Dating of artefacts

Imitation of art objects and relevant raw materials dates back to more than three thousand years ago. When Romans became masters of the Mediterranean world, they were impressed by the beauty of Greek cities. Roman sculptors produced copies of Greek marble statues, and it is unknown if buyers from that time were aware that the objects were counterfeit.

Figure 1 - Aphrodite Of Milos (left) and the Nike of Samothrace (right), at Louvre, France.

Now this has become a multimillion-dollar market. Forgeries imitate the original art pieces in all their materials: ceramic, metals, stones, and so on, and in all their characteristics. Laboratories of archaeometry that apply scientific techniques to the analysis of archaeological materials, receive inquiries about the authenticity of artefacts from museums, art galleries and even from individuals.

One of the key tools to support authenticity investigations is the chronology of events, which is also a major element of reflection in archeological research. All of the materials and the constituents used to make the objects undergo gradual alteration and weathering over time. The chemical composition may change due to the migration of elements, leaching and hydrolysis or other processes. Modern materials, used to make counterfeits, are “aged” but obviously not “naturally”: although they may look exactly like the original, small differences inside them allow us to determine precisely the age of the material (using dating methods) and its provenance.

Figure 2 - Karnak Temple, Egypt.

The dating methods can be divided into two categories: relative dating or absolute dating, depending on whether the exact age can be defined or not.
Relative dating allows to chronologically order events. It includes methods based on the analysis of comparative data, of the context in which the

object has been found. One of the most common methods is stratigraphy analysis. Soil is made up of different layers of sediment, debris, rocks, and other materials which are called strata. At an archaeological site, strata exposed during excavation can be used to relatively date sequences of events: upper strata were formed or deposited more recently than lower strata.

Figure 3 - Cut of soil with different layers.

Absolute dating consists of methods that provide a real estimated age of an object expressed in years. These methods analyze physicochemical transformation phenomena whose rates are known or can be estimated relatively well. Such techniques include carbon dating for bones and wood, thermo-luminescence dating for glazed ceramics and also the tree-ring dating of timbers, called dendrochronology. Let’s see them one by one.

Dendrochronology is a dating method based on the analysis of patterns of tree ring growth. The time at which the rings were formed is directly related to the calendar year. The study of the tree rings provides also information about the past climate, weather conditions, or climate changes that have affected tree growth. This method can determine ages up to 11000 years ago, but its application in dating is limited. It is mostly used to date old buildings, or to determine certain aspects of past ecologies.

Figure 4 - Tree rings.

The most widely used dating technique in archaeology is radiocarbon dating. It is routinely employed to date objects containing carbon in the form of organic material such as bones, clothes and also wood. It can determine the age up to 55 or 60 thousand years ago.
How could the presence of carbon be exploited to date an object? Carbon is one of the most abundant elements in the universe by mass.Three isotopes occur naturally: ¹²C and ¹³C being stable, and the radioactive ¹⁴C, with a half-life of about 5730 years. The physical source of the ¹⁴C is nitrogen gas, which makes up 78% of the earth’s atmosphere. ¹⁴C is originated in the upper atmosphere: when cosmic radiation from space enters the atmosphere, neutrons are created. By a nuclear reaction, these slow down as they collide with nitrogen atoms, producing ¹⁴C atoms and protons.

${}_{\ 7}^{14}N+{}_{0}^{1}n \rightarrow {}_{\ 6}^{14}C+{}_{1}^{1}p$

Then ¹⁴C combines with oxygen to form carbon monoxide and carbon dioxide (¹⁴CO and ¹⁴CO₂), which then mix with the stable carbon compounds in the atmosphere (¹²CO₂ and ¹³CO₂).

${}^{14}C+O_{2} \rightarrow {}^{14}CO_{2}$

Carbon dioxide dispersed in the atmosphere is used in photosynthesis by plants, and from here is

passed through the food chain. ¹⁴C is taken up by plants, animals and human beings during their lives and when living matter dies no new carbon is added. Every plant and animal in this chain will therefore have the same amount of ¹⁴C compared to ¹²C as the atmosphere. However, after they die, the amount of ¹⁴C slowly decreases with time due to radioactive decay. By measuring this quantity and knowing the initial ratio, it is possible to calculate the time elapsed since the organism died.

Figure 5 - Schematic representation of 14C passing through the food chain.

The ratio of ¹⁴C over ¹²C in the atmosphere has remained relatively stable at about 1.2 parts per trillion over the past several thousand years, but atmospheric testing of nuclear weapons during the 1950s and early 1960s caused a rapid increase in atmospheric ¹⁴C content (Fig. 6). This sudden increase is known as bomb pulse and it is an isotopic chronometer of the past half-century. From the peak in 1963, the level of ¹⁴CO₂ has decreased due to mixing with large marine and terrestrial carbon reservoirs.

Figure 6 - 14C/12C ratio relative to the natural level in the atmospheric CO2 as a function of time in the second half of the 20th century.

The fluctuations in the carbon ratio required to calibrate the dates obtained by radiocarbon dating with calendar dates. This was done by means of standard methods such as dendrochronology. The

calibration has been performed by correlating the calendar date of tree-rings with the ¹⁴C age of the rings, and this extends to 11500 years ago.

