We try to actively promote good practice in the care of lead sculpture and with this in mind we publish here our recent experience and findings with regard to lead dioxide formation.
The problem in brief
Over the past twenty years in the United Kingdom we have observed a growing incidence of outdoor lead sculpture and other lead constructed items, such as roofs and rainwater goods, developing a visually disfiguring red-brown to purple-brown surface discolouration.
We have also found this same problem on some lead sculptures in the grounds of the Palace of Queluz, 20 km west of Lisbon in Portugal (where there is a significant collection of mid-18th century lead sculpture), and it is likely to be occuring throughout Europe.
In the majority of cases, the discolouration witnessed in both countries has been found in rural or semi-rural locations, particularly on recently conserved/cleaned surfaces but also (though seemingly less frequently or rapidly) on lead that has had ample opportunity to develop a stable natural patina.
The relatively short time in which this brown colouration has appeared and its visually disrupting nature is causing great concern, mainly for aesthetic reasons.
(Photo left: Location - Cornwall, England)
Unlike bronze sculpture, there has been comparatively little research or analysis on the natural formation of patina on lead sculpture or the environmental factors that influence its development, apart from those extreme conditions that can cause active corrosion and consequently the breakdown of the metal surface.
However there is a reasonable amount of published research on the breakdown and discolouration of lead based pigments that may have some relevance to the surface problem now being experienced on exterior lead sculpture and constructions. In addition, the corrosion coatings found on archaeological lead objects have been studied in some detail.
(Photo left: Location - Warwickshire, England)
Chemical identification of the red-brown formation
The full analysis of this red-brown compound has yet to be completed. Lead forms five oxides: Pb2O, PbO, PbO2, Pb2O3 and Pb3O4. Two of these oxides are of particular interest in this instance: lead dioxide (PbO2) also known as ‘puce oxide’, which is a red-brown colour, and red lead Pb3O4 or ‘minimum’ is 2PbO·PbO2.
It is our opinion that the most likely compound to be producing this formation is lead dioxide PbO2, which is insoluble in water. Red lead is also insoluble in water, however the reactions required for its formation in most environmental conditions would seem to be less likely to be encountered.
(Photo left: Location - Derbyshire, England)
The composition of naturally formed lead patina
Lead is normally exceedingly resistant to corrosion when exposed to the atmosphere, whether it is rural, urban or coastal. On new lead sheet and castings, the formation of a patina takes time to develop an insoluble and strongly adhering coating of lead sulphate (PbSO4), which is silvery grey in colour.
Lead forms its patina in three stages: on initial exposure, a film of lead oxide (PbO) forms, which is then converted to lead carbonate (PbCO3), which is light grey to white in appearance. This is not to be confused with the basic lead carbonate ‘white lead’ Pb3(0H)2(C03)2, the pigment formed by acting lead with acetic acid (vinegar). This lead salt is also found in lead patinas.
The film of lead carbonate does not adhere particularly well to the metal substrate and can wash off, however in reaction with carbon dioxide and sulphur dioxide this slowly converts to the stable and largely insoluble lead sulphate PbSO4, via intermediate tetrabasic lead sulphate and sulphite phases (third stage). Levels of atmospheric pollution play an important role in the rate of formation of a stable end product.
The natural formation of the patina on lead as described above does not take into
account the presence of impurities in the lead alloy, from past surface treatments or from the possible action of other atmospheric pollutants in disrupting this chemistry.
Therefore taking into account the inevitability of at least some of these additional factors taking a part in patina formation, it is likely not only that other lead compounds will be found as constituents of a natural patina but that pollutant particulates and other lead compounds would be found in any detailed analysis of a naturally formed patina.
(Photos left: Examples of natural lead patina, on a lead sculpture group and a lead hopper head, both in England)
Possible causes of the red-brown formation
As already explained, the normal formation of a stable patina on exterior lead is a relatively simple but slow chemical process. However it is clear that, in locations where instances of the red-brown formation have been noticed, this process is being corrupted by some form of abnormal chemical reaction or environmental pollutants. The basis of current thought is that, since around 1990, there has been a dramatic reduction in SO2 immersions while, correspondingly, there has been an equally significant increase in NOX pollution. The theory is that for some reason the SO2 was acting as an inhibitor to the development of the PBO2 formation and, since this is reduced, the NOX gas pollution (particularly from aviation) is causing the conversion of the existing natural corrosion product to lead dioxide. Increased instances of the red-brown PBO2 formation in locations beneath flight paths have been noted, but this correlation has yet to be proven.
As the areas of formation do not exhibit any of the characteristics of active corrosion such as pitting, powdering of the surface or loss of metal, it is assumed that the chemical process being seen is one of the conversion of existing white to grey coloured lead compounds or salts to red-brown compounds. The red-brown discoloured surface areas are found to be thin coatings and insoluble in water but cleaning tests using both dilute acetic acid and hydrochloric acid have shown to remove the formation and, so far, have revealed no damage to the underlying metal surface, further supporting the theory that it is a conversion process that is producing the surface colour change and not corrosion in the true sense.
As mentioned in the introduction, it is important to recognise that the colour change is found to be developing not only on cleaned or new lead but also on lead with an existing sound natural patina.
(Photo above: Location - Warwickshire, England)
Possible causes of the conversion of lead carbonate and lead sulphate to lead dioxide or red lead
1. It is well documented that lead can develop a red-brown formation when exposed to alkalis such as lime or cement, either in direct contact or as airborne dust. In the USA, this discolouration has been attributed to surface contamination with chemical fertilisers. It is possible that both phosphate- and nitrate-based chemicals could cause the formation of PbO2 dioxide.
