Magnesia (MgO) rarely occurs in clay in larger quantities than I per cent., and, so far as known, none of the New Jersey clays are exceptions to this rule. When present, its source may be any one of several classes of compounds, i. e., silicates, carbonates and sulphates.

In the majority of clays the silicates, no doubt, form the most important source, and minerals of this type carrying magnesia are the black mica or biotite, hornblende, chlorite and pyroxene. These are scaly or bladed minerals, of more or less. complex composition, and containing from 15 per cent. to 25 per cent. of magnesia. The biotite mica decomposes or rusts very easily, and its chemical combination being thus destroyed, the magnesia is set free in the form of a soluble compound, which may be retained in the pores of the clay. Hornblende is not an uncommon constituent of some clays, especially in those which are highly stained by iron, and have been derived from darkcolored igneous rocks. Like biotite, it alters rather rapidly on exposure to the weather. Dolomite, the mixed carbonate of lime and magnesia, is no doubt present in some clays, and would then serve as a source of magnesia. Magnesium sulphate, or Epsom salts, probably occurs sparingly in clays, and might form a white coating either on the surface of clay spread out to weather, or else on the ware in drying. It is most likely to occur in those clays which contained pyrite, the sulphide of iron (FeS2), for the decomposition of the latter would yield sulphuric acid, which, by attacking any magnesium carbonate in the clay, might form magnesium sulphate. This substance has a characteristic bitter taste.

It is only recently, however, that the true effects of magnesia in clay have been discovered, for since it was often derived from similar minerals as lime, and resembled it chemically, it was thought to exert the same effect on clays. That it does not has been shown in an interesting series of experiments conducted by Mäckler.1 As his results have been published in a German magazine, and are probably inaccessible to many of the readers of this report, it may be well to quote from them. Mäckler noticed that certain kinds of fireproofing, made from a calcareous clay containing several per cent. of magnesia, behaved somewhat differently from most products made from limy clays, and concluded that the effects were due to the magnesia contents of the material. In order to prove this point, he selected a clay, which was free from lime or magnesia, and in its raw and burned condition had the following composition:

Analysis of clay used by Mäckler in tests on effects of magnesia.

To one hundred parts by weight of this clay, either lime or magnesium carbonate were added in the proportions given below, the percentages given in parenthesis representing the quantity of lime or magnesia contained in the amount of carbonate added. The physical tests of these mixtures are also given below.

It will be seen here that the effect of magnesia was quite different from that exerted by the lime. The mixtures containing magnesia did not vitrify suddenly, as did the limy clays, nor did the magnesia exert as strong a bleaching action on the iron, and the points of incipient fusion and viscosity were also separated.

With a mixture of kaolin and magnesia, similar results were obtained. The mixture of kaolin and magnesia showed a higher shrinkage at the beginning of the burning than the kaolin alone, and then increased but little until a high temperature was reached, when the shrinkage suddenly began again. A hard body was obtained at cone I with the kaolin-magnesia mixture.

The effect of magnesia, therefore, if present in sufficient quantity is to act as a flux and make the clay soften slowly instead of suddenly, as in the case of calcareous clays.

Of the various kinds of clay found in New Jersey, the brick clays usually contain the highest percentages of magnesia, but even in these it is rarely present in sufficient quantity to exert a noticeable effect. The fire clays contain the least.

The range of magnesia in several classes of clays, as figured from a number of analyses, is as follows:1

1 Bull. N. Y. State Museum, No. 35, p. 524. Owing to an error in an analysis of a brick clay the figures in this table have been recalculated.

The alkalies include potash (K2O), soda (Na2O) and ammonia (NH3). There are other alkalies, but they are probably of rare occurrence in clays.

The amount of total alkalies contained in a clay varies from a mere trace in some to 7 per cent. or 9 per cent. in others. The range of alkalies in several classes of clays was determined to be as follows:1

Ammonia is, no doubt, present in some raw clays, judging from their odor, and it may possibly exert some effect on the physical structure of the clay, it being found that the bunches of grains in a clay tend to separate more easily, when the clay is agitated with water, if a few drops of ammonia are added. As ammonia is easily volatile, it leaves the clay as soon as the latter is warmed, and, therefore, plays no part in the burning of the clay. The two other common alkaline substances, potash and soda, are more stable in their character, and are, therefore, sometimes termed fixed alkalies. These have to be reckoned with in burning, for they are present in nearly every clay.

Several common minerals may serve as sources of the alkalies. Feldspar may supply either potash or soda. Muscovite, the white mica, contains potash. Greensand or glauconite contains potash.

Other minerals, such as hornblende or garnet, might serve as sources of the alkalies, but are unimportant, as they are rarely present in clays in large quantities.

Orthoclase, the potash feldspar, contains 17 per cent. of potash (K2O), while the lime-soda feldspars contain from 4-12 per cent of soda (Na2O), according to the species. The limesoda feldspars fuse at a lower temperature than the potash ones, but are also less common.1

Muscovite mica contains nearly 12 per cent. of potash and may contain a little soda. Muscovite flakes, if heated alone, seem to fuse at cone 12, but, when mixed in a clay, they appear to act as a flux at different temperatures, according to the size of the grains. If very finely ground, the mica seems to vitrify at as low a temperature as cone 4,2 but, if the scales are larger, they will retain their individuality up to cone 8, or even 10. The latter is true of the micaceous talc-like clays found in the Miocene formation around Woodstown, which are composed chiefly of white mica. We, therefore, see that the minerals supplying alkalies are all silicates of complex composition. Each has its fixed melting point, and the temperature at which the alkalies flux with the clay will depend on the containing mineral, and also on the size of the grains. If the alkali-bearing mineral grains decompose, the potash or soda are set free and form soluble compounds.3

Alkalies are considered to be the most powerful fluxing material that the clay contains, and, if present in the form of silicates, are a desirable constituent, except in clays of a refractory character. On account of their fluxing properties, they serve, in burning, to bind the particles together in a dense, hard body, and permit a white ware, made of porous-burning clays, to be burned at a lower temperature. In the manufacture of porcelain, white earthenware, encaustic tiles and other wares made from white-burning clays, and possessing an impervious or nearly impervious body, feldspar is an important flux.

1

1 See Seger Ges. Schr., p. 413.

2 Trans. American Ceramic Society, Vol. IV, p. 255.

* See Origin of Clay, Chap. I.