Caring for our cannon collections

If you’ve ever visited one of our castles or forts, chances are you have seen cannons looming overhead, a reminder of the turbulent history of some of our sites. We care for over 150 cannons on open display at multiple locations across Scotland, from Shetland down to Dumfries. Many date to the Napoleonic era, in the 1800s.

There have been multiple projects in the past exploring how we can best care for our cannons. We are now working with stakeholders including the Royal Armouries and Historic England to establish a comprehensive programme of conservation. Building off current research and the maintenance work that already happens at our sites, this project seeks to address the unique challenges presented by caring for cannons displayed outdoors, where weather, pollution, and visitor impact has an ongoing effect on their condition.  

Many of the challenges are due to the nature of the historic iron that our cannons are made of. Unlike modern iron or steel, the age and surface variations of the ironwork means that cleaning and painting needs to be carefully carried out to avoid losing or damaging historic material. By conserving the cannons we are able to reveal more of the historic details, such as maker’s marks and royal insignia. 

This conservation programme also forms part of a much wider research project exploring protective paints and coatings for outdoor ironwork. The cannons at HES present an ideal case study for looking at the effectiveness of different paint systems in a range of environments. 

As part of this, two trials have already taken place. These focused on testing different methods of preparing the surface of the cannons for painting, as well as selecting which modern paint system to apply, based on research from the early stages of this project.

The first trial took place at Stirling Castle and included lifting the cannon from its carriage with a portable gantry to provide better access to the areas being treated.

Multiple methods were trialled to remove areas of flaking paint as part of the surface preparation. Sanding down the old paint layers by hand was found to be the best method as it is the most effective and easiest to control. Areas on the cannon that were actively corroding (rusting) were then treated with a rust convertor, which chemically reacts with rust to create a more stable surface.  

Another key part of surface preparation is salt testing. Salts from the environment increase corrosion, especially at our coastal sites. Making sure to wash off any salts present is an essential part of slowing further corrosion and preparing the surface for painting.  

Following surface preparation, a three-part paint system was applied. This consists of a primer layer to actively protect the ironwork, a ‘build coat’ to help provide a smooth finish, and a topcoat which protects the underlying paint from the effects of the environment, as well as creating a shiny black surface.  

A similar methodology was trialled at Dumbarton Castle, although limited space meant that the cannon could not be removed from its carriage during treatment.

We also discovered the historic use of lead paint on the cannon at Dumbarton, meaning that work had to be carefully planned to ensure safety measures were in place. The presence of lead paint is always tested for, and when identified there are increased restrictions around surface preparation methods and protective equipment must be worn.

This trial also highlighted the challenges of undertaking this work outdoors, where unpredictable Scottish weather can cause delays. Keeping the ironwork dry during treatment is essential to prevent trapping moisture between the paint layers, which can increase the risk of future corrosion. Dry, warm weather also helps ensure the paint dries properly.

Ongoing visual inspections and targeted analysis will help us determine how well the paint ages at both sites. As part of this, an electric probe was used to measure how thick each layer of paint was once it had dried. This will help us understand how the paint performs compared to published data and manufacturer specifications. 

The trials also include testing the use of ‘tompions’, caps placed in the end of a cannon’s barrel. Tompions protect the internal ironwork from the environment and stop debris collecting inside the cannon. These trials will compare the use of traditional wooden tompions with a new prototype developed in partnership with the Royal Armouries by installing one of each type into cannons that are displayed side by side. A monitor placed inside the barrel of each cannon will measure the internal temperature and humidity for a year to see how well each tompion performs.  

These trials in combination with the wider research on paints and coatings for outdoors ironwork will help us establish a comprehensive programme of conservation for cannons at HES.  We will also be able to share our learning with other heritage bodies who have these challenges. Combined with ongoing maintenance to keep the cannons clean and prevent future damage, this project will ensure that the historic cannons at our sites continue to stand guard for the next 100 years.