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Censorship of parts of written text was and is a common practice in totalitarian regimes. It is used to destroy information not approved by the political power. Recovering the censored text is of interest for historical studies of the text. This paper raises the question, whether a censored postcard from 1942 can be made legible by applying multispectral imaging in combination with laser cleaning. In the fields of art conservation (e.g. color measurements), investigation (e.g. Analysis of underdrawings in paintings), and historical document analysis, multispectral imaging techniques have been applied successfully to give visibility to information hidden to the human eye.
The basic principle of laser cleaning is to transfer laser pulse energy to a contamination layer by an absorption process that leads to heating and evaporation of the layer. Partial laser cleaning of postcards is possible; dirt on the surface can be removed and the obscured pictures and writings made visible again. We applied both techniques to the postcard. The text could not be restored since the original ink seems to have suffered severe chemical damage.
One of the most important materials presenting and witnessing human culture is paper. The cleaning of paper is often necessary because contamination must be removed so that the fragile organic substrate can be preserved. The conventional cleaning methods are mechanical or involve the application of chemicals. These methods can damage drawings or print layers to some extent or make the original paper substrate brittle. More specifically, the use of a scalpel blade can cause damage to fibers. Chemical cleaning is difficult to perform locally, can dissolve foreign matter that then migrates into the paper substrate, or involves volatile organic compounds that can be harmful to the conservator. There is, therefore, a need for new conservation technologies aimed at the safe cleaning of paper. Lasers have proved to be an appropriate tool for cleaning as the energy dose and penetration depth at the specific point of contamination can be controlled. Additionally, if used properly, laser cleaning is not destructive to the paper.
Paper is one of the most important materials representing and witnessing human culture particularly as a carrier medium for text and image. As soiling hampers the reception of information, paper cleaning techniques are needed. Traditional mechanical and chemical cleaning methods are used by conservator-restorers. In some cases, a classical cleaning procedure of paper objects yields unsatisfactory results or a conventional treatment is even impossible. Especially, fragile paper objects cause problems due to mechanical instabilities. Laser cleaning as a non-contact method might be a way to overcome some of the limitations of classical cleaning techniques. Laser parameters have to be chosen to achieve removal of the soiling without influencing the artwork. Any immediate as well as long-term effects causing an irreversible change of the artwork have to be avoided. At present, most laser applications are found in stone and metal conservation, while laser treatment of complex organic materials like paper is still not fully developed for application in conservators' workshops. This contribution describes recent work of pulsed laser cleaning of soiled model samples. Pure cellulose, rag paper and wood-pulp paper were mechanically soiled with pulverized charcoal in a standardized procedure to make model samples representing essential characteristics of contaminated real-world artworks. Afterwards, model samples were cleaned using short and ultrashort laser pulses in the nanosecond and femtosecond time domain, respectively. An extensive analysis of the model samples after laser treatment using an optical microscope and a multi-spectral imaging system allows a comparison of the cleaning results obtained with both laser sources.