It is true, however, that some of the very best pavers are manufactured from clays of other ages and recent investigations seem to indicate that most, if not all, clays when properly handled, can be made into pavers of excellent quality. The Holland and Oldenburg clinkers, excellent pavers, are made from glacial clays.
What is Vitrification?—It is well known that clay as it comes from the machine contains a greater or less, but always a considerable, proportion of mechanical water, and that if the clay has been properly prepared this will all pass off in the dryer or during the first water-smoking without inducing any chemical change whatever. We also know that it contains a considerable percentage of combined water or water of hydration which is driven off during the second water-smoking, and that this process leaves the clay anhydrous, nonplastic, and incapable of recovering these properties by any known process. It is also known that after the clay has lost its water, if the temperature of the kiln is sufficiently high, it will shrink slightly, become less porous, and change its structure en tirely. Instead of the granular or but loosely coherent structure shown by the surface of fracture immediately after the conclusion of the water smoking process, we now have a compact glassy or felsitic, all but im pervious, substance which plainly indicates that the body has undergone profound chemical changes that have transformed it from an earthy into a stony material. It is to the sum of these changes that we apply the term vitrification.
We do not yet fully understand their nature, but we think we know some things about them. We believe that the increasing temperature of the kiln induces chemical changes and solid solutions in the clay in virtue of which more easily fusible compounds and mixtures are formed; that when these compounds reach incipient or actual fusion they induce similar changes in other ingredients of the clay which would not undergo chemical transformation at that temperature if it were not for the pres ence of the first formed compounds. When these last compounds reach partial or complete fusion they react on yet more refractory ingredients of the clay, and so on.
When fusion occurs in these compounds they increase in volume and become slightly blebby, they also tend to settle together, and by a com bination of these two processes the pores are filled, the structure becomes more impervious, and the material harder.
Distinction between vitrifaction and fusion.—If a mass made up of a single chemical compound, like pure kaolin or any one of the group of similar substances which are found in clay, be ground to uniform fine ness, moulded, dried, and gradually heated, it will pass through all the changes enumerated above; but as all the particles are of like size and have the same composition each will be affected in the same way by heat, and, consequently, each will pass into incipient fusion at the same tem perature so that the whole mass will soften at approximately the same time and become deformed. In such a mass the temperature of vitrifac tion and that of fusion will usually be so close together that it will be difficult so to control the fires as to produce complete vitrifaction without deformation of the brick.
On the other hand, if, as is usually the case with clays, the mass is made up of several compounds each having a different fusion point and which are also capable of uniting at certain temperatures to form new compounds which have yet other fusing points, the temperature at which incipient fusion or vitrifaction begins and that at which the whole mass softens may be widely different, and will usually be more widely separ ated than is the case with a pure substance.
Again, if the original mass contained both coarse and fine particles the finer particles will be first attacked, then those which are coarser and so on until the largest are involved and in this way the range of vitrifaction will be widened.
Conditions which are essential in a paving brick clay.—From what has preceded we can see that a clay from which paving brick are to be man ufactured must be a mixture of more and less fusible materials, the more refractory particles forming a skeleton which holds the mass in shape, while the more readily vitrifiable portions undergo chemical changes, form solid solutions, become vesicular, and draw together until the pore spaces are partially or wholly filled and the mass becomes relatively impervious.