What's Wrong with this Painting?
Although it appears to be a static object, Guercino's Erminia and the Shepherds has undergone considerable changes due to age, deterioration, and damage in the 355 years since its creation. These changes have necessitated restoration measures in the past, and thus the process you are viewing here may be the painting's third or fourth conservation treatment.
Even though a painting and the materials added during restoration age as one, the materials added to the Guercino during earlier treatments have become problematic. All of the above factors have produced condition problems that fall into two categories, structural and cosmetic.
The present lining fabric, attached to the reverse of the original canvas with an aqueous glue/paste, is failing to support the large painting. Tension on the canvas is very poor, and planar distortions have developed. Old repairs to tears and holes have become visible and distracting.
The present varnish and old retouching detracts from the painting's appearance. The varnish - an aged natural resin - saturates the paint layers poorly, is very uneven in gloss quality, and has discolored markedly. Losses in the canvas, paint, and ground are poorly filled and textured. There are at least two generations of retouching used to conceal abrasion, losses, and other disfigurements. In the older retouching, an oil medium was applied in flat fields of color with no attempt to reconstruct missing areas of design. Additionally, old retouching may cover undamaged, original passages of the painting.