Fort Nelson on Portsdown Hill, Portsmouth houses the Royal Armouries collection of Artillery an important part of which includes a group of Bronze Cannon currently displayed outside on the Parade Ground. The majority of these were transferred from Tower Wharfe Green at the Tower of London in 1991 although several have been at the fort for about ten years.
The cannon display a pale yellow / green patination and the aim of the project was to establish the cause of this by monitoring atmospheric pollutants from surrounding industry, primarily emissions of Sulphur Dioxide and Oxides of Nitrogen from information provided by the Atomic Energy Authority, the body responsible for correlating emission data for the Government. This was interpreted in relation to weather movements to build up an environmental overview of the fort. For the purpose of the project we installed a computerised automatic data logging weather station running on a Microsoft Windows programme. This continually recorded weather patterns which were stored hourly. Wind speed, direction, temperature, rainfall, pressure and relative humidity were all registered and presented in either graph or spreadsheet form for analysis. It would be futile to suggest that Sulphur Dioxide emitted from a known source was a potential corrosion problem if the wind consistently carried the plume away from the fort. Wind direction was also important because the fort is situated in a marine location 1.2 Miles from the Solent and part of the project was to ascertain if windblown Chlorides were present to compound samples taken from the cannon.
The study was confined to six Bronze guns on the parade ground. These are of Turkish, Russian, Indian, Chinese and English origin dating from the mid 15th to the early 19th Centuries. Despite the diversity of the foundries and the time scale over which they were cast all display similar corrosion trends. The representative guns were The Dardanelles Great Bronze Gun cast in Turkey in1464 (Wt: 16 Tons), an 18 Pounder Bronze gun cast in Turkey in 1708 (Wt: 3.75 Tons), a late 18th Century Indian 24Pdr, a Russian Bronze Licorne from 1835, a Chinese 36Pdr from 1856 and an early 19th Century English Bronze 24Pdr (Wt: 2.75 Tons).
Reduced Sulphur Gases
Four Local Sources:
Ozone and Oxides of Nitrogen
Two Local Sources:
To identify the corrosion products present a 10mg scraping was taken for analysis by the British Museums Conservation Research Group using x-ray Powder Diffraction (XRD).
Results of the XRF Analysis
Northern European Copper Ores were often Sulphide rich and the extraction process
required vaporised trace elements Arsenic and Bismuth leaving the extracted Copper
relatively pure. Central European Ores however tended to be Oxide rich so were
more easily extracted leaving a higher proportion of Antimony, Arsenic and Bismuth
in the final product. If the Antimony content is over 0.3% the Alloy will be more
brittle 2. This is applicable to the Chinese 40 Pounder from 1856 with a 0.39%
Antimony content. This is also a notable for being a Brass gun whose major elements
were found to be Cu: 67.7% and Zn: 26.9%.
The Indian 24 Pounder from the late 18th Century is a Tin rich Bronze with 87.5% Copper and 13.4 % Tin. Exposure of a Tin rich bronze to the environment results in the formation of a very thin but resistant Tin Oxide which diminishes the dissolution of the Copper from the Alloy 2. This was found to be the case with the Indian gun which has a greenish blue / black patination and whose major identified corrosion product was Cassiterite a common Tin Oxide.
Nickel is a notable trace element due to its sensitivity to Chlorides, Sulphur compounds and Oxides of Nitrogen. It was found in all of the guns except the Brass Chinese 40 Pdr from 1856.
Three of the guns were found to have Arsenic contents between 0.3 - 0.5%. These are the early 19th Century English 24 Pdr, the Turkish 18Pdr from 1708 and the late 18th Century Indian 24 Pdr. This is significant because at these levels Arsenic increases the density of the Alloy and can increase corrosion resistance 3.
Results of the XRD Analysis
The most prominent finding was that Brochantite was identified as the major corrosion
product on four of the six guns sampled: The Dardanelles Great Bronze Gun cast in
Turkey in 1464, the Chinese 40 Pdr from 1856, the Russian Licorne from 1835 and the
early 19th Century English 24 Pdr. Brochantite is one of the most common outdoor
Bronze corrosion products which forms a soluble, porous, hygroscopic and unprotective
surface in the presence of Sulphurous atmospheres and rainwater. Its physical
appearance is a series of small grey / black areas surrounded by pale green corrosion
products. Because the pale areas are anodic and the black cathodic a self perpetuating
corrosion cycle will continue with selective dissolution of Copper from the Alloy
4.
The important findings were that Brochantite was the main corrosion product concurrent with Sulphur Dioxide and rainwater and that there were no traces of chlorides present to the compound samples analysed. Accelerated corrosion in a marine environment could therefore be eliminated despite strong Southerly winds from the Solent.
Sharon Robinson, Tyne and Wear Museums.
