Some common agents are humidity, light, temperature, dust, insects and humans. Italian Panel Painting, 19th C New York State Bureau of Historic Sites Too-low humidity extracts water from some materials and causes paper and leather to become brittle, glue and paste to dry out, and wood to warp and crack. Too-high humidity results in the absorption of water which corrodes materials, generates mold and softens adhesives. Rapid changes in humidity cause materials to swell and contract. Wood cracks, bends and twists.
Oil paintings crack and flake.
Salts within the object dissolve, migrate and recrystallize on the surface. For arid and semi-arid environments such as New Mexico, where objects have already adjusted to the dry atmosphere, collectors need to protect from accidental contact with water from roof leaks, overhead pipes, and rain from open windows. Avoidance of extreme humidity changes is highly recommended. Fluctuating temperatures cause materials to expand and contract as temperature rises and falls. This is more problematic with objects containing two or more substances, such as a retablo, that respond to temperature changes at different rates.