The skeleton spoken of above may be composed of more refractory material than the remainder of the brick or may simply be made up of coarser particles of the same material as the rest, but in the latter case the range of vitrifaction would usually not be very wide.
If a given clay does not fulfill the conditions named above the defect must be remedied by mixing it thoroughly with more or less refractory material or by coarser or finer grinding of a portion, as the case may demand.
If a clay does fulfill the conditions in a measure, but the range of vitri faction is narrow, this may often be remedied by proper mixing or handling.
If all or a large proportion of the particles are so very fine that the clay settles into a compact, almost impervious mass so that it is difficult to dry and cracks badly, some coarser material must be introduced or some method of handling devised to open the structure of the mass and allow the water to pass out readily. When, as is not infrequently the case, this fine-grained, compact mass shows a decided tendency to split up into cubes instead of disintegrating under the treatment ordinarily given to clays, some special plan such as mixing into a thin paste with water and allowing it to settle, or more thorough grinding and mixing than is usually resorted to will have to form part of the preparation in order that the coarser material may be thoroughly incorporated, or, ex posing the clay to frost during the winter must be resorted to in order to break up the strong tendency to joint. Experience has shown that this tendency can be overcome by any of the methods mentioned, but as they are all too expensive to be used in general practice, some other plan of handling these clays must be devised.* No attempt has been made in the foregoing paragraphs to enumerate all the difficulties that may arise in the preparation of materials for the manufacture of paving brick, but perhaps we have gone far enough to indicate what was meant by the claim that pavers of excellent quality may be made from clays which are not now considered available, if proper admixtures are made and the materials are correctly handled.
Whether it will pay to do these things on a commercial basis is a matter which local conditions alone can decide in each case.
I repeat at this point what I said earlier in this section. We know very little about details in the pyro-chemical behavior of clays. It is a matter which we hope to see taken up in the near future and when this is done I have no doubt that many of the things we now think we know will have to be modified, but although this is true, such an investigation should point the way to the solution of many of the problems which now disturb us.
In the foregoing pages I have tried to show that in the present state of our knowledge it is impossible to answer the question "what is a paving brick clay ?" I have endeavored to show that no special kind of clay is required for the manufacture of pavers, but its fitness for such use is more a question of physical structure than of chemical composition; at the same time it must be realized that certain mineral substances when present in larger quantities unfit a clay for this purpose. I have also tried to suggest the possibility (which we know in certain cases to be a fact) that, while suitable structure has been given to certain shales by nature, we may devise ways by which this lack of structure may be com pensated in clays that do not now possess it, using means so inexpensive as to make the operations commercially successful.
We are now in a position to hurriedly trace the geological history of our State with a view to ascertaining where deposits of those materials which are most likely to be useful to the manufacturer of vitrified brick may be found. We hope the time will come when we can speak definitely of the properties of these deposits, but until a survey of the clays of our State, such as is now in progress in Ohio, shall be made, we must content ourselves with indicating where suitable deposits are likely to .be found.