The economic and technical phases of the clay industry of this State are most important. In a single year (1906) this Commonwealth produced and marketed 1,027,541,000 common brick, valued at $6,586,374, or at the average rate of $6.41 per thousand. It is safe to say that the other clay products of the State were of equal value, so that in a single year nearly $14,000,000 worth of finished product are turned out from our natural clays.

GRADES OF CLAYS.

Of the higher grades we have kaolinite (called also kaolin or china clay), found in Adams, Berks, Carbon, Chester, Clinton, Cumberland, Delaware, Lancaster, Lehigh, Philadelphia and York counties; halloysite, found in Delaware and Northampton; allophane, in Berks, Lebanon and Northampton; and poryphyllite, near Mahanoy city, Schuylkill county. All these are of the distinctly kaolinite group. But it is neither necessary nor desirable to go into a description of all the varieties, the deposits of some of which will be fully developed in many years. Therefore we will only treat of the kaolins, the fire clays, the pottery clays and the brick clays, with brief reference to the slip clays, the brick making shales and slates, and the new method of making sand lime brick.

ORIGIN OF CLAYS.

To many the subject of the origin of clay is as a closed book. When rocks containing silica and alumina decay by the restless and constant action of the air, rain and frost, they crumble into earth or soil. This soil contains alumina, silica, lime, magnesia, iron oxide, potash, soda and other constituents. The soils containing the iron oxide are known as ocherous; and, when there is not too large a percentage of iron (say not more than 35 per cent.), it is employed in painting, under the name of mineral paint. Some of these deposits, gradually hardening by lateral and other pressure, and the weight of years, become shales. Many shales of this Commonwealth are nothing more than indurated clay; and when pulverized, have all the elements necessary for first grade brick. The finer shales, when ground, have the same plasticity as clay, and are often moulded and baked into brick, especially of the vitrified kinds for paving. Shales, however, with much bituminous matter in them, can not be used successfully in brick manufacture.

LOCALITIES OF CLAYS.

Where nearly pure silicious rocks are found, in the same locality kaolin or china clay may be confidently looked for. The clay beds. are naturally and usually at a lower level than the eroded rocks; but this does not invariably apply, for rocks of higher altitude may

have disintegrated entirely away, and the resulting mass be located at a higher point than any silicious or aiuminous rocks of the same neighborhood. This is exemplified in Clinton county, northwestwardly from Renovo, where high grade kaolin clays are found near the crests of some of the mountains. As a rule clays occur in large bodies in broad valleys; also near the base of high hills or mountains. For instance, in the low lands of the Delaware, the Schuyl kill, the Susquehanna, the Allegheny, the Monongahela and the Ohio rivers. Under almost every coal vein or bed there are beds also of clay; some of it of the fire clay variety, and some of it known as residual clay, the latter decidedly gritty, even to the fingers.

Of all the clays mentioned argil (white clay), or potters' clay, of first class quality, has not been located in sufficient quantity in Pennsylvania to prevent its importation from other states and countries. It would be a benefit in several ways to have a systematic expert search for this kind of clay; for it usually ought to be found abundantly in close proximity to the outcroppings of the geologically Silurian age, and the Silurian rocks are found in 26 counties.

Clays of this early origin are usually hard and dense rock, and must be ground for use; and this will apply to the fire clays immediately beneath the carboniferous coal seams.

EXTENT OF CLAY BEDS.

