What is corrosion?
Corrosion that appears on metal objects is a chemical reaction where the metal is trying to return to its natural mineral state. This often appears as green deposits on copper and red/brown deposits on iron. The main cause of corrosion on metals is a humid environment.
How do you remove corrosion?
Some corrosion is good! If your metal object has a thin dark layer on the surface this may be acting as a protective barrier from further break down of the surface. This is called patination or tarnish.
We would recommend you avoid trying to remove heavy build-up of corrosion yourself. This often needs to be done under a microscope in controlled conditions. Excessive use of polishes and chemicals (I’m including Cola and Ketchup in this!) could remove original surface so that details and decoration are lost. For example, you would no longer be able to identify the date of a coin if the surface was abraded and removed. It can also cause the metal to thin and wear down silver plating. Chat to a conservator if you are concerned about the corrosion build up on your object or want to find the most suitable cleaning regime.
How do I care for the object in the future?
One of the most damaging things to metals is the oils and acids from our fingers, which can etch in to the surface of the metal. We would recommend always wearing cotton, nitrile or latex clothes when handling metal objects. This will also reduce marks and therefore the need for cleaning.
Giving the metal object a dust with a soft micro-fibre cloth or soft hog or pony hair brush will help prevent build-up of dust and dirt. Buffing or coating the surface with wax should rarely be necessary and should only ever be attempted after removing dust and dirt. Depending on the object and environmental conditions a wax or lacquer may be applied to protect the surface from humidity and prevent future corrosion development.
Store in an environment with a low humidity and pack using inert materials such as foam or acid free envelopes. A hoard of coins recently in conservation was packaged in individual acid free paper envelopes, sealed in a Polypropylene plastic box (recycling number ‘5 PP’) to provide a controlled environment and a sachet of silica gel to reduce humidity.
So, remember, avoid the cola and gently brush your objects to maintain a healthy patination!
Kayleigh Spring, Conservator, Conservation and Museums Advisory Service
Wiltshire Conservation and Museums Advisory Service is based at Wiltshire and Swindon History Centre. We preservice the Wiltshire and Swindon Archives and provide support to museums, heritage organisations and individuals to care for and conserve historic collections and meet professional standards.
If you have items in need of a little TLC see if our conservator can give you hints and tips for cleaning without damage, advice on storage or a to get a quote to have them repaired.
We hold regular conservation surgeries at WSHC from 2 – 4pm on the second Thursday of every month. The next one will be on May 9th. Book you appointment by calling 01249 705500.