Iron Oxide. Sources of iron oxide in clays.-Iron oxide is one of the commonest ingredients of clay, and a number of different mineral species may serve as sources of it, the most important of which are grouped below: Hydrous oxide. Limonite. Oxides. Hematite, magnetite. Silicates. Biotite, glauconite (greensand), hornblende, garnet. Carbonates. Siderite. In some, such as the oxides, the iron is combined only with oxygen, and is better prepared to enter into chemical combination with other elements in the clay when fusion begins. In the case of the sulphides and carbonates, on the contrary, the volatile elements, viz., the sulphuric acid gas of the pyrite and the carbonic acid gas of the siderite, have to be driven off before the iron contained in them is ready to enter into similar union. In the silicates, the iron is chemically combined with silica and several bases, forming mixtures of rather complex composition and all of them of low fusibility, particularly the glauconite. Several of these silicates are easily decomposed by the action of the weather, and the iron oxide which they contain combines with water to form limonite. The range of ferric oxide as determined from a number of clay analyses is as follows:1 Effects of iron compounds.-Iron is the great coloring agent of both burned and unburned clays. It may also serve as a flux and even affect the absorption and shrinkage of the material. Bull. N. Y. State Museum, No. 35, p. 520. Coloring action of iron in unburned clay.—Many clays show a yellow or brown coloration due to the presence of limonite, and a red coloration due to hematite; magnetite is rarely present in sufficient quantity to color the clay; siderite or pyrite may color it gray, and it is probable that the green color of many clays is caused by the presence of silicate of iron. In New Jersey this is specially true of glauconitic clays. The intensity of color is not always an indication of the amount of iron present, since the same quantity of iron oxide may, for example, color a sandy clay more intensely than a fine grained one, provided both are nearly free from carbonaceous matter; the latter, if present in sufficient quantity, may even mask the iron coloration completely. The coloring action will moreover be effective only when the iron is evenly distributed through a clay in an extremely fine form. It is probable that the limonite coloring clays, is present in an amorphous or noncrystalline form, and forms a coating on the surface of the grains. Coloring action of iron oxide on burned clay.—All of the iron ores will in burning change to the form of oxide, provided the clay is not vitrified, and so affect the color of the burned material; if vitrification occurs, the iron oxide enters into the formation of silicates of complex composition. The color and depth of shade produced by the iron will, however, depend on 1st, the amount of iron in the clay; 2d, the temperature of burning; 3d, condition of the iron oxide, and 4th, the condition of the kiln atmosphere. 1. Clay perfectly free from iron oxide burns white. If a small quantity, say I per cent., is present, a slightly yellowish tinge is imparted to the burned material, but an increase in the iron contents to 2 or 3 per cent. produces a buff product, while 4 or 5 per cent. of iron oxide makes the clay burn red. 2. If a clay is heated at successively higher temperatures, it is found that, other things being equal, the color usually deepens as the temperature rises. Thus if a clay containing 4 per cent. of iron oxide is burned at a low temperature, it will be pale red, and harder firing will be necessary to develop a good brick red, which will pass into a deep red and then reddish purple. 3. Among the oxides of iron, two kinds are recognized, known respectively as the ferrous oxide (FeO) and ferric oxide (Fe2O3). In the former we see one part of iron united with one of oxygen, while in the latter one part of iron is combined with one and one-half parts of oxygen. The ferric oxide, therefore, contains more oxygen per unit of iron than the ferrous salt, and represents a higher stage of oxidation. In the limonite and hematite the iron is in the ferric form, representing a higher stage of oxidation. In magnetite both ferrous and ferric iron are present, but in siderite the ferrous iron alone occurs. In the ultimate chemical analysis the iron is usually determined as ferric oxide, no effort being made to find out the quantity present as carbonate or sulphide. Iron passes rather readily from the ferric to the ferrous form and vice versa. Thus, if there is a deficit of oxygen in the inside of the kiln, the iron does not get enough oxygen and the ferrous compound results, but the latter changes at once to the ferric condition, if sufficient air carrying oxygen is admitted. Similarly if ferric oxide is present in a clay containing considerable carbonaceous matter, the latter will, if it cannot get enough oxygen from the kiln atmosphere, take it from the ferric oxide and so reduce the latter to the ferrous condition. The same change may be produced by smoky