In an on-going study of the materials and techniques of twentieth-century Canadian painters, similar conservation issues in oil paintings by various artists have been noted. These include delamination and lifting paint, zinc soap protrusions and surface efflorescence or accretions. Examples of these phenomena are presented. Delamination in an oil painting from 1956 was found to be related to an underlayer with a high concentration of zinc fatty acid salts (zinc soaps). In two paintings that date from 1936 and 1937, zinc soaps have aggregated and formed protrusions that have broken through the paint surface. The protrusions were analysed using a combination of SEM-EDX, GCMS and FTIR. The FTIR spectra were compared to those of synthetic zinc palmitate, stearate, azelate and oleate. The combined GCMS and FTIR results indicate that the protrusions contain primarily zinc palmitate and stearate. Peak splitting in the FTIR spectrum, which is not observed in synthetic zinc palmitate, stearate or binary palmitate-stearate salts, is likely due to structural distortion. The final example describes a disfiguring surface accretion on a 1952-1954 painting caused by the reaction of zinc with a low molecular weight carboxylic acid (2-hydroxypropanoic or lactic acid).

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Corrosion noted on a number of nineteenth and twentieth century indoor bronze sculptures in the collection of the National Gallery of Canada was the cause for a major condition survey of its Canadian, American, and European bronzes. The results led to a comprehensive study, which included material analysis. This article outlines the steps undertaken to determine the extent and type of corrosion, its possible cause(s), and potential remediation. The survey included 136 bronzes, of which 40 were selected for an in-depth study. Indoor air quality tests of the current building, housing the collection since 1988, were carried out. The dossiers and database records of the selected sculptures were reviewed to investigate their condition, treatment, and storage history. The metal, patina, and corrosion products were subjected to analysis. More areas of corrosion were generally associated with predominantly green patinas and chloride-containing compounds identified in either patina or corrosion samples. The most frequently identified compounds in the patina samples were atacamite, a copper(II) chloride hydroxide, and cuprite, a copper(I) oxide. The most frequently identified compounds in the corrosion samples were atacamite and rouaite, a copper(II) hydroxide nitrate. The analysis determined that the current environment is not contributing to the surface alterations of the sculptures; however, past storage conditions, chemical residues from artificial patination, and likely casting residues are the main causes of the current condition.

. 63, 3433 (1985). Results of kinetic studies of the racemizations and radio-bromide exchanges with I -arylbromoethanes in acetone with added LiBr are reported. The results are direct empirical evidence supporting the ion-pair mechanism for bimolecular nucleophilic displacement at saturated carbon. For the unsubstituted and less electron rich substrates, behaviour expected for a classical SN2 mechanism is found, but for 1-(4-methylphenyl)bromoethane, the racemization rate is greater than twice the exchange rate, indicating internal return with racemization from a solvated ion pair. For 1-(3,4-dimethylphenyl)bromoethane, the carbocation is sufficiently stabilized that the reaction shows intermediate behaviour; the racemization rate is significantly less than twice the exchange rate, for example. Through the series from the electron withdrawer to electron donor substituted substrate, the activation parameter AH) progressively decreases and AS9 becomes rapidly more negative. ALLAN R. STEIN et ELIZABETH A. MOFFATT. Can. J. Chem. 63, 3433 (1985). On rapporte les r6sultats obtenus lors d'ktudes cinttiques relatives aux ractmisations et aux Cchanges de radio-bromures d'aryl-1 bromoethanes dans de I'acCtone B laquelle on a ajoutC du LiBr. Les rksultats obtenus correspondent B des donnees empiriques directes qui apportent un support au mtcanisme par paire d'ions lors des reactions de substitutions nucltophiles bimolCculaires au niveau de carbones saturks. Dans les cas des substrats qui ne sont pas substitues ou qui sont moins riches en Clectrons, on observe le comportement attendu pour un mecanisme SN? classique; toutefois, avec le (methyl-4 phCny1)-l bromotthane, la vitesse de ractmisation est au moins deux fois plus grande que la vitesse dlCchange et ceci suggkre la presence d'un retour interne avec ractmisation dans une paire d'ions solvatCs. Dans le cas du (dimtthyl-3,4 phCny1)-1 bromotthane, le carbocation est suffisamment stabilist pour que la reaction prksente un comportement intermediaire; par exemple, la vitesse de racemisation est au moins deux fois plus faible que la vitesse d'tchange. Lorsque I'on passe des substrats portant des groupements Clectroaffinitaires i ceux portant des groupements Clectro-donneurs, il y a une diminution graduelle du AH) alors que le AS+ devient rapidement plus negatif.[Traduit par le journal]Since Sneen and Larsen (I), in 1969, proposed an elaboration of the Winstein ion-pair scheme (2) as a "unified mechanism" for nucleophilic substitutions at saturated carbon, it has slowly been gaining respectability, so much so that the ion-pair mechanism now receives extensive discussion in undergraduate textbooks (3).The ion-pair mechanism can perhaps best be approached by considering the process of ionization and dissociation of a substrate RX. This can be considered a stepwise process of ionization to a contact or intimate ion pair within a solvation sheath or cage followed by separation of the two ions, R+ and X-. The separation of the ions would occur with layer by layer of solvent inserti...