Saturday, March 11, 2006

Fugitive Colour

My parents were in Morocco on holiday last week and when I rang them up the other night they told me how interesting and aesthetically pleasing the geology of Morocco was. Some of the hills they saw were bare of vegetation “with colour streaks like contour lines” and others looked as if they were covered in grass, but on closer inspection turned out to be made of green rock.

If you go down to your local DIY hypermarket, you will undoubtedly discover a paint colour labelled ‘stone’. You can probably imagine what this colour would be like: a warmish tone of grey.

In reality, rocks, minerals and soils can be almost any colour you care to imagine. Geologists have their own colour charts for describing rocks, the standard one being the Munsell Rock Colour Chart. The Munsell Corporation make two versions of their chart that I know of, one with 115 different colours used for describing rocks and another with 395 different colours for the description of soil.

Find any rock colour on the chart and the Munsell notation underneath gives you the value (degree of lightness), the hue (colour) and the chroma (degree of saturation) for that colour. The idea is that geologists can use these notations in their writing to communicate exact colour information that anyone can understand (so long as they have a Munsell Colour Chart).

The Munsell Corporation takes its name from A.H. Munsell who, in 1941 published “A Color Notation: An illustrated system defining all colours and their relations”. The Munsell system dictates that color is three-dimensional, consisting of value (or degree of darkness and lightness), chroma (or degree of color intensity), and hue (or location on the color spectrum). There are five primary hues: red, yellow, green, blue and purple . Munsell also established intermediate hues of yellow-red, red-purple, purple-blue, blue-green and green-yellow.

While researching this post, I arrived upon an article in which a soil scientist describes having an argument with another soil scientist about the piece of soil they needed to describe. The debate seems to have been of some significance since its outcome related to the land being classed as a protected wetland, or not, depending on whether the soil was hydric. Eventually they had to agree to disagree when they discovered, on close comparison, that the supposedly standard colour tables that each had were slightly different when overlaid. In fact they were both right, it was one, or both, of their colour tables which was wrong.

Colour is a fugitive phenomenon, not easy to pin down. I have often tried turning a piece of stone in the light to see its true colour. In reality there is no 'true' colour. I remember watching the red Hopeman Sandstone cliffs near Elgin in Scotland where I grew up, turning from dark red to livid pink as the evening sun settled lower and lower in the sky. We try to impose order on the world, but in some ways, the more we try to pin down the characteristics of a thing, the more elusive that thing becomes.

Last week I also attended a lecture about seismic attribute analysis. A substantial part of the lecture concerned the apparently simple matter of choosing a colour scale for the attribute that is being displayed on the monitor screen. The subject is more complex than it first appears, since all colour display depends on the fitting of a colour scale of a certain range to a set of number data that is to be displayed using those colours. If the two ranges do not fit, or are unsuited to each other (perhaps, for example, because the number data is logarithmic in character) it will not be possible to display the data in a convincing way.

In addition, a colour scale which looks good on the computer screen may not look good on paper since the RGB colour scheme used for screens will be mapped onto the CMYK colour scheme used for printing and this mapping may result in loss of certain tints.

The lecturer also told us that the colour magenta was named after the town of Magenta in Italy since “someone had been there and picked up a magenta-coloured rock”. Well, it would be great if it were true, but the truth, according to Wikipedia, is rather more convoluted. Magenta, which is a hue of purple, was one of the first aniline dyes and was discovered shortly after the Battle of Magenta in 1859 which took place near the town of Magenta in northern Italy. The color is named after the battle, and hence only indirectly after the town. It appears it has nothing to do with the rocks that occur there.

There are actually very few colours named after rocks, perhaps because rocks make such unreliable colour scales. The poet George Szirtes, who trained as an artist, has frequently used the names of artists' colours as titles (and touchstones) for his poems: Chinese White, Romanian Brown, Solferino Violet, Kayenta Black, Payne's Grey... These names, like the name of magenta, hint more of history, specific moments in time, than specific natural properties, as if these hues were fleeting, their moments past, connected to a sort of permanent nostlagia for a particular historical moment.

Like the colour ‘khaki’, a word originally used to describe the sandy brown colour of British soldiers’ uniforms during the time of British rule in India. The word comes from the Urdu meaning “dusty”. But the colour 'khaki' has now changed to a greenish tinge, because soldier's camouflage unforms have adapted to more temperate climes. Another colour derived from the stone it describes is ‘turquoise’ which derives from ‘pierre turquoise’, the french for ‘Turkish stone’. But again, it describes a certain historical moment, when stones were brought from Turkey, rather than the properties of a particular rock. Who knows now if the rock was pale bluey green or pale greeny blue?

Emerald and ruby are also used as colour signifiers with the strange result that Ireland, which contains no emeralds at all, is known as the Emerald Isle just because of the ‘fresh verdure of its herbage’.

Attempts are sometimes made to dissuade people who are colour-blind from becoming geologists, but I did once work with a geologist who was colour-blind. It is a typical characteristic of geological correlation diagrams (correlating the same stratigraphic horizons in wells for example) to use particular colours for types of surface with common genesis. Unconformable surfaces will often be shown with a red line while transgressive surfaces might well be indicated with a green line. Unfortunately for my colour-blind colleague, both these colours looked exactly the same to him.

His inability to identify these colours had a powerful effect on his ability to comprehend particular moments of geological history. He would often have to ask: “And so, the transgression occurred when exactly?”

In the end, it has to be said that geologists are generally somewhat suspicious of colour as a property. They know that the colour of a rock need not adhere to it at the moment of formation, but may have stained it at some point in its subsequent history. It is only when the colour can be proven to be related to the processes of deposition that it really becomes of interest.

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