Historic wrought iron cleaning and painting
Historic Scotland is responsible for the welfare of considerable amounts of historic iron work. This may occur as structural and decorative features at heritage sites and also in public areas. The iron occurs in both cast and wrought forms, with both often being painted for decorative and protective reasons. Some research has been carried out at Historic Scotland into best practice for recoating cast iron, but this data cannot be directly extrapolated to procedures for wrought iron. It has differing physical properties and corrosion phenomena due to its metallography being significantly different to that of cast iron, consequently coating systems for wrought iron must be assessed separately. Optimising surface preparation can lead to increased longevity of paint layers, which impacts on the costs of preserving heritage. With this in mind David Watkinson in Cardiff Department of Archaeology and Conservation was commissioned to investigate surface preparation of wrought iron to receive coatings. The study will be carried out by Nicola Emerson, an MSc and BSc Conservation graduate from Cardiff, under the supervision of David Watkinson.
Multiple paint layers on historic wrought iron samples.
Industrial preparation of corroded ferrous metal surfaces to receive protective coatings has received considerable attention, and many international standards exist for methodologies. Within heritage conservation iron is often contaminated with chloride and corrosion may be an integral part of an object’s shape, which introduces certain ethical and operational constraints to the process, as does the need to be aware of paint chronology within layers to identify the original paint. In some contexts international standards are used to prepare surfaces, such as the SA 2.5 used to clean the above water wrought iron plates of the ss Great Britain (ref to ss GB project page) (Watkinson at al 2005). A number of cleaning processes are loosely in use within heritage conservation and this project sets out to compare a selection of five for their performance.
Tests by Historic Scotland on surface preparation of cast iron (Wilson 2010)
Heritage wrought iron will be treated by surface preparation methods and then examined by SEM and optical microscopy to assess surface morphology following cleaning. The influence of cleaning on corrosion rate will then be quantitatively assessed for individual historic iron samples by recording oxygen consumption in controlled RH and temperature. Once the best performing method has been identified two paint systems will be tested for their performance. Overall the data will contribute towards best practice for protective power and longevity of coatings, which will directly impact on cost effectiveness of painting to maximise resources available for heritage preservation.
This Project is funded by Historic Scotland at £24,000 over one year and begins in October 2011.