None is infallible and before embarking on an extensive dating survey, due thought must be given to what might be achieved and which methods might be the more successful. Whilst earlier types of wooden joints may be copied in later buildings and earlier styles may be reintroduced in later periods to confound the conservationist or historian, any reuse of older materials should become obvious by the use of the chronometrical methods described here.
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In order to obtain an accurate match and hence a date, it is important to have at least 80 rings on the specimen that is to he dated.
With fewer rings the pattern might have repeat matches at different points in the time scale and so give rise to multiple possible dates.
Dendrochronology is the oldest method, having been introduced over a century ago by an American astronomer, Professor A E Douglass.
He wanted to know whether the number of sunspots affected weather on Earth.
(It should be noted that there is no direct linear relationship between ring width and, say, sunshine, or other weather components) Thus a 'good' or 'poor' growing season is defined with reference to the amount of growth produced.
For example, the year 1976 had a gloriously hot, long summer with most rainfall arriving in autumn but the trees did not appreciate it and all oaks produced a distinctively narrow ring.Again the summer of 1915 was cold and wet, quite different from 1976, yet the trees also produced a distinctly narrow growth ring.So it will be seen that seasons that are hot and dry as well as those that are cold and wet will produce a narrow ring so that such a ring is not diagnostic of the weather.The youngest patterns are obtained from living trees, where the felling date of the final ring is known.Progressively older patterns are obtained from trees in recent buildings, older buildings, archaeological sites and ancient bog oaks.The method relies upon the response of trees to the weather during the growing season, which runs from March to October.