The Challenge of Cleaning Spanish Colonial Paintings Containing Unknown Materials

TLDR: In the early Colonial period' colo res de Castilla' were imported from Spain but later almost all the pigments were produced in America as mentioned in this paper The restoration of these paintings is a constant bundle of surprises, and sometimes a great challenge.

Abstract: Colonial painting techniques of the seventeenth and eighteenth centuries are very different to those encountered in Europe and North America The restoration of these paintings is a constant bundle of surprises, and sometimes a great challenge Paintings were executed on one or more pieces of canvas sewn together, either hand-woven locally or reutilized pieces of merchandise bundles from Spain The ground usually consisted of a mixture of chalk, brown or red earth, ashes (used as a fungicide) and very strong animal glue This layer is usually thick, very hard, sensitive to water, and sometimes presents deep cracks The paint layer consisted of colours made from organic and inorganic pigments They were extracted from plants (eg indigo), insects (eg cochineal) and minerals (eg 'alballalde' or lead white) During the early Colonial period' colo res de Castilla' were imported from Spain but later almost all the pigments were produced in America [1] Glazes were transparent pigments dissolved in resins or gums that are very sensitive to water Since pigments were mostly locally produced and varied from area to area, often pigments which appear similar react very differently to solvents during the cleaning process [2] Mediums were also locally prepared by the painters The most common combination was a mixture of egg yolk with linseed or walnut oil Others were gums extracted from fruit trees, gum tragacanth, gums or mucilages extracted from cactus plants, and resins such as copals Very often the paintings were not varnished Both the paint layer and the protective layer contained substances like polysaccharide gums and mucilages Among those commonly found are acacia gum (gum arabic) and gum tragacanth, as well as Senegal gum, cherry gum, plum-tree gum, willow-tree seed and other unknown gums awaiting further research Some of them were most likely mixed with alcohol Their most important characteristics are that they dry out, do not crystallize and are only partially soluble in water Moreover they are prone to attack by fungi and other microorganisms Proteinaceous or albuminous materials like white of egg, casein and egg yolk are found mixed with oils or gums, which become very hard They darken and give a grey tone that 'kills' the brilliance of the original colours, leaving an almost irreversible layer over the entire painting When oils were used, they often contained impurities like dust or plant mucilages that appear as small rough dark spots deeply ingrained in the paint layer The resins found include Peruvian balsam and Tolu balsam,