The Nd:YAG lasers have gained the attention of many restorers as a useful tool, and many advantages, as well as disadvantages, of the laser techniques are well-known in advanced restoration/conservation research centres and studios. Ongoing studies of laser cleaning efficiency and of the long-term effects of laser irradiation periodically yield important results. The largest part of the studies is focused on the development of cleaning equipment and the establishment of procedures, or on the monitoring of remaining particles and surfaces.

The main question generating this study is the behaviour of the biological sample under laser treatment. More precisely, there is uncertainty concerning whether the biological particles are only removed by laser cleaning, and remain alive after the operation, or whether other modifications are induced and their natural evolution is halted or even destroyed. This study, therefore, complements analyses of laser beam interaction with various contaminated materials - stone, walls, frescos, paper, wood, etc.

Selected substrates contaminated with various species were cultivated in a laboratory, and monitored before and after irradiation. All the species selected for testing - Penicillium sp., Cladosporium sp., Ulocladium sp., Chaetomium sp., Alternaria sp.- are among the common agents that cause the biological degradation of old buildings, frescos, paper, wood, etc. Cleaning tests have been developed using the Nd:YAG laser at its basic setting (1064nm) and N2 laser (337.1nm). The study indicated that there were several cases when, after a cleaning operation, biological growth was stimulated by contaminated particles displaced by the irradiation. We feel that these fine removed particles deserve more attention. The Nd:YAG laser irradiation with appropriate fluences for safe cleaning did not reduce the number of units forming colonies on paint layers, or paint layers covering plaster.

The present work shows how an improper cleaning procedure may only appear to be efficient. Furthermore, results demonstrate that, at least for tested biological contaminations, a simultaneous vacuum cleaning is required.