A Preliminary Investigation on the Identification of Artificial Irradiation in Thermoluminescence Pre-Dose Dating of Ancient Chinese Porcelain

Abstract

This study investigates the identification of artificial irradiation in thermoluminescence (TL) pre-dose dating of ancient Chinese porcelain to address the challenges posed by sophisticated counterfeiting techniques. While TL pre-dose dating is effective for authenticating ceramics, modern imitations artificially irradiated to mimic ancient doses complicate accurate age determination. By analyzing the TL characteristics of five historical porcelain samples (Song, Yuan, Ming, and Qing Dynasties) and artificially irradiated modern replicas, distinct differences were observed. Natural irradiation samples exhibited lower TL sensitivity, less smooth glow curves, and reduced linear regression fit (R² < 0.97) compared to artificial counterparts, which showed higher sensitivity, smoother curves, and superior linearity (R² > 0.97). The following methodology was proposed: annealing samples to erase natural signals, applying equivalent artificial doses, and comparing TL responses. The results demonstrated significant disparities in TL behavior between ancient and irradiated samples, enabling discrimination. This approach enhances the reliability of TL pre-dose dating for porcelain authentication, offering a practical solution to combat forgery in cultural heritage preservation.

1. Introduction

Accurate estimation of the firing date is not only crucial for ceramic archaeology, but also important for the study of the authenticity of historical ceramics, anti-counterfeiting, collectors, museum owners, and auction houses, and thermoluminescence (TL) dating is one of the most effective methods for establishing the temporal context of ceramic production. TL is a type of phosphorescence that occurs when a solid releases the radiation energy accumulated by ionizing radiation excitation as light energy in the form of photons when heated [1,2]. Traditionally, researchers have relied on the TL property of quartz for dating, especially the 375 °C TL peak, which has a high sensitivity and is widely used for dating archeological pottery [3,4,5,6,7,8,9,10,11]. This measurement of TL peaks of quartz requires extracting quartz grains from pottery fragments and determining the paleodose using either fine-grain [12] or coarse-grain techniques [13]. However, the high-temperature TL peaks of quartz (above 250 °C) are difficult to use for dating ancient Chinese porcelain, because porcelain is fired at much higher temperatures than pottery, and fine-grain extracts are mixtures of quartz and mullite [11]. The TL peaks of mullite overlap with those of quartz, making the dating results ambiguous and hard to interpret [14]. Fleming [15] discovered the TL pre-dose effect, which means that the 110 °C TL peak of quartz at room temperature only lasts 1–2 h, but can be reactivated and detected after receiving an approximate 100 mGy radiation dose. The TL sensitivity of the reactivated 110 °C peak is proportional to the total accumulated radiation dose [15,16,17,18]. Stoneham [19] was the first to use the pre-dose technique to date ancient Chinese porcelain, and since then, this method has been widely used for dating and authenticity identification of ancient porcelain. At present, a mature TL pre-dose method for dating porcelain fired less than 1500 years ago has been established and there are many application examples in ceramic archaeology [20,21,22,23,24,25,26,27,28]. In addition, the TL pre-dose method is also employed for accidental personal dosimetry through the measurement of porcelain materials commonly used in dentistry [29].

TL technology has been applied and developed to identify a large number of modern imitations of ancient ceramics, which has somewhat reduced the disorder in the antique market [30]. However, some adept counterfeiters have learned to artificially increase the TL age of modern imitations by irradiating them with exact doses, creating the impression that the imitations are ancient authentic products. For pottery, the artificial irradiation dose can be determined by comparing the paleodose measured by the fine-grain method and the pre-dose method. In the field of artificial irradiation, the range of α particles is very short, and external irradiation is unlikely to irradiate the α dose into the interior of the porcelain, so it is very difficult to artificially forge the alpha dose. If the two paleodoses are roughly equal, it means that there is no α dose component in the paleodose, indicating that this is an imitation that has been artificially irradiated [31,32]. However, for porcelain, the fine-grain method is problematic because the TL peak of mullite interferes with the high-temperature TL peak of quartz [14]. The paleodose obtained by the fine-grain method has a very large error, so it is difficult to use the same methodology as pottery to identify artificial irradiation. Therefore, how to distinguish whether the paleodose measured in the porcelain is caused by natural radiation or artificial irradiation has become an important topic in TL pre-dose dating of porcelain.

In this paper, we used the TL pre-dose method to obtain the paleodose of five ancient Chinese porcelains of different periods and one modern porcelain irradiated with different artificial doses. By comparing the TL experiment results of natural and artificial irradiation samples, we found that some data of the two types differed, and proposed that this could be a feasible method to identify artificial irradiation of porcelain.

2. Samples and Experiment

2.1. Equipment

TL measurements were conducted on a Risø TL/OSL-DA20 dating instrument at the Research Center of Ancient Ceramic, Jingdezhen Ceramic University, Jingdezhen. Irradiation was carried out using an attached ⁹⁰Sr/⁹⁰Y β source. The dose rate was 97 ± 3 mGy/s delivered to the grain on stainless steel discs. A Hoya U-340 filter compatible with the instrument was employed in the experiment. The heating rate was 5 °C/s. All of the measurements were performed in a nitrogen atmosphere.

