Oh dear! Not the sort of telephone conversation to have with a client when one is a few hundred miles away on half-term holiday with the family (see that story here).
However, I was reminded of a strikingly similar case a few years ago on a house in Mayfair – one which I was able to resolve by using the techniques employed for the analysis of historic decorative schemes.
On that occasion the building was situated on the corner of the busy intersection of two streets in Mayfair. Our client had recently bought the eighteenth century house whose brick façade had been painted black for many years. The house had been beautifully finished – the façade in an off-black masonry paint which we had colour-matched for the client and the windows in an off-white gloss. The overall effect was similar to that in this photograph:
Some months later I received a complaint that the façade streaked whenever it rained and that the paint was being washed off and settling on the window cills. I explained that, in my experience, dark-painted facades in London tended to show airborne dirt more than lighter colours. I was aware that the house was less than 200 metres from Park Lane (the A4202) which experiences some of the highest traffic flow in London. What I didn’t know at the time was that in a recent study of the influence of meteorological factors and traffic volumes upon suspended particles the kerbside of Park Lane was chosen as one of the four sites in London. This was selected because it has the highest traffic flow in the city.1
The first thing that I did was to check the formulation of the paint. The colourant used to tint the masonry paint consisted primarily of carbon black, which is characterised by its low particle size and intense black colour. There were also small amounts of white and blue in the formulation.
Carbon black is the generic name given to the product resulting from the partial oxidation of hydrocarbon. Approximately 70% of carbon blacks go into the manufacture of vulcanised rubber tyres. A further 10% goes into the production of ink, plastics and paint. The remaining 20% is used in a variety of products including belting, hose, moulded elastomers and footwear.2
Although related, carbon black is slightly different to soot, which is the unwanted byproduct from the combustion of carbon-based materials for the generation of energy or heat. Large amounts of soot can be found in cities – the majority from the consumption of fossil fuels – particularly diesel fuel, coal, jet fuel, natural gas and petrol. Cars airplanes and central heating boilers are responsible for much of this.
As we retain a sample of all specially-mixed colours this was tested for ‘chalking’ by wiping with a damp cloth – no problem was found.
I reported back to my client that the paint mixed for him had been correctly formulated and that there were no indications of it being faulty in any way.
The client did not believe me and insisted that I make a site visit, which I duly did and found nothing other than a light coating of dust on the painted surfaces. This was reported.
He was still not happy so I made a second site visit immediately after the next patch of rainy weather. Certainly I could see some streakiness on the off-black façade and there was a thin brownish black deposit on the window cills. This appeared to be dirt; however, I took a number of samples in order to examine them under the microscope.
Samples of the residue removed from the window cills were placed on glass microscope slides. Five dispersions were made and these were examined and photographed using a compound microscope.
Photomicrographs of a number of the dispersions are included below:
This dispersion is composed largely of silica, iron oxides and a couple of agglomerates coated with black paint. The combination of silica and iron oxides is found in soil, bricks and stone. In this instance they were not found in the masonry paint. The large agglomerates appeared to be particles of a cementitious nature covered by paint. These lumps were many times larger than particles of carbon black. This was probably sand and cement pointing that has been washed off. It will be understood that as a result of natural attrition the facade of a building is constantly being worn down by wind and rain.
This dispersion shows further lumps of silica, iron oxides and agglomerates coated with black paint. The large lump in the top centre is an organic brownish black that had been partially dissolved by the microscope slide mountant and the heat required to melt it. The organic brownish black appeared to be tar. This had either been washed down from the roof or had been blown up from the street. It is possible that there had been road works in the neighbourhood or roofing work on this or on nearby buildings. Lumps of this nature can be found in the smoke generated by both activities. Most of the particles are many times larger than carbon black.
It is also possible that some of the particles are tiny pieces of rubber that have been detached from vehicle tyres by the abrasive and shearing forces exerted while driving. A study in the United Kingdom finds that about 10 to 20 percent of a tyre’s total weight is worn off during its lifetime, which works out to about 58,000 tons a year in the UK alone. The heavy particles quickly settle on the road and pavement while the lighter particles become airborne.
The particles shown in this dispersion are largely composed of the partially dissolved organic brownish black seen in slide 2. Large agglomerates coated with black paint can be seen together with very small translucent particles of silica. This slide appears to be largely tar and sand.
A certain amount of vegetable fibre can be seen in all the slides, although it is best shown here. The staining caused by the partial dissolving of the organic brownish black can be seen clearly. The vegetable fibre has probably been blown over from Hyde Park or from nearby trees.
This dispersion shows many large particles. These consist of translucent silica together with lumps of the partially dissolved organic brownish black. This slide is mainly tar and sand.
If the masonry paint had been under-bound or over-pigmented there would have been signs of “chalking”. Tiny particles of an intense black pigment would be washed down the facade and would collect on the horizontal surfaces such as window cills.
The samples showed many different substances on the exterior of the building. These probably included sand, stone, brick, soil, tar, soot, vegetable fibre, and small lumps of the painted facade itself. No doubt particulate matter from the brakes and tyres of the traffic passing close to the house could also be found.
Put simply the deposits on the window cills were dirt.
That house was situated on a cross roads near one of the busiest streets in London. In such a location a recirculating vortex is created leading to heavy concentrations of particle mass. It is for that reason that a roadside monitoring point was established during the recent study. The local conditions are different throughout London, but here they are particularly demanding. A dark-painted facade will invariably look streaky once airborne dirt settles on it – whether in the centre of London or in the suburbs.
This was a particularly time-consuming exercise, which unfortunately didn’t even merit a response from the client – all too eager to find someone to blame for a fact of life.
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1 Roy M. Harrison, Alan M. Jones and Roger Barrowcliffe. Field study of the influence of meteorological factors and traffic volumes upon suspended particle mass at urban roadside sites of differing geometries. Atmospheric Environment. Volume 38, Issue 37, December 2004, Pages 6361-6369. Amsterdam, Elsevier.
2 Peter Lewis. “Carbon Black” in Pigment Handbook. Vol 1 (ed. Peter A. Lewis). Pages 743-758. New York, John Wiley. 1988.
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