Carbon-14, thermoluminescence TL and OSL: which dating method to choose?
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Choosing the right dating method in an archaeological context is an essential step. Depending on the nature of the material and the discovery context, scientists use different techniques. Carbon-14 dating, thermoluminescence (TL), and optically stimulated luminescence (OSL) each address specific issues.
Discover their principles, their fields of application, their limitations, and the criteria that allow choosing the most suitable method for each archaeological context.
In summary:
- The choice between Carbon-14, thermoluminescence (TL), and OSL depends on the material to be dated, the archaeological context, and the study's objectives.
- Carbon-14 dating is used for organic materials, thermoluminescence (TL) for heated materials (ceramics, terracotta), and optically stimulated luminescence (OSL ) for buried sediments.
- Each method has its own areas of application, limitations, and levels of accuracy.
Three dating techniques, three distinct scientific approaches
Carbon-14 dating
Carbon-14 dating (radiocarbon dating) is based on measuring the radioactive decay of the ¹⁴C isotope, naturally present in living organisms. Upon the organism's death, carbon exchange with the environment ceases, and the isotope begins to decay with a half-life of approximately 5,730 years. By measuring the residual amount of ¹⁴C relative to stable carbon ¹²C, the organism's date of death can be estimated.
It is used in archaeology as it offers an average precision of ±20 to 200 years. It allows dating:
| Charcoal | Bones | Textiles or plant fibers | Any organic residue datable up to about 50,000 years |
Carbon-14 dating only applies to organic materials, thus excluding ceramics, stones, metals, or mineral sediments. It is also sensitive to modern contamination (bacteria, roots, infiltrations) which can falsify the results. It is therefore crucial to carefully carry out sample collection to ensure reliable scientific dating.
Thermoluminescence (TL)
Thermoluminescence dating is based on measuring the light emitted by a mineral when it is reheated in a laboratory. This technique thus analyzes electrons trapped in the crystalline structure of the material since its last exposure to high temperatures, generally during its manufacture or its use in a hearth. These electrons accumulate due to the natural radioactivity of the environment and retain a measurable "signal".
In the laboratory, controlled heating is carried out, releasing this signal in the form of luminescence. The more intense the signal, the longer the period that has elapsed since the last heating.
We will use thermoluminescence to date:
| Terracotta | Ceramics | Tiles | Old bricks | Any heated element such as hearthstones, kilns, or fragments of thermal plaster |
Thermoluminescence covers a chronological range between approximately 300 and 500,000 years, with an average accuracy of ±10%.
However, this technique has certain limitations:
- It is destructive: a significant fragment of the material is taken for analysis,
- It is sensitive to potential subsequent heating, whether natural or anthropogenic,
- It requires precise knowledge of the ambient radioactivity in which the material was buried to accurately calculate the dating.
As with any method based on accumulated signals, the quality of the sample and context data is crucial to ensuring the reliability of the results.
To learn more: Absolute dating by thermoluminescence: some archaeological questions.
Optically Stimulated Luminescence (OSL)
Optically stimulated luminescence (OSL) dating is based on measuring the light signal emitted by minerals, generally quartz or feldspar, when they are exposed to controlled light in a laboratory. This signal corresponds to the accumulation of trapped electrons in the mineral's crystal lattice since its last exposure to light.
In other words, OSL dating makes it possible to estimate when the sediment was buried, i.e., the last time it was exposed to daylight. It is a direct scientific dating method for sedimentary events. This technique is particularly useful in archaeological contexts where organic matter is absent.
The OSL will thus be used for dating:
| Buried soil layers or archaeological levels | Sediments related to structures (foundations, embankments, circulation levels) | Geoarchaeological or paleoenvironmental contexts (alluvia, dunes, loess) |
OSL dating covers a time range from approximately 1,000 to 700,000 years, with an average accuracy of ±5 to ±15% depending on the context of the object.
However, it requires a very strict sampling protocol. Samples must be protected from light as soon as they are extracted, as even brief exposure to daylight can erase the signal. They are usually collected using opaque tubes that are sealed immediately under controlled conditions.
This method also requires specific equipment for sample preparation and analysis, as well as great rigor in data processing to account for the site's natural radioactivity. As with other luminescence dating techniques, the reliability of the scientific dating depends directly on the quality of the sample collection, sediment preservation, and control of environmental parameters.
Which archaeological dating method should you choose: Carbon-14, TL, or OSL?
Comparison table of methods
The table below summarizes the main differences between these three methods to facilitate their comparison.
| Method | Material type | Measurable age | Average accuracy | Limitations |
| carbon-14 | Organic material (bones, wood, coal, textiles, etc.) | Up to ~50,000 years ago | ±20 to ±200 years | Not applicable to mineral materials, highly sensitive to contamination |
| Thermoluminescence | Heated minerals (ceramic, terracotta, etc.) | 300 to 500,000 years | ±10 % | Rear heaters, destructive method |
| OSL | Sediments, quartz/feldspar exposed to light | 1,000 to 700,000 years | ±5 to ±15% | Collected away from light, strict protocol mandatory |
Which criterion for which context?
The choice of the most appropriate method depends on several factors:
- Nature of the material: a fragment of wood → C14, pottery → TL, sediment under occupation soil → OSL
- Conservation: if organic matter is absent or too degraded, TL or OSL become the only options.
- Research question: dating an event (e.g., burial of a level, last use of a hearth)
- Stratigraphic context: choice of a non-destructive or complementary method
However, the study context within excavations can include various elements to analyze. Combining these methods often leads to a more robust chronology. A specialized laboratory can thus recommend the most relevant analyses based on available materials and study objectives.
Example: on a Neolithic site with a hearth, pottery, and sedimentary layer:
- Charcoal → carbon-14 dating
- Terracotta → thermoluminescence dating
- Sediments → OSL dating
In conclusion, choosing between radiocarbon dating, thermoluminescence (TL), and optically stimulated luminescence (OSL) depends primarily on the material studied, the archaeological context, and the scientific question asked. These methods are not mutually exclusive: they are complementary and can be combined to refine the chronology of a site or an object.
In case of doubt, CIRAM experts will guide you in choosing the most suitable analyses and in interpreting the results to establish a reliable chronology consistent with your archaeological context.
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You may be asking yourself these questions about dating methods in an archaeological context?
Yes, but only if the object comprises several materials suitable for different techniques. For example, an object combining wood and ceramic can be dated by carbon 14 for the wood and by thermoluminescence for the ceramic. By comparing the results, their consistency can be verified.
There is no universally more accurate method. Reliability depends above all on the material analyzed, the quality of the sample, and the archaeological context. A method perfectly suited to the material will yield better results than a technique used outside its field of application.
No. A dating provides a chronological range corresponding to a specific event (death of an organism, last firing of a ceramic, or last burial of a sediment). Its interpretation must always be placed within the archaeological context.
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