Table 2 Typical deteriorations in Guangyuan Thousand-Buddha grotto

Stability deterioration

Scaling off

Surface of the stone is peeled off as a slice or crust due to salt dissolution and crystallization, temperature fluctuation, freezing or weathering

Detachment

Surface of the stone is out of the grotto body and forms a cavity but is not peeled off

Water deterioration

Water leakage, water incrustation inside the grotto due to rainfall, surface runoff and capillary water

Body loss

The grotto or sculpture show a certain volume loss from its initial state

Cracks

Includes three types: one is weathering fracture, often occurring on the surface; second is stress fracture, often extending to the inner stone, and may cause body loss or complete breakage; last is structural fracture, often closing, smoothing and appearing as a group

Plant damage

Plants grow in the cracks of grotto; the root may lead to more cracks

Weathering deterioration

Rock powdering weathering

Surface of the stone is softened and peeled off because of temperature and humidity fluctuation, freeze–thaw cycles, water and salt activity

Deep loss

The plaster layer of paint in the grotto is peeled off from the stone

Salt crystallization

Soluble salt dissolved and crystallized because of temperature and humidity changes

Pigment layer peeled off

Paint layer of grotto flaked and broke away from the grotto or sculpture

Paint layer craquelure

Paint layer of grotto cracked but not peeled off from the grotto or sculpture

Scratch or graffiti

Scratch or graffiti by human activity

Improper repairing

The grotto was repaired by improper materials or methods in the past

Surface pollution

Sootiness on the grotto or sculpture by smoke from the past. Dust and weathering products cover the surface of the grotto

Microorganism pollution

Microorganism grows on the grotto