Conservation Science

222 Chapter 9

Surface etching

Formation of depression

Formation of

hardened crusted

Extreme flaking Black crust

Crust regenerated

Flaking

Tafoni

Scaling

Breaching

layer

Removal of

crust

Figure 5Potential degradation pathways for sandstone
(Based on Inkpen and Petley, 2001)

Critical Damage

Threshold

Damage/Pollution

Level

Time

Fast rate of

damage Slower rate

of damage

Figure 6Damage function for stone. Stone decay proceeds, often at an assumed linear rate,
to a critical threshold point. Beyond this point the damage is unacceptable. Unlike
some phenomena to which this form of analysis has been applied, reducing
pollution does not reverse the effects of damage. Certain suites of processes may
operate at a more rapid rate to alter the stone, even under the same pollution
conditions, than other suites of processes. This could result in differential damage
on a building