2. The electrolytic process used in the lead acid storage cell battery which utilises the potential difference between lead metal and lead dioxide in an acid electrolyte. The cathode is made of lead metal, the anode made of grid filled with red lead which is converted to lead dioxide PbO2 in a dilute solution of sulphuric acid. In our view there is a strong possibility that the presence of moisture on lead with a coating of lead oxide PbO and/or lead sulphate PbSO4 could, by electrolysis, produce PbO2 dioxide using the same reduction principle as the lead acid battery. Furthermore the addition of heat in the form of sunlight will aid the reduction process. Given that the sculptures and surfaces that have developed this conversion formation are positioned in the open and receive strong direct sunlight this possibility needs to be investigated further.
3. At the Palace of Queluz it is known that the sculptures were once painted with lead based paints, the undercoat of which was a red lead Pb3O4. Acid will decompose this compound to both the dioxide PbO2 and the monoxide PbO. Past cleaning of the sculptures, which now have little remaining paint, may have initiated the areas of dioxide formation or as the original paint gradually failed, the process might be one of natural decomposition and surface conversion.
4. There is an additional possibility that should be considered, that of microbiological induced conversion of the lead surface. In the study and conservation of murals and wall paintings, both interior and exterior, it has been found that many strains of bacteria and fungus could affect both the pigments and binders of paintings. Bacteria (especially of the genus ‘arthrobacter’) were found to reduce lead containing pigments to lead dioxide. Further sulphur reducing bacteria, which play a role in stone deterioration, may in environments prone to industrial pollution be able to colonise the surface of lead sculptures and objects. If present in the environment the pollutant sulphur dioxide could catalyse a reaction to convert existing lead compounds to brown-red dioxide.
5. Red lead Pb3O4 or 2PbO.PbO2 can be produced by the annealing of lead carbonate in air 6PbCO3+O2 => 2Pb3O4+6CO2 or the oxidative annealing of lead white 3Pb2CO3(OH)2+O2 => 2Pb3O4+3CO2+3H2O. Whether conditions are right for either of these reactions to take place needs to be assessed.
The photos below show the development of the formation on a lead statue between 2002 and 2008:
(Location - Kent, England)
Research to identify the conditions under which red-brown formation is produced
As explained, so far there has been little research undertaken into the composition and potential causes of discolouration of the patina on lead sculpture. The possible causes that we have outlined are derived from research in related areas of fine art and buildings conservation and as such only provide the basis from which a more focused research programme could be devised. It is possible that the conditions that favour the production of this conversion formation are relatively simple in chemical terms, however the identification of the causes could be complex and involve a unique set of conditions and chemical reactions to prevail to initiate the conversion. Further, the necessary combination of conditions may only be encountered within certain environmental parameters that could be seasonal and hence more difficult to identify.
Initial research is required to establish by analysis the chemical make up of the patina layers on the affected sculpture; this may provide good clues to the likely chemical reactions required to produce what is assumed to be formation by PbO2. Following this, analysis of the air quality to identify any pollutant gasses or particulates that could act as a catalyst to the reaction might further eliminate certain possible causes. This will allow further research to be focused on the most likely causes.
We are monitoring the existing formation on sculpture in the various locations in the gardens of the Palace of Queluz already affected by this problem. Additionally we have placed test lead coupons to which a variety of surface finishes have been applied to assess which are vulnerable to PbO2 conversion formation.
Future surface treatment of lead sculpture
It is clear that in terms of conservation there needs to be a concerted effort to establish the chemical and environmental factors which result in this visually disfiguring discolouration. This being said, it is also necessary to look at how the surface of lead sculpture and objects can be protected in the future, particularly after conservation works.
To rapidly form a stable patina that replicates the slow natural formation is potentially a difficult problem. Lead used in roofing is usually given a coat of patination oil to aid the production of natural patina by reducing the dissolution of the initial, more soluble lead compounds.
Until now the traditional conservation treatments for producing a patina on cleaned or repaired lead sculpture has been confined to chemical treatments using very dilute sulphuric acid to aid the production of PbSO4, proprietary nitric acid based lead blacking products used in the stained glass industry or as previously mentioned, patination oil.
However in ideal circumstances it would be best to allow the lead to naturally form a patina. One way of achieving this, while at the same time providing a visually unified surface colouration, is to apply a very thin coat of paint, matching the colour of the natural patina, which will degrade and be washed off in time; this allows a natural patina to form gradually as this process takes place. The sculpture group illustrated left at the Palace of Queluz has been treated in this way after receiving full conservation, and its ongoing condition will be monitored.
In conservation, cleaning lead with diluted hydrochloric acid (HCL) is a process used quite frequently, however the patina film formation produced during this process is not entirely insoluble and can wash off. Film solubility increases quite rapidly with small rises in temperature.
Depending on the results of future research it may be necessary to consider other chemical methods for patination, or the use of clear surface coatings (such as oil or wax) to provide additional protection, or in extreme circumstance the use of paint coatings to prevent the red-brown formation occurring. It is important to remember that historically the majority of lead sculpture was, at the point of manufacture, fully painted to resemble stone or bronze, and in some cases gilding was applied. If, after research, no reliable solution can be found to this relatively recent problem then repainting in the traditional way might be the only way forward if the present disfiguration is aesthetically unacceptable.
(Photos above: Location - The Palace of Queluz, Sintra, Portugal)
The assessment of the red-brown formation problem given here is in the main part empirical but is intended to apply some logical perspective towards finding a solution to this visually unacceptable discolouration.
We hope the above information is helpful. Please do not hesitate to contact us for further advice