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Contamination of Archaeological Artifacts

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• Sources of contamination can be natural or artificial
• Contaminants affect the age of archaeological samples
• Pretreatment done to remove contaminants depends on the type of sample

Anything that affects the radiocarbon or carbon-14 content of a sample is considered a contaminant. The soil surrounding a bone or a piece of paper used to wrap it contains carbon that may compromise the age of the bone sample. Anything carbon-containing that comes in contact with an archaeological artifact would affect its radiocarbon content.

Natural Contaminants

Limestone, humic or fulvic acids, and plant roots are examples of natural contaminants. They are introduced into the artifacts from the surrounding material, e.g. soil. They are considered contaminants because they introduce additional radiocarbon on the sample.

Limestone, which is geological in origin, should be removed before radiocarbon dating because it makes the sample appear older than its true age. Plant rootlets that grow on samples introduce modern carbon, thereby making the sample appear much younger. Humic or fulvic acids, which are products of microbial degradation of plants and animals, can make an artifact significantly older or younger depending on the age of the organisms.

Artificial Contaminants

Artificial contamination happens during post excavation particularly during collection, conservation, and packaging of the artifacts. A bone labeled using paper and animal glue has already been contaminated. Other examples of artificial contaminants are biocides, conservation chemicals like polyvinyl acetate and polyethylene glycol, and cigarette ash.

Archaeological samples that have been artificially contaminated usually appear much younger than their true age. This is due to the introduction of modern carbon on the samples.

Pretreatment Methods

Before a sample is radiocarbon dated, it is pretreated to remove contaminants. Physical pretreatment starts with the removal of visible contaminants like plant rootlets and scraping off the exterior layer. This is often followed by size reduction through crushing and other methods.

Chemical pretreatment follows physical pretreatment to ensure removal of contaminants. The chemicals used, their concentration, and the number of chemical washes depend on the type of sample and the probable contaminants. There is no standard chemical pretreatment applicable to all samples for radiocarbon dating.

Important Note on Pretreatment - It is important to understand the pretreatments which are going to be applied to samples since they directly affect the final result. You are welcome to contact us to discuss the pretreatment or request that we contact you after the pretreatment (and prior to dating) to discuss.

Materials such as charcoal, wood, peat, and textiles typically undergo the acid-alkali-acid (AAA) method before radiocarbon dating. Learn more

Materials such as sediments and soils typically undergo acid washes (no alkali) before radiocarbon dating. Learn More

Materials such as shells and other materials where a date on the inorganic carbon (carbonate) is to be done typically undergo acid etching before pretreatment. Learn More