"We have on our property 36-foot vein of potters' clay, or clay of the semi-fire clay nature. Owing to the ability of this clay to stand intense firing, we commenced mixing it with a variety of clays which we have, the idea being to secure such a mixture of materials as would result in the production of a thoroughly vitrified brick, suitable in every way for the paving of public roadways. After a variety of experiments, we finally secured a mixture of clays which under intense heat made a paving-brick of great strength and toughness." Mr. George S. Tiffany, of Tecumseh, Mich., in speaking of the manufacture of vitrified street paving-brick, said : " I could tell you the best way to make vitrified brick, but the question at the root of the matter is : What clay will make the best pav ing brick? I can only answer that question by saying that the clay must be ascertained by experiment and trial. The differ ences in clays are just as infinite as the characters of men. Of fire-clays there are a vast number of varieties, some of which clays possess scarcely any refractory power. There are clays that are called common clays that are superior in refractory power to some clays that are called fire-clays. A clay that will make good sewer pipe is good clay from which to make vitrified brick. In the process of vitrification, you want to stop just before the process is completed. At that point it is liable to go into the molten state and your whole kiln come down upon your hands. You must ascertain by the most careful ex periments, if you attempt to make vitrified paving-brick, how high you can set your brick, or else you will endanger your whole kiln and lose thousands of dollars by foolish trials.
"There are many of our surface clays that are simply de composed shale which has become plastic clay under atmos pheric action. Such clays take the salt-glaze as a general thing best. They have the proper elements, containing suffi cient silica and iron, and they are free from infusorial earth, lime, etc. Take a clay that will rapidly effervesce under the action of acids, and you cannot get any kind of a glaze upon it, and I suggest the acid test to you before you make any other ex periments. You can also judge something by observation of the effect of fire upon your own kilns of brick, whether you have any prospect of making paving-brick. If the heads of your arches —if the under brick, the key brick are glazed, that is an en couraging sign. There was a friend of mine asked me not long since to visit his yard. He wanted to talk with me about put ting in a stiff-mud machine to make paving-brick. I went to see him. Of course I wanted to see his kilns and his clays, es pecially his clays. I went out to the yard and we examined the burned brick in the kiln. They were removing the brick and he took out a brick from the arch and showed it to me. One end of that brick was burned to a good hard heat with no sign of vitrification whatever.
"The other end of that brick was burned and shriveled out of shape. Now, there was not a point between the two ends that had the elements of a vitrified brick that was in a fit condition as a body for a paving-brick. There are a thousand such cases
all over the country. It is absurd for any one to attempt to make paving-brick from clays of that kind." Brick for roadway paving, as will be seen by the foregoing description of the clay to be used in their manufacture, are en tirely different from common brick. Clay for such brick should be able, without fusing, to withstand a sufficient degree of heat and for a long time, to render the brick hard and im pervious to water.
Lime, magnesia and the alkalies in the clay render it fusible, . and they are to be avoided : iron, beyond 6 to to per cent., if the clay is silicious, also renders it fusible. Silica and alumina constitute the refractory parts of brick, and the other parts named, the fluxing part.
From the experience of the leading manufactures of paving brick, it is concluded that fineness of grain is most essential. When fine-grained clay has a hard, compact structure, no pains should be spread to reduce it in a dry state to an impalpable powder.
When the clay is neither dry nor fine, but of desired com position, it may be calcined and then finely ground.
The clay used at Galesburg, Ill., is of a shale formation and is known as soapstone. It lies near the surface of the hills, and varies in depth from fifty to nearly three hundred feet. It is almost free from grit, and when pulverized and moistened it is sticky or doughy in the fingers. A lump taken fresh from the bank without crushing or drying takes water very slowly, and if rubbed on a smooth surface before drying will present a slick, glassy or oily surface.
The Diamond Brick Company, of Kansas City, Mo., exploits a clay similar to the clays above described, except that it is found at a depth of from fifty to two hundred feet below the surface, and is therefore mined by driving entries and drifts the same as coal is mined. The texture of this clay is very fine. It is almost free from grit, and can be polished with a smooth surface when taken fresh from the bank. In the green state, or fresh from the bank, it is darker than that found at Galesburg.
The clay at Atchison, Kan., is also classed among the soap stone clays, and is very similar to the three above-mentioned, except that it runs from a light gray to a dark gray in the bank. It also has a slight trace of sand or grit, and does not polish quite so smooth when green. It vitrifies at a high de gree of heat, and stands up well under fire.
At Sioux City, Iowa, the clay is also found in very high bluffs. Viewed from a distance, it resembles more the appear ante of large or thick layers of stone than either of the former clays. The physicial appearance of these clays is different from all other clays that we have seen. In its natural state it is very hard and dense. Has quite a per cent. of sand, also streaks of a semi-fire-clay. The clay, when dry, becomes very hard ; it does not have so much of the oily or soapy appear ance. It requires very heavy machinery to handle it properly, and is one of the hardest clays to burn I have seen. It stands well under the fire and makes a very superior paving-brick.