Figure 7 - Radiocarbon Age Calibration Curves 0–50000 Years, Mike Christie - CC BY-SA

How is the measurement performed? Because the decay rate of Carbon-14 is known, the amount remaining in the sample is typically determined by measuring its activity with appropriate liquid or gas counters. Then with a calculation depending on the technology used, it is possible to determine the time elapsed since the death of the organism. This technique requires samples of sufficient size, containing several grams of carbon. By using accelerator mass spectrometry, ¹⁴C atoms are detected and counted directly, which allows dating of samples containing only a few milligrams of carbon. Years are reported as “years before present”, referring to 1950 as the “zero” date, and with an uncertainty of 30 radiocarbon years.
Analogously to carbon dating, another naturally occurring radionuclide that can be exploited for dating is Lead 210. This is used to determine the accumulation rate of sediments in lakes, oceans and other water bodies. ²¹⁰Pb is part of the ²³⁸U decay series. Since the half-life of uranium is very long, 4.5∙10⁹ years, it can be considered to be present at an unchanging concentration in the earth's crust. In the decay series of ²³⁸U, ²²²Rn is produced. This is a gas that is released in the atmosphere and naturally decays to ²¹⁸Po, a metallic radionuclide which, over a period of hours or days, falls to the earth with dust and rain. Over some minutes, a number of subsequent radioactive decays produce ²¹⁰Pb (half-life = 22.3 yr). The ²¹⁰Pb which falls into a lake or ocean tends to end up permanently fixed on sediment particles. Within 2 years, ²¹⁰Po, the daughter of ²¹⁰Pb, is in secular equilibrium with lead. The measurement of the alpha emitting ²¹⁰Po provides more accurate estimates of the ²¹⁰Pb quantity. Due to the ²¹⁰Pb half-life of about 22 years, this dating method covers only up to 100 years ago.

Another absolute dating technique is luminescence dating. This refers to the method that determines how long-ago mineral grains were last exposed to sunlight or manufactured at temperatures above 500°C. It allows for the determination of when pottery was made but also the age of porcelain, bricks, tiles, terracotta, natural sediments, coastal sand dunes, lava, and so on. It does not require a contemporary organic component of the sediment to be dated. Luminescence techniques are used mostly for artefacts of pre-radiocarbon age. The time span of years determined goes from some hundreds to 300 thousand years or more with errors of 5 to 10%

Figure 8 - Arkose rock with feldspar grains.

Minerals act as natural radiation dosimeters. All sediments and soils contain trace amounts of naturally occurring radioactive isotopes from the uranium or thorium series, or from potassium-40. These slowly decay over time, emitting radiation which adds to the ones derived from cosmic rays. This radiation is absorbed by mineral grains in the sediments such as quartz and potassium feldspar.
How is it absorbed? Electrons are excited from the valence band to a higher energy state, creating a hole. After a short time, most electrons recombine with holes. However, some electrons are trapped in defect sites. This trapped charge accumulates over time. When these minerals are heated, such as while a pot is being baked, the traps formed by

their crystal structure are emptied and the clock is reset to zero. Then, the trapped charge starts re-accumulating over time, at a rate determined by the amount of background radiation present in the location where the sample was buried. The trapped electrons are light sensitive. Stimulating mineral grains either optically or thermally releases electrons, and when they recombine with the holes a luminescence signal is emitted, the intensity of which varies depending on the amount of radiation absorbed while the object is buried and on the specific properties of the mineral. The techniques based on luminescence dating depend on the type of stimulation: we will talk about thermoluminescence (TL), where the sample is thermally stimulated, while optically stimulated luminescence (OSL) or infrared stimulated luminescence (IRSL) refer to samples optically stimulated by light, blue/green or infrared respectively. The light emission is plotted against temperature for thermoluminescence, obtaining a glow curve, or against exposure time, obtaining a shine down curve. The age of the sample is determined by dividing the absorbed dose (as equivalent dose, ED), measured in grays, by the dose rate (DR), expressed in grays per thousand years.

Figure 9 - Typical quartz TL curve, GeoGammaMorphologe - Own work, CC BY-SA 3.0

For the dose estimation, some properties have to be assumed, such as the signal intensity grows linearly versus the dose received. The estimation of the absorbed dose, as equivalent dose, is calculated most commonly with an additive method: several aliquots of the sample are irradiated at known doses in order to increase the signal. The initial value is then extrapolated from the signal growth curve.
A similar technique that is based on the same principle of TL and OSL is Electron Spin Resonance dating (ESR). This method allows to estimate the number of trapped electrons remaining in the sample (equal to the number of paramagnetic centers) by using a combination of microwaves and a variable magnetic field, without evicting the unpaired electrons from their trap. The disadvantage of ESR is that it is much more complicated and has larger uncertainties than luminescence techniques. However the measurement can be repeated different times and it is particularly useful for notably non translucent material, whose electrons cannot be stimulated to come out. ESR has been successfully applied to the dating of materials such as speleothems, spring deposited travertines, mollusc shells, corals, and tooth enamel.
Dating techniques are employed to distinguish forgeries from originals and are very important for art authentication. While carbon dating requires the presence of carbon derived from organic sources in the sample, luminescence dating became the chosen technique for ceramic artefacts. In some cases, when the forgeries have an exceptionally sophisticated quality, a combination of two or more laboratory approaches has to be applied.