Essential oils mixtures as an eco-friendly biocidal solution for a marble statue restoration

In order to achieve eco-friendly restorations, researchers and stone conservators are shifting towards low environmental impact biocides like plant natural extracts. The application of essential oils mixtures was tested in order to select and carry out a biocidal treatment on the Statue of Silvanus (National Archaeological Museum of Florence). Four essential oils (Coridothymus capitatus L., Syzigium aromaticum (L.) Merr. & L.M.Perry, Cinnamomum zeylanicum Blume and Origanum vulgare subsp. Hirtum Link Ietsw.) were chosen and blended, obtaining two different mixtures used for the treatment of the statue. They were applied by using three different carriers (agar-agar, Politect® and Carbogel) for ensuring a better biocide-patina contact. The poultice method allowed to use low concentrations and long times of application. This methodology required corrective actions, linked to the peculiarity of the stone masterpiece, like the pH adjustment of the compound. The results are an important precedent for the use of essential oils as biocidal treatment on outdoors stone heritage. In particular, the low concentration that have been used are non-toxic for humans and entail no environmental bioaccumulation. Moreover, the statue will be further constantly monitored in order to document the long lasting effect of the applied treatments.