Cleaning of corroded iron artefacts using pulsed TEA CO 2- and Nd:YAG-lasers

Abstract

Archaeological iron artefacts are often covered with a crust, containing organic fibres, mineral particles, dirt, etc. Before conservation, this crust must be carefully removed. With traditional mechanical and chemical methods, there is an obvious risk of over-cleaning. These methods also increase the risk of the oxidised or primary corrosion layer breaking away which will lead to loss of information about the form of the object and other important surface details. The main advantage of using pulsed lasers for cleaning metallic archaeological artefacts is that the removal of the crust is well controlled and can be carried out layer by layer. To determine the most suitable irradiation conditions, the wavelengths and the fluence for successful cleaning must be evaluated carefully. As test samples, corroded utility articles such as nails and hand tools have been used. Depending on the composition of the crust, different lasers have been used: TEA CO 2 lasers have been successfully used for removal of organic materials and rust. EO-Q-switched Nd:YAG laser at IR and at green wavelengths, which have different absorption characteristics, has also been tested. The Nd:YAG laser was able to clean the corroded samples; however, the risk of surface damage was higher than when using the TEA CO 2 laser. Analysis of the surfaces cleaned by lasers has been performed with optical and scanning electron microscopy, X-ray spectrometry and Raman-spectrometry.