All bronzes more than about one thousand years old which are intact have come down to us because they were buried in the earth, acquiring characteristic signs which cannot be faked.

The most evident indication of authenticity is furnished by the patina. The so called patina on excavated copper and bronze objects is caused by chemicals contained in the earth. Initially a very conspicuous, significant and non falsiafiable phenomenon occurs. As soon as it is discharged from the alloy, the copper forms a strong, brick-red coat of copper oxide, this red oxide combines slowly with the acids in the ground water to form very visible greenish-blue metal salt crystals. This process must be understood and looked for by collectors of excavated bronze, in order to avoid buying fakes.

The colour of these often very beautiful crystals depends on the kind of acid that happens to be in the ground. Carbonic acid produces bright green malachite crystals and other acids produce blue crystals, whilst ferrous alloys form crusts that range from yellow to brown. All these deposits may include solid elements contained in the earth, forming a complex, variegated crust having different colours and often various layers.

A genuine patina can be recognized at first glance. An artificial patina will have been produced by dipping the object into chemical bath or coloured paint. In both cases only a thin, monochromatic coating is obtained which, once removed, reveals the bare, corrosion free metal. A patina of this kind is flat, smooth and amorphous.

Modern forgers are skilled in reproducing patinas which at first sight look convincingly genuine. But a magnifying glass reveals that they do not consist of mineral salt crystals but of a mass of coloured synthetic material, glue and miscellaneous additives to make the surface look very rough.

Recently Chinese forgers have developped a new technique by applying crystals of the correct composition onto the outer side of bronze objects; the essential characteristics of authenticity, that is the red layer of copper and the deep corosion of the bronze body, are however missing