Native or metallic copper occurs in Michigan, Australia and elsewhere and has been thus preserved through the ages due to the absence of corroding influences such as acidulated waters. By far the greater proportion of copper found in the crust of the earth occurs as the sulphide either alone, as chalcocite and covellite, or, more commonly, in association with other sulphides, as chalcopyrite, bornite and others. These sulphides are, how ever, only permanent "at depth," in the lower strata, away from corroding influences. In the upper strata where the minerals have come into contact with rain water or with moist air containing carbonic acid, the sulphides have been transformed into the more stable malachite. The very large copper deposits of the Belgian Congo are a good example of such occurrence of sulphides in the lower layers and carbonate or malachite in the upper.
Other minerals of copper that are met with in patinas are the basic chloride, atacamite, the basic sulphates, such as brochantite, the blue carbonate, azurite, and cuprite, the red cuprous oxide. All of these minerals when exposed to moist atmosphere decom pose and turn into malachite. Among the valuable bronze col lections, we do meet with examples of patina composed wholly or in part of one or more of these unstable compounds, i.e., un stable in climates such as those of London, New York or Paris. Thus, there are a large number of instances on record where a natural blue patina (azurite) has been preserved for years by taking proper precautions. On the other hand, collectors have often lamented that a patina originally of a very attractive blue was gradually changing into a green (malachite) patina. The red copper oxide patina is highly prized. The large bronze cat, of Egyptian origin, on exhibit at the Metropolitan Museum of Art (New York) has large irregular patches of this beautiful red patina. By suitable protective measures the transition into
the carbonate, malachite, can be very much retarded.
Particular attention should be paid to the basic chloride of copper, atacamite, of frequent occurrence in Egyptian bronze patinas. This chloride is perfectly stable in an absolutely dry atmosphere such as is met with in the sealed tombs of Egypt. However, as soon as this chloride-bearing patina is brought into contact with moist air not only is the transformation into malachite very rapid—very often within but a few weeks—but the chloride in presence of moisture and carbonic acid rapidly corrodes the entire bronze. A chloride, such as copper chloride, sodium chloride or ammonium chloride, is a very dangerous constituent of the patina and is one of the most active agents giving rise to the dreaded bronze "disease." The presence of chlorides in the soil in which a bronze object lies buried will greatly hasten the formation of a patina. How ever, the rapid growth of a patina brings about a poor, loose structure of little or no permanence. The characteristics of the best or "noble" patina are a very slow formation, compact, solid texture, gradual transition from metal through oxide to carbonate, the malachite of fine crystalline structure, the hair line fissures of the bronze directly under the patina tightly packed with oxide or carbonate, and a smooth almost enamel-like surface free from warts and nodules.
Artificial Patinas.—The production of artificial patinas has been practised since time immemorial. Unfortunately, however, nearly every one of the hundreds of formulas prescribed involves the use of sal ammoniac or some other chloride—one of the most destructive compounds to introduce into a patina. Chlorides are very treacherous and should never be resorted to in spite of their "convenient and rapid action." Years of experimentation and testing have led us to condemn all procedures involving the use of chlorides or other strong salts. It is to be borne in mind that the ultimate compound of lasting quality which is sought after is the green basic carbonate compound of copper. The final patina produced must be free of all mineral acids such as mu riatic, sulphuric or nitric since these acids will cause a rapid destruction of the entire body of the bronze.