General Knowledge
Why do old paintings crack in a fine net of lines?
- Paint and canvas shrink at different rates
- Sunlight burns them
- They are cleaned too often
The answer is Paint and canvas shrink at different rates. Craquelure - layers age and move by different amounts.
Because the layers age at different rates. Oil paint continues to harden and shrink for decades, while the canvas or panel beneath expands and contracts with every change in temperature and humidity - so the brittle top layer is pulled about by a support that moves more than it does, and it fractures.
The pattern is called craquelure, and it is diagnostic. Paintings on wood panel crack differently from paintings on canvas; Italian, Flemish and Dutch practice each produced characteristic networks; and the cracks in a genuinely old painting run through every layer, including the varnish, in a way that is very difficult to fake. Forgers bake canvases and roll them to induce it, and conservators look precisely there.
It is also why climate control in galleries matters more than lighting. A stable relative humidity keeps the support still; swings are what tear the paint. The Mona Lisa's poplar panel has a crack running down from the top that has been monitored for centuries, and the painting is kept in a sealed case at a fixed humidity largely to stop it moving.