Use of high-strength electromagnetic radiation to remove phototrophic biofilms from terracotta artifacts

Abstract

A novel technique, effective in eliminating biodeteriogens from biofilms encrusting terracotta artifacts, is presented here. This method is based on the use of high-strength electromagnetic radiation (EMR) in the radiofrequency band. Shards of terracotta from historical pots at the Botanical Garden of Naples, Italy, were used. The shards, after sterilization, were inoculated with several phototrophic microorganisms previously isolated from whole terracotta pots. The newly formed biofilms were exposed to EMR amplitude modulated by a train of rectangular pulses with Tr = 200 ns repetition time and 10% duty cycle. The exposure protocol consisted of three applications of 2 h each, every other day. Denaturing gradient gel electrophoresis analyses conducted on the newly formed biofilms showed that, after the first exposure to EMR, all species in the biofilms but one were still alive. The second exposure resulted in the disappearance of 9 out of 13 species that were initially present on the samples. After the third exposure, all species disappeared. Superficial layers of terracotta from the exposed samples, transferred to a culture medium at 24 °C for 72 h, did not show any re-growing of organisms. Petrographic analyses of the sampleswere carried out before and after the treatments; they showed that exposure to EMR did not cause structural alterations in the treated substrates. Moreover, the amplitude of the EMR that samples were exposed to was not high enough to cause any significant increase in the temperature of the substrates; that is, no thermal effect, which is the most relevant effect when matter or organisms containing water are exposed to EMR, was observed. Finally, the field strength of the EMR showed to be non-invasive for the artifacts and non-dangerous for operators and the environment as compared to other techniques adopted in the field of conservation of cultural heritage.