Fire clay beds are numerous and large. The great Pittsburg coal seam, covering in this State, Ohio and West Virginia an area of 17,000 square miles, has brick making material (clays or shales) lying over or under it, and in many places next to it, to more than twice the amount of coal; and this will apply with equal force to the various bituminous coal beds of western Pennsylvania, known as the Nineveh, Dunkard, Jollytown, Washington, Little Washington, Waynesburg B, Waynesburg A, Uniontown, Sewickley, Redstone, Upper Freeport E, Lower Freeport, Kittanning Upper C, Kittanning Middle C, Kittanning Lower B, Clarion, Brookville, and Mt. Savage; one of these clay and shale beds a short distance below the Pittsburg coal being at various places about 100 feet thick. There is no official data to the effect of a thorough testing of these clays for commercial use; probably because most of them would have to be secured by shafting and drifting; but, if any of them were found to be equal or superior to the best imported clays, it might well repay the expense of the investigation. In some parts of Pennsylvania, notably Allegheny, Butler, Clearfield, Clinton, Cumberland, Lawrence and Somerset counties, private enterprise has had much to do with successful fire clay development, and the products have well repaid the outlay. There are hundreds of other beds within our borders that are as yet untouched, and may be for

many years, unless this Commonwealth takes up the subject in the way of economic geology, mineralogy and chemistry, or in the same manner that our sister states of New York, New Jersey and Ohio have done.

It will be interesting to know that in the deepest drill hole in Pennsylvania,-at West Elizabeth, Allegheny county,-where the earth was penetrated a distance of 5,575 feet, the aggregate thickness of the clays, shales or slates, capable of being manufactured into brick, is 4,188 feet; the thickest bed being about 200 feet, and there being a thickness of 125 feet of brick slate from the depth of 5,450 feet to the bottom of the hole. It is not supposed that the vast bulk of these beds will ever be used for brick-making purposes, but the facts are stated that the reader may have some conception of the vast clay deposits of Pennsylvania.

VARIETIES OF CLAYS.

The purest clay found in nature, as has already been intimated, is kaolinite (frequently called kaolin or porcelain clay), composed of silica, alumina and water, and in about the following proportions: Silica, 46.5 per cent.; alumina, 39.5 per cent., and water 14.0 per cent.; total, 100.00 per cent.

Another chemical formula of pure clay has been given as follows: Silica, 46.30 per cent.; alumina, 39.80 per cent.; water, et cetera, 13 90 per cent.; total, 100.00.

The dehydrated pure clay (that is, with the water all driven off), contains silica 57.42 per cent., and alumina 42.58 per cent.; total, 100.00 per cent.

Decomposed kaolinite is properly called kaolin. Pure kaolin according to the foregoing formulae, has not been found, so far as we know, in this State. The nearest approach to it were small beds found in Berks, Carbon, Chester and Schuylkill counties; the first and third in Silurian, and the second and fourth in much younger rocks.

Nearly all kaolins are more or less mixed with free quartz, and generally there is a small percentage of iron oxide; also, lime, potash and soda. A very nearly pure kaolin was found in two localities in Indiana county; one known as "Indianaite" (Cox), made up as follows: Silica, 45.90 per cent.; alumina, 40.39 per cent.; lime, trace; alkalies (potash and soda), 0.40 per cent.; water, etc., 13.26 per cent.; total, 99.86 per cent. Also Indianaite, from the property of the Pennsylvania Salt Company, as follows: Silica, 39.35 per cent.; alumina, 36.35 per cent.; lime, 0.40 per cent.; water, etc., 22.90 per cent.; total, 99.00 per cent.; The product of each deposit was almost snow white. All the kaolins mentioned above could have been used successfully in the manufacture of the best porcelain or china

ware; but, as already stated, the beds were small, and nearly all of them have been worked out, or so nearly, that Pennsylvania manufacturers have been compelled to go to other states for their supply.

CLAY CONSTITUENTS.

As heretofore intimated, silica is present in all clays,-it is a necessary constituent. Silica appears in clays in three forms,-first, as quartz; secondly, that which is combined with the alumina and water as in kaolinite; and, thirdly, that which is combined in silicate minerals, with one or more bases. In the second combination it is sometimes as soft seemingly as putty, and frequently it is not altogether indurated (hardened) in the second type. Quartz appears to some extent in all clays; and in some examinations of clays the first and third varieties are combined as sand. This examination is made in what is called a rational analysis, to distinguish the two from the second type.

