Context of study Historically, wrought iron was in ubiquitous use for structural, decorative and utilitarian items. Maintaining this material is a headache for heritage bodies due to a lack of quantified data for producing best practice guidelines to support evidence based preservation processes for use in cost benefit assessments, management, and future maintenance planning. The application of coatings is normally central to most wrought iron preservation programmes. Much published information and several international standards guide the preparation of modern ferrous metal surfaces for coating application. This is absent for corroded heritage wrought iron, where choosing a preparation and coating method relies upon anecdotal evidence from conservation practitioners and very limited reported or published research. Historic Scotland and Cardiff University are undertaking research to address this need for a best-practice protocol for surface preparation of historic wrought iron. This will involve generating experimental data on corrosion rates to consider in conjunction with evidence from current research and practice in industry. Methodology Investigation of surface preparation methods currently used in wrought iron conservation identified five methods appropriate for further research: flame cleaning/wire brushing; chemical cleaning (NaOH)/wire brushing; and air abrasion with aluminium oxide/glass beads/crushed walnut shell. In keeping with the project focus on real life contexts, corroded historic wrought iron samples are being used rather than analogues. Surface morphology of the cleaned samples was investigated and recorded by SEM and optical microscopy, then classified according to a criteria anchored terminology designed to assess the suitability of surfaces for coating. The samples are undergoing accelerated corrosion at high static RH in sealed reaction vessels. Remote sensing of oxygen consumption records corrosion rate for each sample. With the aid of controls this will determine whether surface preparation method influences the corrosion rate of the uncoated wrought iron. This can potentially identify which method(s) produces the least corrosion and will be considered in conjunction with the practicalities, health and safety concerns, environmental impact, and cost effectiveness of employing each method in order to advise on a best-practice methodology for heritage wrought ironwork. The next phase of the study involves a preliminary investigation of the relationship between surface preparation and surface coating by re-cleaning the samples, coating them with a protective coating system (yet to be selected) and exposing them to a high RH. This paper verbally reports results of the surface preparation experiments and progress on the coating tests.
展开▼
