Verdigris
In colloquial terms the patina, consisting of green copper salts, is often described as "verdigris". This is inaccurate. Verdigris is only caused by the chemical reaction of copper materials with acetic acid and is a mixture of basic copper acetates. In contrast to copper salts which form a natural patina, verdigris is water soluble. Visually it may be recognized by its strikingly green color. (reference)
Verdigris pigment
Copper acetates of different chemical composition make up the pigment verdigris. This pigment ranges in color from green, to green-blue, blue-green, and finally just blue. Reactions with copper acetate vary among substances such as the following: copper acetates dissolve in mineral acid, alkalis convert them into blue copper hydroxide, oils, resins and proteins react to form green transparent copper oleates, resinates, and proteinates. Of the many different types of verdigris each type can be classified into either basic or acidic. (reference)
Neutral Verdigris is neutral copper acetate which occurs when basic acetates are dissolved in acetic acid, or when basic verdigris is ground up with strong acidic acid. Decomposition of neutral verdigris occurs when a solution is boiled. This verdigris is dissolved in acidic acid. The shape of neutral verdigris is hexagonal and rhombic with distinct boundaries.
Basic verdigris forms from the combination of air, water vapor, acetic acid vapor, and copper or copper alloy mix. It forms a solid of blue, or blue-green. It is often made up of fine needles. The chemical formula for basic verdigris can include all of the following:
[Cu(CH3COO2]2. Cu(OH)2. 5H2O (blue)
Cu(CH3COO)2. Cu(OH)2. 5H2O (blue)
Cu(CH3COO)2. [Cu(OH)2]2 (blue)
Cu(CH3COO)2. [Cu(OH)2]3. 2H2O (green)
In the presence of hydrochloric acid (HCl) verdigris is soluble and forms a green solution. From interaction with caustic it is much less soluble and precipitates. Sulfur compounds in the air darken all forms of verdigris. In the first three months of use the verdigris formulations can change from blue-green to green. All verdigris reacts with resin to form copper resinates. This copper resinate is rather transparent and often used as an overpaint to increase depth of saturation of an opaque green.