Cappagh Brown

Synonyms:

Cappagh Brown, Euchrome, Mineral Brown

This earth was found on the estate of Lord Audley, in the Cappagh Mine, which was opened in the year 1814, and is situated about ten miles west of the town of Skibbereen, in the county of Cork. In composition and general characters it resembles raw umber, but has a more reddish hue. It contains ferric hydrate and ferric oxide, with a considerable amount of one of the oxides or hydrates of manganese. It gives off a good deal of water when heated to 100° C., and acquires a rich reddish-brown colour, not unlike that of burnt sienna, and almost identical with that of Caledonian brown. The specimens examined contained mere traces of organic matter, so that it cannot be regarded as a kind of bog-earth or peat, although it is, of course, possible that there may be another mineral found in the same locality which might be correctly so designated. The following analysis represents the composition in 100 parts of a characteristic specimen of this pigment:

It is probable that a part of the manganese really exists in the form of the red oxide (Mn₃O₄), and a part of the iron as ferrous oxide (FeO). I have, however, calculated both these metals into their higher oxides. In this way it happens that the added percentages exceed 100, even when the traces of magnesia and potash present in Cappagh brown are not included in the total. The large quantity of water present in this mineral in a loosely-attached form (hygroscopic), amounting to nearly one-fifth of the weight of the pigment, indicates the desirability of cautiously drying the substance previous to grinding it in oil. A temperature of 60°C. should not be exceeded.

Cappagh brown works well in oil, particularly if it be dried at a heat below that of boiling water before it be ground in oil. It is an innocuous pigment, but its permanence has scarcely been sufficiently tested. A rub of Cappagh brown in oil, exposed to strong sunlight for one month, lost a little of its yellow, assuming a somewhat ruddier hue; the change, however, was very slight, and did not appear to have increased after continued exposure for five months more.