Using Survival Analysis on Conservation Metadata to Benchmark Treatment Frequency

摘要

This research addresses the question of what is an acceptable period between conservation treatments. Taking the cleaning of impressionist paintings as a case study, survival analysis is carried out on metadata from conservation records for the Tate, UK, the National Gallery in Oslo, Norway, and the National Trust, UK. Currently the only guidance available is offered by the National Trust. Based on cost-benefit analysis, it suggests an interval of 25 years for light surface cleaning and 100 years for major treatment. The majority of paintings held by the National Trust are pre-20th century, with a varnish layer protecting the paint surface. Arguments are put forward that this guidance cannot be applied to unvarnished paintings with exposed ground and a fragile paint layer. The cleaning process for these types of paintings poses a greater risk as it inevitably causes some loss of paint. Any cleaning of paintings in this category is more akin to a major conservation treatment. The Kaplan Meier Product Limit estimator uses metadata collected from the conservation records to estimate an average inter-cleaning period for the Tate, UK, and National Gallery, Oslo. The result of the theoretical analysis, although highly dependent on the quality of conservation records available, is supplemented by evidence from practical experience. Current views on acceptable frequency of cleaning and the associated risks of treatment are sought using a survey of conservators with experience of treating impressionist and post-impressionist paintings. This is applied to a case study of the Munch paintings in the University Aula, Oslo, that are currently unacceptably dirty. In the past, these paintings have been cleaned on average once every 14 years, which is clearly too often. The results suggest that the intercleaning period should be increased by a factor of three. Criteria are discussed for improving the environment within the University Aula, Oslo, to achieve this goal.