Sampling and Sample Contamination

Analytical chemists know that results of any analysis cannot be better than the quality of the sample used for the analysis. This is also true for 14C dating and particularly for its application in dating museum artifacts. We can get meaningful radiocarbon data only if the dated sample of organic material is indisputably the material that was removed from the live carbon cycle during the time that corresponds to the time of creation of the artifact. This sounds very logical, but without full realization of what this means, there is substantial potential for interpretation error or overinterpretation of radiocarbon data.

Radiocarbon dating is indifferent to the creative force of the artist or the artist's methods. Radiocarbon dating "sees" only organic materials used during the creative process. Radiocarbon dating cannot provide information about when or by whom a given artifact was created. It can only provide information as to when different organic materials found today as parts of the given artifact were removed from the live carbon cycle. For example, if a baroque wooden sculpture was carved from an old wooden beam taken from a destroyed Gothic church, the radiocarbon dating would not provide information about the date of the carving or the date of the Gothic church, but only about the ages of individual tree rings found in the wood sample. We would not even be able to say when the tree was cut down (t = 0 for radiocarbon dating) because the peripheral tree ring under the bark (the last ring grown before cutting) probably would be removed by a carpenter to shape a beam. We would be able to extrapolate our data to provide at least an estimate of when the wood was used as a building material in the church. But in this case, we have to realize that this estimate might be quite misleading if the wooden beam was a later addition or replacement, or if the original tree was "dead" long before it was used to make the beam (natural causes or storage for later use). Sampling of artifacts has to be done with deep understanding of materials (origins of different materials, their participation in the live carbon cycle, and their practical "shelf life"), artist techniques (mixing of materials of different origins), and prevailing customs in handling materials for artistic use (wood aging before use as a support material for panel painting or the tendency to recycle materials for later use). It is also critical to sample a site of the artifact that has a high probability of being part of the original structure of the artifact (part of a panel painting could be a later addition needed to fit a painting to a new frame; a paint sample could be a later overpainting or restoration). A decision on sampling strategy should not be made without consultation with art conservators and art historians involved in the project. If there are any doubts about the originality of the sample site, several samples should be taken from different areas of the artifact. It is better to refuse to work with bad or inadequate samples than to risk sampling error, misinterpretation, or overinterpretation of radiocarbon data. There is also practically no museum artifact untouched by other artists, restorers, or object conservators. Many museum objects have a long history of successive cleaning, repairs, and restorations. There is a high probability that these treatments have changed the original 14C makeup of the object. For example, paraffin wax treatment or acrylic varnish coating represents the introduction of "dead" carbon material and makes the object apparently "older." Treatment with newly produced beeswax or natural resin varnishes would introduce "modern" carbon material, resulting in a positive shift in the apparent radiocarbon dates. A detailed knowledge of past conservation or restoration treatments (very seldom available) or detailed chemical analysis is needed to prepare the ground for a successful sampling strategy and proper sample pretreat-ment prior to the radiocarbon dating. The majority of museum historical objects containing organic materials are composed of complex natural materials (cellulose, oils, proteins, waxes, natural resin) and their mixtures (egg tempera-pigment, oil, protein, carbohydrates, natural dyes, and inorganic salts). Detailed knowledge of materials used to create the art object helps to design the pretreatment needed to remove sample contaminations or to identify a component that is the least affected by contaminations and suitable for more reliable age determination.

In the sample preparation step the pretreated sample is transformed into a sample compatible with the radiocarbon dating procedure. Carbon dioxide or volatile hydrocarbons are used for gas counters, scintillating liquid-soluble organic compounds for scintillating counters. For AMS measurements the organic material is transformed to make a small target of graphitic carbon [K9,K10]. Special care has to be taken that sample pretreatment and target preparation steps do not change the carbon isotope composition of the original sample and do not introduce further contaminations [K11].

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Responses

  • elisa
    Can all contimation be remvoed from a sample before dating?
    13 days ago

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