The downside to the sampling is that it is a destructive method. Taking the samples is rather tedious because the preferred method is to look for flakes of pigment that are about to fall off. You then pluck these off with fine pointed tweezers. You also try to sample from all over the same piece of rock art rather than concentrating on one spot. Before and after pictures are taken and it is often impossible to tell that the rock art was sampled. The sample size is measured in milligrams.
One of the easiest samplings that I did was on some rock art from a cave called Naj Tunich in Guatemala. The rock art was actually an inscription panel that was vandalized and badly smeared. That rock art was a prime candidate because it was vandalized and because the inscription recorded the date of a Maya ruler visiting the cave. We knew the exact date of the inscription so it was a good test for validating the method. The decision on sampling rock art is not taken lightly. The rock art must have some really good reason for being sampled and it must have a good potential for providing new information. This sampling is not widely done on rock art. Allan
