and carbonaceous character of the organic matter found in the clay is due to its having decayed, out of contact with the air, such conditions bringing about a slow carbonization of the material. In some clay beds a considerable layer of organic matter may accumulate, producing a bed of coaly material, which represents the early stage in the formation of true coal.1

Many surface clays contain organic matter in the form of plant roots that have grown down into the clay from the surface, but the effect of these is probably not very material. The amount of organic matter which a clay contains is usually small, and 3 per cent. or 4 per cent. may color a sandy clay deep black. The depth of color will, however, be influenced by the way in which the organic matter is distributed in the clay, finely divided and evenly distributed particles producing a deeper and more uniform tint. As long as any organic matter remains in the clay, it will tend to exert a reducing action during the burning, because the carbon, in changing to carbon dioxide, requires oxygen, and takes it greedily from the atmosphere within the kiln. Under such conditions, there is little opportunity for other compounds requiring it to get any, until the organic material has been consumed. The organic matter in clays will, however, pass off during the early stages of burning, if sufficient air can enter the kiln, and the clay is not heated too rapidly. In fact, if the clay contains considerable organic matter, the combustion of the latter may even add to the temperature of the kiln. In burning clays, which by their color clearly show that much organic matter is present the burning should proceed slowly until all the organic matter is driven off, for, if this is not done, the surface of the clay may burn dense, and form a skin which the oxygen cannot penetrate. Consequently, the carbonaceous matter will not be consumed, but will remain in the interior of the ware. Moreover, the clay may swell up and become porous, due to the presence possibly of hydrocarbons (compounds of hydrogen and carbon), which are decomposed by the heat, and the gases, in endeavoring to expand and escape, bloat the clay. Many of the Clay Marls show this phenomenon, if heated too rapidly.

It has not been determined definitely whether finely divided organic matter exerts any influence on the plasticity or tensile strength, but it, no doubt, increases the absorptive power of the clay for water.

In most chemical analyses the organic matter is rarely determined separately, but is included in the "loss on ignition," for, in fact, the amount is commonly so small as not to require separate determination.

Soluble Salts.

Origin. It has been pointed out in Chapter I (Origin of Clay) that in the decomposition of mineral grains in clay, soluble compounds are often formed. During the drying of the clay the moisture brings these to the surface and leaves them there when it evaporates, thus forming a scum on the air-dried ware and sometimes a white coating on the clay after it is burned. Those found in the clay are commonly sulphates of lime, iron or alkalies, and their formation is generally due to the decomposition of the iron pyrite frequently contained in the clay. A much greater quantity of soluble sulphates will be formed if the pyrite is in a finely divided condition and evenly distributed through the clay, but these compounds may also be formed without the aid of pyrite, as when the carbonates are set free by the decomposition of silicates such as feldspar. When the soluble compounds have formed in the green clay their presence can often be detected by spreading the dug clay out to weather, which will result in their forming a crust on the surface of the mass.

Their formation does not cease, however, when they are removed from the ground, for in some cases fresh pyrite grains remain in the clay after mixing, and if the clay is stored in a moist place these may decompose, yielding an additional amount of soluble material. One means of preventing this would seem to be to use the clay as soon as possible after mixing.

In some cases soluble sulphates may be even introduced into the clay by the water used for tempering, for distilled water is the only kind that is free from soluble salts. All well and spring waters contain some at least, and if these flow or drain from clays

or rocks containing any pyrite they are almost sure to contain soluble salts. Those flowing from lime rocks are usually “hard” on account of the lime carbonate which they contain. Still another source of soluble salts in raw clay lies in some of the artificial coloring materials which are sometimes used.

Soluble sulphates are sometimes formed, in burning, through the use of sulphurous fuel, that is, coal containing more or less iron pyrite. When the coal is burned part of the sulphur in the pyrite is expelled, and, uniting with the oxygen, forms sulphuric acid gas (SO). This passes through the kiln, and, if it comes in contact with carbonates in the clay, converts them into sulphates, because many substances, such as lime (CaO), have a stronger affinity for sulphur trioxide (SO) than for carbon dioxide (CO2).

It frequently happens that clay products come from the kiln apparently free from any superficial discoloration or coating, but develop one later on if subjected to moisture. In this case. the salts had been formed within the body of the ware during burning, and are brought to the surface after the ware is exposed to the weather. The moisture, which is then absorbed, brings the salts to the surface upon drying out.

The coatings thus far mentioned are all white in color. In some instances, however, the product becomes covered with a yellow or green stain, which is caused either by the growth of vegetable matter on the surface of the bricks or by soluble compounds of the rare element vanadium. Some white coatings seen on brick, however, come from the mortar.

Quantity of soluble salts in a clay.-The amount of soluble salts present in a clay is never very great, but less than 0.1 per cent. is often sufficient to produce a white incrustation. A number of soluble salt determinations were made on clays from different localities in the State in order to obtain some idea of the quantity to be found in the clays of different formations. In making these tests a five-grain sample of clay was taken and heated in distilled water for about one-half hour, after which the solution was forced through a small pasteur filter connected with a force pump, and the clear filtrate evaporated to dryness in a

platinum dish. The results obtained on a number of samples

'These numbers as used here and elsewhere in this Report refer to localities as indicated upon the maps, Plates X, XI, XII, XIII.

These numbers wherever used refer to numbers of the test, on file in Dr. Ries' laboratory.

From these it will be seen that the samples of Clay Marl I showed the highest percentage of soluble salts, and that as a series the Raritan clays showed the lowest percentage of soluble salts.

The Raritan samples tested, with the exception of locality 222, are of the higher grade clays, and do not include any of those used for fireproofing. It is probable that these, if tested, would yield percentages as high as Clay Marl I, since they contain considerable pyrite.

Prevention of soluble salts. If a brick is vitrified the soluble compounds are rendered insoluble, but in the manufacture of many grades of ware the clay is not carried to vitrification, and therefore the soluble salts must be rendered insoluble if possible. This is most effectively done by adding some chemical to the wet clay which will react with the soluble salts in it, and either render them insoluble or else change them into some very easily soluble compound that can be readily washed from the surface of the

ware.

The substance commonly added is either barium chloride or barium carbonate. When barium salts come in contact with soluble sulphates, barium sulphate is formed, a combination which is absolutely insoluble in water. This is expressed by the first of the following chemical reactions if barium carbonate is used, and by the second if barium chloride is employed.