Hall Conservation Ltd


  • Oculus before treatment. Image provided by the Stained Glass Department, Canterbury Cathedral

  • Lamination in the wrought iron, uncovered during cleaning

  • One of the old repairs to the ferramenta


Date:18 March, 2013

Project Information

Hall Conservation Ltd was selected as conservators to the charcoal wrought iron* ferramenta at Canterbury Cathedral due to our historical knowledge of, and expertise in blacksmithing. The South Oculus ferramenta dates back to c.1180, it is the support frame for the glazing and is an incredible example of Medieval engineering. What makes it truly extraordinary is that it is a ‘space frame’, and the space frame is assumed to be a 20th century invention, however, this light but strongly braced structure would clearly have fulfilled the same role. It is constructed using technology usually associated with timber technology; mortices, tenons and wedges. From the way it is assembled, it is clear that it would have been constructed in the forge prior to being lifted as once complete unit, 20m up the South Transept at Canterbury Cathedral.

In its more recent history the ferramenta had been painted in order to protect the iron from the elements, this paint had broken down allowing water to creep beneath it, trapping water which was causing corrosion and had to be removed. The old paint and window putty was picked off using scalpels, brass brushes and Wishab™ sponges. This minimal intervention approach ensured that the surface of the iron was not damaged during the treatment. After cleaning the ferramenta was coated with Waxoyl™ in order to protect it from the elements.

During this project we instigated research into the metallography of the oculus. Please see our research page for more information.

*Long before coke and coke fired furnaces were developed in the first decade of the 18c, wrought iron was produced in charcoal fired furnaces and is referred to as charcoal iron.