Wednesday, January 13, 2010

Color me 10YR5/8

At any archaeological site it is absolutely crucial to analyze the soils as well as the artifacts. Investigating the soils can reveal how different strata formed and how processes such as erosion shaped the landscape. This is especially important at Port Tobacco, where the movement of soils and the silting up of the river had a major impact on the town. In the field we record the depths of different soils and note their color values. Now, without some sort of color standard we would end up taking a look at excavation notes and finding all sorts of color descriptions...with names like "brownish red" or "mouse brown." While these sorts of descriptions may be somewhat accurate, they are quite subjective. How do I know that what I consider to be light brown is the same thing you consider to be light brown? What if I have two different shades of light brown? Since everyone sees colors differently, it would be near impossible to create a clear set of soil descriptions for an archaeological site...but no need to fret! This is where the handy-dandy Munsell book becomes our reference (a tool of sorts) of choice.

This brilliant blue book contains 322 color chips on 9 different charts, identified based on hue (a color's relation to red, yellow, green, blue, and purple), value (lightness or darkness of a color), and chroma (the purity of a color). Created in the early 20th century by an artist and professor named Albert Munsell, it was adopted by the United States Department of Agriculture in the 1930s as the golden standard for conducting soil research.

When reading a page in a Munsell book, the vertical numbers denote the value of a color, while the horizontal numbers specify the chroma. The hue is designated by what page you are on. For example, the Munsell notation of a particular soil is 10YR5/8. The color name of this notation is yellowish brown. The YR is the abbreviation for yellow-red, and the 10 refers to where on this particular letter range (yellow-red) the color is. The 5 represents the value, and the 8 is the chroma.

In the field a soil should be matched to the most similar color chip, and its texture and any inclusions should be noted. The best way to compare colors is to either hold a bit of the soil behind each cut-out hole by a color chip, or simply by holding it next to the chip.

While there will still be a little bit of variation in how people see the colors of soils, the Munsell book established a universal system for describing colors that gets us much closer to an accurate soil description of a stratum. The book is also handy for estimating proportion of mottled soils and looking at a soil's granular structure. Munsell books are not only used for soils, but are a standard for describing hair and skin colors in forensic pathology, as well as describing colors of beer in breweries! Someone has to make sure that amber ale is actually amber!

So, when on a site learn to love the Munsell book. Identifying soil colors quickly and accurately requires practice, practice, and more practice!


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