Friday, 16 July 2010

Grey Granite (II)

Often, Aberdonians are confronted by an outsider with the usual preconceived opinion:
"Oh, Aberdeen; it's a really grey place, isn't it? So... grey",
And, inevitably, we reply with the time-honoured old saw:
"Actually, the city's nickname is 'the Silver City', and while it's true that, on a driech day, the city is grey, (which city isn't?) if you see Aberdeen on a fine day, you'll notice the mica in the granite a-glistening and a-glinting in the sun, truly shining silver... a radiant beacon in the north... beautiful... twinkling... shimmering... etc etc..."
But now, we're afraid to say, some of the city's buldings are not so much 'silver' as 'urine-yellow'. The most glaring example is the City of God Church at Babbie Law.


It is as if some leviathan late night reveller has chosen (the city lacking pubic conveniences) to relieve his brobdingnagian bladder all over this place of worship. It looked great the moment the scaffolding came down, but within a week, it started developing a tinge of pigment. Now it's just yellow. Yellow.

We've noticed this phenomenon on other buildings all over the city-centre, and notably on some properties in residential terraces of the prestigious, exclusive west end. As the height of summer came, and the days became brighter and brighter, we could no longer ignore it and just hope it would go away - it's getting worse all the time. Up close, it's possible to see what look like brush strokes on the yellowed stones. What's going on?

My old dad says that traces of the corrosive chemical used as the first part of a two-stage cleaning process have been left in situ, and are now reacting with airbourne pollution (Aberdeen having some of the worst air-quality in the UK) to turn this unsavoury colour. The second stage of the process involves ablation of the corrosive agent along with the now dissolved dirt by a grit-entrained high pressure water jet. It is a fine line between using enough pressure to remove the corrosive agent completely, and using too much pressure; necessitating re-pointing of the stonework. He thinks that the cleaning contractor has erred on the side of lesser pressure.

But.... we think he's talking rubbish. We can remember the cleaning that went on in the 1980's with this two-stage process, and it was a big deal. It took ages. Streets were closed, buildings shrouded in scaffold and tarp for weeks on end. So no, we don't think that's what happened at the City of God church.

We think they chose Q-Switched Nd:YAG Laser Cleaning.

For about 15 years, debate has raged within the conservation community as to the benefits and drawbacks of this technique. One thing is for sure, under certain conditions, the phenomenon of 'yellowing' occurs. The phenomenon is not well understood, but it is real and has been studied in the cleaning of granite.
The a*-parameter, or red-green component, is the most affected, leading to a change in hab (hue) and was interpreted as a result in variations in the Fe compounds, which strongly condition stone color.
That study concludes that the laser treatment can chemically change the make-up of the stone - resulting in a permanent change in the colour - the iron content has been oxidised; in effect, the laser treatment has rusted our granite.

Other studies have suggested alternate mechanisms which might be causing the yellowing:
Yellowing is a well-known phenomenon after laser cleaning of marbles, especially in case of Nd:YAG lasers operating at 1064 nm (Klein et al. 2000b, 2001, Vergès-Belmin & Dignard 2003, Pouli et al. 2006). It has not widely been reported in case of sandstone. In principle, analogous explanations may apply. The colour effect may be due to (organic) remnants of soiling, whether or not transformed by the energy of the laser, or, alternatively, represent former surface treatments (based on organic oils) that came into view again after cleaning.
We have seen that a laser cleaning contractor is at present working on the rennovation of Marischall College. Let's hope they don't **** it up.


Actually, the yellower Alexander Marshall Mackenzie's Marischal frontage (1904-6) becomes, the more it will resemble Charles Barry and Augustus Pugin's Palace of Westminster (1840-70), of which it is of course, a pale yet pompous plagiarism.

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