In addition to the surface cleaning of sheet X/18, Plan of the Villa Madama, the drawing also needed to be conserved to reduce planar distortion.
Conservation to reduce planar distortions
One problem with treating a paper with several different pasted sections is that the sections may respond differently to humidity.
When paper absorbs or loses moisture, and expands or contracts more in cross direction than grain, it can expand up to 10mm. There can be problems humidifying architectural scale drawings, as these may alter.
The humidification process can be used to remove old linings, backings and old repairs and to reactivate water soluble adhesives. The drawing can benefit since as the paper relaxes the fibres swell and to some extent rebind on drying.
Humidification will help an aged, weakened paper to recover because the hydrogen bonds between the paper's fibres which are partially broken become restored. This can give the paper a greater mechanical strength. Through swelling, the paper's structure becomes more open and soluble impurities can be pushed out.
Humidification can be very rapid depending on the weight and how heavily the object is sized.
A Gore-tex® sandwich will ensure that the humidity can penetrate the object equally from both sides, preventing an un-equal expansion of the drawing.
Prior to humidification, the drawing material of sheet X/18 was spot-tested with water and blotter to assess the material's vulnerability to moisture and estimate for how long it needed to be humidified.