fires. The necessity for recognizing these two forms of iron oxide is because they affect the color of the clay differently. Ferrous oxide alone is said to produce a green color when burned, while ferric oxide alone may give purple or red, and mixtures of the two may produce yellow, cherry red, violet, blue and black.1 Seger2 found that combinations of ferric oxide with silica produced a yellow or red color in the burned clay. We may thus get a variation in the color produced in burning clay depending on the character of oxidation of the iron, or by mixtures of the two oxides.3 It is found sometimes that bricks after burning show a black core, due to the iron in the centre of the brick being prevented from oxidizing, but this should not be confused with the black coloration seen on the ends of many arch brick, which is caused by the slagging action of the impurities in the fuel.* 1 3 1 Keramic, p. 256. See "Flashing of brick," Chap. X, and “Burning pottery," Chap. XV. 'See "Burning bricks," Chap. X. 4. Since the stage of oxidation of the iron is dependent on the quantity of air it receives during burning, the condition of the kiln atmosphere is of great importance. If there is a deficiency of oxygen in the kiln so that the iron oxide, if present, is reduced to the ferrous condition, the fire is said to be reducing. If on the contrary there is an excess of oxygen, so that ferric oxides are formed, the fire is said to be oxidizing. These various conditions are often used by the manufacturer to produce certain shades or color effects in his ware. Thus, for example, the manufacturer of flashed brick produces the beautiful shading on the surface of his product by having a reducing atmosphere in his kiln followed by an oxidizing one. The potter aims to reduce the yellow tint in his white ware by cooling the kiln as quickly as possible to prevent the iron from oxidizing. Fluxing action of iron oxide.-Iron oxide is a fluxing impurity, lowering the fusing point of a clay, and this effect will be more pronounced if the iron is in a ferrous condition or if silica is present. A low iron content is, therefore, desirable in refractory clays, and the average of a number of analyses of these shows it to be 1.3 per cent. Brick clays, which are usually easily fusible, contain from 3 to 7 per cent. of iron oxide. Effect of iron oxide on absorptive power and shrinkage of clay. -So far as the writer is aware no experiments have been made to discover the increased absorptive power of a clay containing limonite, although the clay soils show that the quantity of water absorbed is greater with limonite present. This greater absorptive power may be accompanied by an increased shrinkage. The fire shrinkage might also be great because of the increased loss of combined water due to the presence of limonite.1 Lime. Lime is found in many clays, and in the low-grade ones may be present in large quantities at times. Quite a large number of minerals may serve as sources of lime in clays, but all fall into one of the three following groups: See tests, under Fire shrinkage, Chap. IV. I. Carbonates. Calcite, dolomite. 2. Silicates containing lime, such as feldspar, garnet. 3. Sulphates. Gypsum. Whenever the ultimate analysis of a clay shows several per cent. of lime (CaO), it is usually there as an ingredient of lime carbonate (CaCO3), and in such cases its presence can be easily detected by putting a drop of muriatic acid or vinegar on the clay. When present in this form it is apt to be finely divided, although it may occur as concretions or limestone pebbles; in either case, it is usually restricted to drift clays, especially in New Jersey. When lime is present as an ingredient of silicate minerals, such as those mentioned above, its presence cannot be detected with muriatic acid. It is doubtful, however, if many calcareous clays contain much lime in this combination, and the fact that practically all limy clays, shown to be such on chemical analysis, give a strong test with muriatic acid, strengthens this theory. Gypsum, which is found in a few clays, is often of secondary character, having been formed by the action of sulphuric acid on lime-bearing minerals in the clay. Since these three groups of minerals behave somewhat differently, their effects will be discussed separately. Effect of lime carbonate on clay.-Lime is probably most effective in the form of the carbonate. When clays containing it are burned, they not only lose their chemically combined water, but also their carbon dioxide, but while the water of hydration passes off between 450° C (842° F.) and 600° C (1112° F.), the carbon dioxide (CO2) does not seem to go off until between 600° C. (1112° F.) and 725° C. (1562° F.). In fact it more probably passes off between 850° C. (1562° F.) and 900° C. (1652° F.). The result of driving off this gas in addition to the chemically combined water is to leave calcareous clays more porous than other clays up to the beginning of fusion. If the burning is carried only far enough to drive off the carbonic acid gas, the result will be that the quicklime thus formed |