How are buildings dated?
Extracting a tree ring core
The group has completed a pilot study in which two of Royston’s older buildings had tree ring cores extracted and dated.
We are now starting phase two of the project which, we hope, will provide evidence of the town’s development in its earlier years.
Core samples have be taken from a number of cafefully selected proprties both north and south of The Cross where Ermine Street and the Icknield Way meet. Click this link to read about our progress.
Dating buildings using Dendrochronology
Tree rings - What are they?
Each year, as it grows, a tree will lay down new cells in its structure. These are visible as rings when viewed in a cross-section through the wood.
During the spring and summer rapid growth rings will usually appear as lighter and wider concentric bands and slower growth in autumn and winter is usually darker and narrower.
As the weather is different in each year, with some summers being long and hot and others cooler, it is possible to use these rings, and their comparative sizes, to date when a tree was living, right up to the point where it was felled.
Obtaining the evidence
Image : Hannes Grobe/AWI, CC BY-SA 2.5
The process itself is nondestructive. One or more small cores, 7mm in diameter, are carefully extracted from selected timbers using an 11mm drill. These cores are used to find the range of dates when the tree was living and, hopefully, when it was felled.
What is Dendrochronology?
Tree ring samples prepared for analysis
The differences in growth patterns between rings in a given tree will only help date it if they can be matched to other samples with known dates.
Over time, dendrochonologists have amassed large databases of patterns for different tree species, called a “Master Chronology" and these can be used as a point of reference when checking the pattern of rings in any one sample.
Factors which affect a sample's dating process include -
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The tree species as some grow faster than others.
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The geographical location of the sample taken. Local growing conditions can affect the patterns in different parts of the country.
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The number of rings within the sample, ideally at least 100. If it has a small number of rings their pattern may be repeated in several places in the dating records making accurate dating difficult.
Dating when a tree was felled
Knowing when a tree was a living organism doesn’t necessarily tell us when it was felled. For this we need to know the last growth ring.
Ideally, dendrochonologists look what is known as a “waney edged” timber. This is most likely to be found in timber which has not been sawn or otherwise shaped after felling. If its bark is still attached, so much the better. It helps ensure that the final ring added by the tree before it stopped growing has been identified. With this information a more accurate date acn be ascertained for the building which used the timber in its construction.
The Group is very fortunate to have the expertise of Cambridge University’s Dendrochronology Laboratory helping us with this part of the project.
What can make tree ring dating difficult?
Trees add rings each year but, as the tree grows, the width of each ring becomes narrower because the same amount of cells added by the tree each year has to be added to the larger diameter of the tree. This has to be taken into account when comparing a sample against the Master Chronology.
To overcome this difficulty a proceedure known as “standardization” is applied. This makes it possible to more accurately compare each sample core against the Master Chronology and produce the results.
Accurate dating also relies on the availability of datasets for each species of tree. The oak dataset is well established. However, many of Royston’s timber buildings used elm in their construction and this dataset is not as well populated.
What is Stable-Isotope Dendrochronology
A new method of dating timber is now being developed. This combines tree-ring width with stable oxygen isotope analysis to produce a more precise date than tree-ring comparison alone.
The technique shares many similarities with ring-width dendrochronology, but rather than measuring tree ring-widths it is the chemical composition of the wood that is compared against a reference chronology to establish a date. Oxygen isotope dendrochronology works well on fast grown trees with wide invariant rings. There is also evidence to suggest that the method can be used on species other than oak such as chestnut, ash and elm.
More information on the method can be found by following this link -
Dating Core Samples Using AMS Carbon-14 Decay
It is not always easy to date the samples using denrochonology. For example, there is a much larger database for Oak then some other species of tree, such as Elm. Where there are gaps in the reference data other methods may need to be used.
How does AMS Carbon-14 (or Radiocarbon) dating work?
Most measurements are now taken using Accelorator Mass Spectrometers. This one is at the Lawrence Livermore National Laboratory in California.
Carbon-14 is constantly being created in the earth’s atmosphere by the interaction between nitrogen and cosmic rays from the sun. This then combines with oxygen to produce radioactive carbon dioxide which is absorbed by plants through photosynthesis.
This method of dating measures the level of radioactive carbon dioxide which remains in organic tissue once it has stopped absorbing it. This typically occurs when the organism dies. As there is a known rate of decay, it is possible to infer how old a sample is based on the amount which remains.
Where dendrochronology fails to produce a result, or the result is ambiguous we hope to use Carbon-14 dating to help clarify the date of the samples taken. We are fortunate to have a partnership with Bristol University’s AMS Carbon-14 laboratory to undertake this work.
Behind the Radiocarbon Dating Process
AMS Carbon-14 dating measures the ratio of radioactive C14 and non-radioactive C13 in a sample. As time progresses the proportion of C14 will decay at a known rate called the “half life”. This is the period of time required for half of the initial amount of a radioactive material to decay. For C14 this is 5,730 years.
Results from C14 dating are expressed as a period of time “Before Present”, or BP, which is usually taken to mean before 1950 when significant amounts of the isotopes were produced by above ground nuclear testing.
What can make C14 dating difficult?
C14 dating can produce spurious or equivocal results, for example where a sample falls on a plateau on the calibration line or where there are two or more places on the curve which return the same value as the sample’s.
However, as the C14 dating is being used to confirm, or otherwise, the dating evidence from the dendrochronological dating which has been carried out on a sample it is hoped that any such anomolies can be overcome.
It should also be noted that different laboratories use different analytical practices to resolve dates. Results can therefore depend on the laboratory used to process the samples.
Architectural Surveys
While dendro chronology and carbon-14 dating can provide likely date ranges for the wood used to construct the buildings, there is always the possibility that timber has been reused or is not in suitable condition to provide certainty when considering the date of their construction.
The exterior of a building can provide a good idea of its possible construction date, based on it overall style and details which are still visible. However the interior can give much more information, especially where it has not been exposed to renovation.
Where possible, the selected buildings have been surveyed both externally and internally to determine their likely date of construction and how they may have been developed over time.
The project is extremely fortunate to have had access to leading experts in the construction of historic buildings, both royal and vernacular, and who have produced reports detailing their findings.
Supported by the Brian Leslie Racher 2004 Charitable Trust
The Royston Heritage Group is extremely grateful to the Trust for the support it has provided, without which much of the work carried out to date could not have been accomplished.
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