It is conveniently situated between the large and wealthy cities of New York and Philadelphia, and being contiguous to the seaboard, and in easy communication by rail with all the developing cities of the country, this section should become to us what the Staffordshire district is to England.
The color of the rich New Jersey clay, denuded of the soil and often exposed, varies in shade from a light cream-color, al most white, to a soft buff, and sometimes the clay will be of a dark-red color owing to the abundant presence of the oxide of iron, a very light trace of which impregnates all the clay in the circumjacent region.
The red clays containing oxide of iron in abundance are used only when it is desired to give the terra-cotta a deep-red brick color, which is sometimes done for friezes, panels, tiles, and other architectural requirements.
For a long distance the way between Woodbridge, Perth Amboy and New Brunswick is marked by many hollows and excavations which are sometimes of great depth. From the bottom of these, winding wagon roads lead through banks of clay in which large gangs of laborers are regularly at work dig ging material to be used in the production of terra-cotta and fire-brick, and removing that which is unsuitable for these pur poses.
The surface of the country is undulating, and is but thinly settled, and often a heavy growth of birches, maples, and young pines spreads over it, giving no indications of the riches it con ceals, for underlying it is one vast bed of terra-cotta clay, which for fineness of texture and plasticity has no equal in the world.
In applying the term plasticity to this clay, we do not mean it in the common acceptance of that term ; but in addition to the quality of receiving and giving form, that also of retaining it, not only while it is being moulded, but in that most trying time to all clays, which is the period that it is yielding its chemical water, " going through the sweat." It may not be generally known that all things made of moulded clay, although they may appear to be perfectly dry when they go into the kiln, again become softer and almost as plastic as they were when first moulded, and it is this stage of burning that is so destructive to form in the production of ar tistic and architectural terra-cotta. In describing this critical period in burning, we have used the common parlance of the laborers employed about kilns, for two reasons, the first being that there is no technical term applicable to the same condition of things, and the second is that " going through the sweat" is a most accurate and literal description. Should the adobes or sun-dried bricks of Egypt, which have been exposed to the in fluences of that moisture-extracting climate for more than three thousand years, be placed in a kiln and burned, the result would be the same ; they would " go through the sweat" and become soft and plastic before they were burned into hard bricks.
The mechanical water has been extracted from them, but the chemical water contained in the clay has never been driven out by burning. The adobe before burning could be soaked in water, and in a few hours it would be just as plastic as it was when first made, thousand of years ago, but after burning its plasticity is forever lost.
The vitrifying ingredients usually added to the terra-cotta clays are pure white sand, old pottery, and fire-bricks finely pulverized, and clay previously burned, termed " grog ;" these are employed in various proportions, sometimes amounting to nearly thirty per cent. of the mass.
The alkaline salts contained in the clays yield an efflores cence, which acting upon the silicates of the surface, vitrify to a greater degree the exterior of the terra-cotta, and this harder face should remain intact, and under no avoidable circum stances be allowed to be chipped, chiseled, or broken.
Almost any clay that will harden under the action of fire without cracking, providing it is free from stones, will make good common building brick ; but it is not so with the making of terra-cotta.
Terra-cotta being made of hollow and larger forms than common brick, demands a material which has a minimum shrinkage in the process of drying and burning. Where the contraction is too great, the liability to crack in drying or to fire check in burning, is increased to such an extent as often to make the cost exceed the value of production.
For this reason the expert manufacturer will seek for such clays orscombinations of clays as will contract or shrink as lit tle as possible, in reaching the condition of hardness and text ure which constitutes good terra-cotta.
The best standard texture and hardness for architectural terra-cotta is any good sandstone—test with sharp steel.
Terra-cotta should be gritty in texture and slightly porous. If it is vitreous, it will also be brittle. A small confined piece may sustain a great weight, but a large exposed portion is li able to fracture under climatic extremes when heavily weighted in part.
The slightly porous quality enables it to absorb enough water from the mortar in which it is set so as to remain where the mason places it. If vitreous instead of porous, it is liable to be slipped out of place before the mortar has sufficiently set of itself to hold it. It is a safe rule to use clays that will shrink only one-eighth in bulk during the conversion from plastic clay to terra-cotta, but this ratio of shrinkage must always be ac companied by such a tenacity or strength as will serve to hold all its particles together during the process, so that there may be no cracks or flaws in the final product.