PURPOSE: To acceleratedly form a patina film having fine appearance and superior adhesion by coating the surface of copper with a treating liq. contg. an aqueous resin, a compound forming a complex salt with a copper ion, a compound causing a chelate bond to a copper ion, an alkali metallic salt and silica and by spraying water on the coated surface.
CONSTITUTION: A treating liq. is prepd. by adding 10W300g/l (expressed in terms of solid matter) aqueous resin such as acrylic resin composition, 40W500g/l in total of a compound forming a complex salt with a copper ion, e.g., NH 4 OH, a compound causing a chelate bond to a copper ion, e.g., EDTA and an alkali metallic salt such as NaCl and 5W40g/l silica. The clean surface of copper or a copper alloy is coated with the treating liq. by 5W30g/m 2 (expressed in terms of a dry film) and the coated surface is forcedly dried. Water and/or steam is then sprayed on the coated surface in an atmosphere at ordinary temp.W80°C at 1W12ml/m 2 /min rate for 5W120min. The coated surface may be exposed to an atmosphere at ordinary temp.W80°C and ≥90% relative humidity for 30W120min. Thus, the coated surface is allowed to uniformly develop a bluish green color in a relatively short time and finally the surface is dried.