricks the clay particles are not in such close contact with one another as they would be if the ware were formed by the stiff-mud method. In soft mud bricks the excessive amount of water used prevents the clay parti cles from corning into as intimate contact with one another as in the stiff mud manufacture. As a result of these differences in the degree of compactness of the grains, it is found that not only a more easily vitri fied and fused mass is formed, but also that the resultant ware is very much stronger when made by stiff-mud methods. For the same reason this same difference is found between the pressed and jiggered pottery wares.