A monopteros temple, sheet X/4
Age of paper: circa 1550
Laid lines: 35mm apart
Watermarks: two crossed arrows and a six-pointed star
Weight of paper: heavy
Conservation condition of the drawing: fair; adhesive residue and discolouration, staining, undulations and localised cockling; deterioration of animal glue
Animal glue is a protein acquired by boiling animal skin and bone. The glues are full of impurities and therefore of poor quality, although animal-based glues do have very strong binding adhesives.
Degradation of glue is caused by:
- increase in humidity/temperature
- environmental pollutants
- light and UV radiation
The glue breaks down with age, discolours and darkens causing staining to the paper. Eventually, the paper can become brittle and can break away. Total removal of either the residue or stain is not always possible.
Animal glues are very sensitive to environmental fluctuations and one of the factors in the deterioration is the formation of acid within the paper during ageing.
Drying out of an adhesive joint can cause internal stresses within the paper which can ultimately lead to the breakdown of the join. Glue shrinks with age and decays by the process of photo-degradation (i.e. decay by light exposure).
Discolouration of the glue is caused by oxidisation and slow evaporation of properties within the glue. The extent of the discolouration is clearly visible in the images of sheet X/4 using magnification, as shown here.
Wheat starch paste was used for new repairs to many of the Palladio drawings. Starch is ground to produce flour, mixed with water and heated to produce a viscous paste. On cooling, the paste turns into a gel and can be used as an adhesive. It is pH neutral but susceptible to biological attack. However, unlike animal glue it does not discolour with age.