Blogs on metals conservation and research

Having spent too many years working in conservation I thought it was time to share some of my experiences with a wider audience. This periodic blog will look at current projects and conservation challenges as well as providing updates on my research into paint coatings for historic ironwork. Hopefully, it will be of some interest and I would welcome feedback.

In 2017 I decided to start a part-time PhD looking at the use of modern paint coatings for historic ironwork. During my time at Dorothea Restorations Ltd and afterwards I have increasingly been involved in specifying paint coatings for the protection go outdoor historic ironwork, both architectural and industrial; a necessity due to the lack of other environmental control options available to our portable heritage. The coatings used were originally developed for use on modern steel structures and often require a high level of surface cleanliness before they are fully effective. As conservators we are looking for the maximum amount of protection from the minimum amount of intervention so I thought it was time a proper evaluation was made of the common types of coatings used; single pack alkyd paints and two-pack epoxy and polyurethane/acrylic systems. I was also interested to test the long standing assumption that lead paints, the original paint of choice for ironwork, were still perhaps the best that one could use. This feeling was based on comments made and the occasional discovery of original paint layers still adhering well to a historic iron surface.

I was pleased to be offered the opportunity to carry out this research as a part time student back at Cardiff University, where I originally qualified back in 1986! Their knowledge and experience of iron corrosion and research into mechanisms of deterioration is well known in the profession.

22 October 2020

It was decided to test the three paint coating types in an outdoor environment over a period of 5 years rather than using accelerated ageing. A number of tests would be carried out every 12 months to see how each coating type was performing. These included oxygen consumption, electrical impedance spectroscopy, pull off testing and colorimetry. In addition, the weather where the samples are exposed is being recorded for the duration: temperature, relative humidity, surface wetness of samples and light & UV exposure. Hopefully, all this data should provide some meaningful information on how these paints are performing and when their performance starts to decline.

Pieces of genuine historic wrought iron sheet were cleaned prior to painting either to Swedish Standard ST3 (hand cleaned) or Standard Sa2.5 (abrasive blast cleaned). The coatings were then applied by brushing to the manufacturer’s minimum recommended dry film thickness. It proved very difficult to get hold of lead paints due to licensing issues so in the end I had to employ a specialist to supply the painted samples.

In March 2020, just after Covid-19 struck, the first batch of samples were removed from exposure for testing. As these coatings are designed to have a life of several years, no real deterioration was expected. However, the lead painted samples already showed a degree of failure! The finish coating showed obvious deterioration, the surface having cracked and shrunk on the exposed face of all samples! The underlying red oxide primer layers were still intact. The top coat failure was put down to the paint mix and there is a plan to repaint another set (reduced in number) with a revised top coat formula.

The failed top coat layers to a large lead painted sample.

The pull-off tests produced some interesting results. How the coatings failed (after 12 months) related to the level of cleanliness the samples were prepared to. For all those prepared to standard ST3 the failure occurred within the remaining compact rust layer (right image). Those cleaned to Sa2.5 generally failed within the coating itself (left image).

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