Cupping is a distortion of the paint surface on canvas resulting in concave sections of paint and raised areas of paint adjacent to cracks (Kirsh and Levenson 2000).
Crackle patterns on surfaces of paintings can sometimes form islands of paint that can lead to cupping (Canadian Conservation Institute 2011). The cracks in the paint layers allow for air and environmental factors to penetrate in between and under the paint layers. As a result of fluctuations in temperature and relative humidity, paint may delaminate, curling away from the surface along the cracks, forming a concave center with raised edges, like a cup or crater (Michalski 2014). Prolonged cupping can lead to the the distortion of the fabric support of the painting due to tension (South Florida Art Conservation 2013).
Condition Reporting - Paintings. Part III: Glossary of Terms - CCI Notes 10/11. 2011. Canadian Conservation Institute. Retrieved 3/24/2014. http://www.cci-icc.gc.ca/publications/notes/10-11-eng.aspx
Kirsch, Andrea and R.S. Levenson. 2000. Seeing Through Paintings: Physical Examinationin Art Historical Studies. New Haven: Yale University Press.
Michalski, Stefan. "Agents of Deterioration: Incorrect Relative Humidity." Last modified March 13, 2014. http://www.cci-icc.gc.ca/caringfor-prendresoindes/articles/10agents/chap10-eng.aspx
South Florida Art Conservation. "Craquelure." August 19, 2013. http://www.sflac.net/blog/tag/craquelure/