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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.
The pattern of ring widths on a specimen taken from a building is matched, using a computer with a 'master chronology' often several centuries long for the particular area.
This regional chronology will have been painstakingly built up from many thousands of measurements and by cross-matching many overlapping patterns of timbers.
Because of local, non-climatic causes of change of growth width, the chronologies around the country vary somewhat, and the best dating match is always obtained from a local regional master chronology.
The dendro-date is thus the year in which the final ring of the specimen grew (the year in which the tree was felled, but not necessarily the year in which the building was constructed).
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.
The incorporation of ancient bog oak into a building, no matter how intricately carved or jointed, would immediately become obvious to the chronologist, as would timber renovations.
Each method has a distinct role in the investigation of historic buildings.The method relies upon the response of trees to the weather during the growing season, which runs from March to October.In a 'good' growing season the trees within a large climatically homogeneous region all respond by putting on a wide growth ring within the cambium which separates the sapwood from the bark.Unfortunately, after many years of analysis he was not able to confirm the correlation he sought.Nevertheless, the laboratory was able to demonstrate many interesting properties of ring widths and their relationship with various aspects of climate and other natural phenomena and, of course, their use in the accurate dating of timber.