Friday, 1 October 2021
Sometimes, even apparently innocuous things can cause significant conservation problems. This summer our conservation team dealt with an unexpected issue caused by a slip-up from one of our visitors.
During one of our daily artwork checks, a banana skin was found on top of Goslar Warrior (LH 641). While it might not sound very serious, this single piece of litter had quite a dramatic effect on the surface of the work, as Senior Technician James Copper explains:
“The weather was extremely hot, and the bronze itself had become warm/hot, cooking the banana skin. When the skin was removed, the result was a chemical burn mark on the bronze, creating a dark stripped patina in that area.
This was caused by the chemical make-up of the banana, which is packed with various minerals including magnesium, phosphorous, sodium, zinc, and potassium. All of these chemicals are used in the patination of bronzes, and each gives a different colour effect to the bronze.
The amount of potassium in a banana outweighs the other minerals by between 16-300 times, so it would be the most active ingredient in a banana skin. Potassium creates a dark brown/black patina to a bronze, and this is what was happened when the banana skin was left on the sculpture. ”
James and his team undertook a multi-stage process to restore Goslar Warrior to its former glory.
The chemical burn caused by the banana skin resting on the bronze surface of Goslar Warrior
Photo: James Copper
Using a blow torch to activate an application of Potassium Sulphate during the restoration process.
Photo: James Copper
The damaged area of 'Goslar Warrior' mid way through the restoration process, showing a darkened area of bronze.
Photo: James Copper
- Firstly, the blackened/ burnt area was rubbed back with water, and a fine scotchbrite pad to remove the darkened area, and open up fresh bronze to receive a new patina.
- Then the first layer of this new patina was applied: a new base coat. This coat was an application of Potassium Sulphate, which was applied by brush to the area, and heated up with a blow torch. (Although this sounds exactly what was done with the banana, this was a controlled method).
- The next step was to apply a solution of ammonium chloride to the surface, again by brush, and again with heat. This again helped speed up the process, and create a base colour. This colour was allowed to stabilize and develop overnight.
- The following morning, this area was reactivated with a damp sponge and the excess chemicals were removed. Then another application of ammonium chloride was applied, this time cold. This was allowed to dribble a little down the surface to keep a natural and randomised look. This was left for a couple of hours, then given another application, which was allowed to settle overnight.
- The next day the excess chemical was lightly sponged off. Ferric chloride was then added to the ammonium chloride solution, . The ammonium chloride creates a blue patina, and the ferric chloride turns the blue, to green. Several applications of this mix were again applied to the surface, careful not to create a flat colour.
- Again this was left, and excess removed in the morning.
- A further application of ammonium chloride was then given, to bring some more blues back in.
- Later that day, a weak wash of potassium sulphate was applied to this area to soften the green/blues.
- Finally, the various mixes were also applied to the sculpture immediately around the repair to blend in the newly patinated section.
As you can see the damaged section of Goslar Warrior's neck is now all but invisible. Thankfully this kind of damage to sculptures in our gardens is quite rare, however it's always best to remember the following from Dr. Hannah Higham, Senior Curator of Collections and Research:
“Although the sculptures may look tough, they are actually very delicate. The works are hollow and made from bronze which is only about 1cm thick. Each has a unique surface patina, in gold green or brown, which Moore and his assistants created through the application of different chemicals. When touching the sculpture, please be mindful of anything sharp like jewellery, watches, belt buckles or clothing that could inadvertently scratch or dent the bronze. Please also be careful not to step on the plinths, as these are part of the artwork. Thank you for helping us to preserve these works of art for future generations.”
You can read more about Goslar Warrior in our Collections pages, including details of the other casts of this artwork.