The Causes of the Difference in the Color of Bricks.

Manufacturer and builder 1, 1869

A great many erroneous views exist among manufacturers in regard to the chemical combinations to which the color of certain kinds of burnt bricks is to be ascribed. Especially strong is the belief, that white or light-colored bricks can only be produced under the influence of reducing processes, which would change the peroxide of iron into the protoxide; although it is a well-known fact that the protoxide, in its combinations with other bases, forms a far greater variety of more intensely colored compounds than the peroxide. If it were supposed that the metallic protoxide could remain in an uncombined state, a more or less black-colored mass would be the result. It has been surmised that sulphurous acid, resulting from pyritiferous fuel, was an important agent in these supposed deoxidizing processes, although sulphurous acid can never play the part of a reducing agent in the high heat of a brick-kiln. The sulphuric acid, on the contrary, which might have been contained in the clay, would, under these conditions, through the mere influence of heat, be decomposed into sulphurous gas and free oxygen. On the other hand, it is maintained by some manufacturers that white bricks, which assume a green color when half-fused by high heat, owe this change of color and an increase of absolute weight to an addition of oxygen through the influence of protoxide of iron. In this case, as in the former, the reality of facts has been totally reversed; for, according to our investigations, the white bricks contain only peroxide of iron, and the green glazed ones, on the contrary, a considerable quantity of protoxide; the latter, therefore, most have less absolute weight than when they were in the white state, and only the specific gravity of compact pieces has been increased, as, in consequence of a semi-fusion, most of their pores are closed.

Some time since, we analyzed several bricks with reference to the above questions, and recently have given this subject renewed thought. The main question turns upon the determination of the quantity of iron present, and its state of oxidation; since this metal is not only the cause of the various tints; but even of the very existence of color in bricks. The most particular attention, therefore, had to be paid to the determination of the existence of protoxide of iron. This determination, however, presented some difficulties, since hard-burnt bricks are the most refractory of all complex silicious substances. Not even the color of pulverized bricks is materially changed by treatment with sulphuric acid, in a closed glass tube, according to Alex. Mitcherlich's method, at 350 ° centigrade, for eight to ten hours. By treating with hydrofluoric acid, however, a total decomposition is effected, and the protoxide of iron can then be determined In the diluted solution by hypermanganate of potassa. We have made a number of tests to prove that pure hydrofluoric acid (free from arsenic) does not affect the action of the standard potassic solution. J. cooke (Journal of Practical Chemistry, vol. cii., page 456) has also shown that protoxide of iron in a solution of sulphate of iron may be accurately determined in the presence of hydrofluoric acid by hypermanganate of potassa.

The specimens which were first subjected to analysis were two kinds of bricks from the works of Mr. Oppenheim, near Rüdersdorf, and two others from the works of Dr. Kuhnheim, at Freienwalde
on the Oder. The physical properties of these materials and their total contents of iron were as follows:

Rüdersdorf Bricks.
No. 1. Pale red outside, more highly colored inside; moderately hard; manufactured at cherry heat; contain 3.78 per cent of sesquioxide of iron.
No. 2. Pale yellowish white, reddish inside; glazed, harder and firmer than No. 1; manufactured at a considerably higher temperature, verging on that of white heat; contain 4.26 per cent of sesquioxide of iron.

Freienwalde Bricks.
No. 3. Lively red; moderately hard and firm; manufactured at cherry heat; contains 3.79 per cent of sesquioxide of iron.
No. 4. Darker red; more compact and of greater hardness than No. 3; manufactured at a temperature near white heat; contain 4.28 per cent of sesquioxide of iron.

All these four specimens showed, during analyses, reactions so slight of protoxide of iron that its existence, or trace even, is dubious.

These facts show:
1st. That a relatively small percentage of peroxide of iron is sufficient to impart a lively red color to bricks.
2d. That with a perfectly equal percentage of iron, certain kinds of brick remain red when burned at nearly a white heat, whilst others turn pale without a reduction of their peroxide of iron into protoxide.

The last difference is merely based upon a variation in the percentage of lime contained in the raw material. According to analyses made by Mr. Hey, at the laboratory of the Mining Academy at Berlin, under the supervision of Mr. Finkener, the clay of Steinitz-See, from which the Rüdersdorf brick are made, contains 8.69 per cent, that of Freienwalde only 2.47 per cent of lime, while the quantities of magnesia and alkalies are very minute and equal in boils cases. The oxide of calcium, (lime,) if contained in considerable quantity in the raw material, acts, under the influence of high heat, upon the peroxide of iron, predisposing it to enter the combination of a polybasic white silicate. It can not be supposed that a ferrate of lime is formed while silicic acid is present in such decided excess. Besides, the pure compounds of this kind are reddish-brown. The magnesia, which exists mostly in insignificant quantities, does not seem to play an important part. According to the experience of many manufacturers, pale bricks are produced, especially in those parts of the kiln where smoke has had a chance to collect or to touch the clay. The cause of this is the higher local temperature, resulting from a more intimate contact with combustible particles.

If a brick containing but little lime changes in color from light red to dark red, an alteration only of the physical state of aggregation of the free uncomnined peroxide of iron takes place; and this substance is the only cause of color in all kinds of brick.

My next communication will treat of green and dark-colored clinkers.