Copper compounds in metals and colorants: oxides and hydroxides

Abstract

Cuprite and tenorite are important as corrosion products of copper and as alteration products or colorants. In the case of tenorite this alteration is usually undesirable, resulting from degradation of azurite or decomposition of copper(I) chloride (cuprous chloride) under very alkaline conditions. The mode of occurrence of cuprite in ancient bronzes is reviewed. The use of cuprite as a pigment in ancient glasses is discussed and the intermediate compound, copper(II) hydroxide (cupric hydroxide), is briefly described.

Application of Modified Tafel Analysis to the Identification of Corrosion Products on Archaeological Metals Using Voltammetry of Microparticles

“…À0.40 V [53]. The appearance of tenorite can be associated to the prolonged contact of the coins with calcareous materials, since it is known that this mineral can be formed from azurite and other copper corrosion products in contact with CaCO 3 (implying high local pH) and/or relatively high levels of CO 2 [60]. ATR/FTIR spectra confirm the presence of CaCO 3 crusts (see Figure 5a) and the coexistence of several copper corrosion products in the greenish corrosion layers.…”

Dating Archaeological Copper/Bronze Artifacts by Using the Voltammetry of Microparticles

“…[11] The aerobic oxidation of cuprite into tenorite (2 Cu 2 O + O 2 !4 CuO) is a thermodynamically spontaneous process (DG8 f (Cu 2 O) = À146 kJ mol À1 ; DG8 f (CuO) = À130 kJ mol À1 ) [12] favored by the prolonged contact of the artifacts with a CO 2 -rich atmosphere as well as with calcareous materials. [13] Such conditions hold for samples of coins, thus making these pieces favorable candidates for building a calibration graph. Accordingly, three series of coin samples from different collections (Museum of Prehistory of Valencia (MPV), Museum of Archaeology of Xàtiva (MAX), and Antonio DomØnech-FrancØs private collection (ADF); see the Supporting Information) were studied.…”

The Influence of Conservation Treatments and Environmental Storage Factors on Corrosion of Copper Alloys in the Ancient Athenian Agora

“…Although there are no specifically documented cases for the use of sodium sesquicarbonate in the Agora, it was very likely to have been used, as it is mentioned in some conservation notes (Witherill 1955 Pollard et al 1990;Scott 1997).…”

“…It generally forms on top of the corrosion crust, which is very brittle and readily breaks off, exposing a smooth cuprite surface below. This compound is described as duckegg blue, unstable in the presence of carbon dioxide and water, and poorly crystalline (Scott 1997). In the presence of carbon dioxide it has a tendency to change into copper hydroxy carbonates or, in the presence of chloride, into copper hydroxy chlorides (atacamite, paratacamite) (MacLeod 1999).…”

“…Treatment with a 10% solution of sodium sesquicarbonate can cause copper to redeposit as cuprite on the surface of the object, which has been made alkaline by the treatment (Scott 2002). In electrochemicalor electrolytic reduction, any nonreduced secondary copper corrosion products, such as malachite, form cuprite (CU20) and tenorite (CuO), which appear as a black, powdery product (Scott 1997 While the dark brown and blue corrosion products often occur together, they seem to form independently from one another. The only common factor in their formation appears to be sodium, for example from sodium sesquicarbonate, which could have contributed to the formation of cuprite from copper (I) chloride in the dark brown corrosion (Scott 1997) and of sodium copper carbonate in the blue corrosion.…”