Alumina is an absolutely necessary element in clays. In importance it comes next to silica, water or moisture taking third place. Alumina is not only a necessary factor in all clays; but the presence of a large amount of it has almost the same effect as lime in destroying the red color or iron. An excess of alumina in connection with lime and magnesia leads to a bleaching action of the iron, and a buff tint is the product.

Clays contain two kinds of water; the first known as hygroscopic or moisture; and the second is chemically combined water. One variety of moisture is held in the pores of the clay by capillary attraction, and the other adheres to the surface of each clay grain in a very thin filmy manner. The first is the most important on account of its connection with the shrinkage and plasticity of clays. The moisture of clays reaches from thoroughly air dried of of 1 per cent. to 30 or 40 per cent. when freshly taken from a clay deposit. Clays shrink or lose in weight, as a rule, about in proportion to the amount of water driven off. Those most highly plastic (having the power of cohering without other constituents) generally show the highest shrinkage. Clays non-plastic also have large percentages of water. All clays have combined water ranging from 3 per cent. to 14 per cent. The plasticity in plastic clays is not altogether due to water or moisture; but the property comes partly from silica fractionally indurated or hardened; alumina; and often in connection with the alkalies. With these constituents, and the thin plates of kaolinite collected in little bunches, and the capillarity of the moisture acting thereon we have, as it is now believed, this plasticity, or the property which permits the clay to be moulded into any desired form when wet, and to retain that form when dry.

Iron oxide (sometimes called ferric oxide or sesquioxide) is the agent which produces the various shades of yellow and red in clay products both before, during and after burning. (This with the exceptions hereinafter noted.) With iron oxide in a clay, very hard burned, the color may become greenish. Iron oxide (Fe2O1), alone may give a purplish or reddish color, depending upon the quantity; a mixture of iron oxide and ferrous oxide (FeO) (sometimes called protoxide of iron) may produce yellow, cherry red, violet, blue or black brick, depending upon the percentage of iron in the mass, and the degree of heat; the higher the temperature the more pronounced or deeper the color. If a white or nearly white color is desired, there must be less than 1 per cent. of iron oxide in the clay, unless there is lime also; but it takes three times as much lime as iron to destroy the red color. If not, then there will remain a yellowish color. Less than one per cent may produce a grayish tint. Too much iron will give a dirty brown color.

Brick clays, if red brick are wanted, are improved by from 1 to 32 per cent. iron oxide, with an average of not more than 6 per cent. iron. Fire clays ought never to have more than 7.25 per cent. of iron oxide, and they should not average more than 1.5 per cent. Kaolins can not contain much more than 0.01 per cent., if a clear white product is sought for.

Iron in the form of ferrous oxide (FeO) alone will produce a green color when the clay is burned.

Manganese, which occurs in some form in many Pennsylvania clays, will color the clay while burning a light green to a blackish green. In some instances it will give the product an unattractive purplish black color.

Clays containing sufficient lime to lead to effervescence (4 or per cent. lime carbonate), when hydrochloric (muriatic) acid is dropped upon it, ought to be rejected for any kind of brick, if the lime occurs in lumpy or pebbly form; but, if in a finely divided condition, it may be an advantage. Such clays containing as high as 20 per cent carbonate of lime can be used for common and even for pressed brick, as well as earthen ware, and especially so in the manufacture of glazed ware.

Magnesia in clays has nearly the same effect as lime; but with sulphur it may lead to a white coating on the surface of the product. This is noticeable at times on the exposed parts of bricks in buildings. If an analysis shows more than 2.30 per cent. magnesia, and 0.50 per cent. sulphur, the material should be discarded for such work as exterior of buildings or exposed interior work, as this whitish appearance will be manifest after a few weeks or months exposure to the atmosphere. In pottery clays, fire clays and kaolins, the less magnesia the better for the product.