Stone samples are labeled by ⁸⁵Kr in low energy (hundreds of eV) plasma beam. Radioactivity and its changes during any surface reaction leading to the liberation of trapped krypton ions back to the gas phase is measured by G.M. counters. The methods enables researchers to follow the slight interaction of stone with an atmosphere containing around 100 parts per million of sulfur dioxide or nitrogen oxides. The technique can even be used for evaluating treatment efficiency because the diffusion processes during which the krypton ions are freed from the mineral lattice are only slightly influenced by conservation materials like siloxanes or acrylic resins.