2.2. Samples and Preparation

This experiment used five pieces of ancient Jingdezhen porcelain from the Song, Yuan, Ming, and Qing Dynasties, provided by the Yuan Blue and White Museum of Jingdezhen Ceramic University (Figure 1a–d,f). Among them, we specifically chose a piece of porcelain bearing the inscription “大明嘉靖年制”, indicating that it was produced in the Jiajing period of the Ming Dynasty (1465–1487 A.D.). Additionally, a modern replica of Jingdezhen Yuan blue-and-white porcelain was selected (Figure 1e).

The sample preparation process consisted of the following steps: (1) removing a block from each porcelain piece with pliers; (2) peeling off the glaze layer, crushing the block, and sieving to obtain grains of 125~200 μm; and (3) dividing grains into several 10 mg aliquots each and storing in the dark.

For the porcelain featuring Chinese characters (Figure 1f), we divided the extracted grains (Sample JJ) into two portions. One portion was subjected to TL pre-dose dating to determine the paleodose, while the other portion underwent rapid annealing at 700 °C for 10 min, resulting in Sample JJ-A. An aliquot of Sample JJ-A was selected to verify its natural irradiated dose had been completely zeroed. Subsequently, Sample JJ-A was artificially irradiated to yield Sample JJ-A-AR, with the irradiation duration calculated by dividing the paleodose of Sample JJ (2133.6 mGy) by the dose rate of ⁹⁰Sr/⁹⁰Y β source. TL pre-dose dating was performed on Sample JJ-A-AR to obtain artificial paleodose relative to the natural paleodose of Sample JJ for comparative analysis (Figure 2). Meanwhile, some of the modern replica grains were exposed to a ⁹⁰Sr/⁹⁰Y β source for 20 s (1939.6 mGy), 40 s (3879.2 mGy), and 60 s (5818.8 mGy), respectively, simulating the similar effects of irradiation doses to samples from the Ming Dynasty, Yuan Dynasty, and Song Dynasty.

Table 1. The grain sample number and descriptions.

No.	Description
S	White porcelain from the Song Dynasty
Y	White porcelain from the Yuan Dynasty
M	Blue-and-white porcelain from the Ming Dynasty
Q	Blue-and-white porcelain from the Qing Dynasty
JJ	Blue-and-white porcelain featuring Chinese characters, produced in the Jiajing period of the Ming Dynasty
JJ-A	Sample JJ underwent rapid thermal annealing
JJ-A-AR	Sample JJ-A exposed to 90Sr/90Y β source for 22 s (2133.6 mGy)
X	Modern replica porcelain
XR20	Sample X exposed to 90Sr/90Y β source for 20 s (1939.6 mGy)
XR40	Sample X exposed to 90Sr/90Y β source for 40 s (3879.2 mGy)
XR60	Sample X exposed to 90Sr/90Y β source for 60 s (5818.8 mGy)

shows the grain sample number and descriptions.

2.3. TL Measurements

Before conducting the paleodose analysis, the porcelain was examined on the thermal activation characteristic (TAC) curve to determine its potential to generate high pre-dose sensitivity and achieve full activated temperature. The sample was heated from 400 °C to 700 °C (the instrument’s maximum heating temperature) in 20 °C increments, and the TL sensitivities at each temperature were recorded. The TAC curves of all samples showed that the temperature of the highest sensitivity was between 640 °C and 680 °C. Hence, the TAC temperature for this experiment was set at 680 °C uniformly.

The paleodose of the aliquot was determined using the exponential saturation regression method in the pre-dose technique. Wang et al. [25] explained the detailed principle of the method, and

Table 2. Measurement sequence to evaluate the paleodose.

Run No.	Procedure
1	Heat to 160 °C to remove possible hybrid peaks, return to room temperature (25 °C)
2	Give 2 s (194 mGy) test dose (β0), heat to 160 °C to measure S0
3	Heat to TAC temperature (680 °C), return to room temperature (25 °C), give 2 s (194 mGy) test dose (β0), heat to 160 °C to measure SN
4	Give 60 s (5818.8 mGy) laboratory β dose, measure SN↓
5	Heat to TAC temperature (680 °C), return to room temperature (25 °C), give 2 s (194 mGy) test dose (β0), heat to 160 °C to measure SN+β
6	Give 60 s (5818.8 mGy) laboratory β dose, measure SN+β↓
7	Heat to TAC temperature (680 °C), return to room temperature (25 °C), give 2 s (194 mGy) test dose (β0), heat to 160 °C to measure SN+2β
8	Give 60 s (5818.8 mGy) laboratory β dose, heat to 160 °C to measure SN+2β↓
9	Heat to TAC temperature (680 °C), return to room temperature (25 °C) give 2 s (194 mGy) test dose (β0), heat to 160 °C to measure SN+3β

presents the measurement sequence. To account for statistical fluctuations and errors associated with TL pre-dose dating, three parallel aliquots were tested for each specimen.

The paleodose was calculated using the radiation quenching method, and the detailed methodology and calculation formula derivation process can be found in Wang [33]. It must be noted that some old porcelains may exhibit sensitivity saturation after multiple irradiation doses [17]; hence, Wang recommended measuring only two laboratory β doses [33]. However, for porcelains since the Northern Song Dynasty, using three laboratory β doses and a three-point linear regression relationship could enhance the reliability of the experimental results [34]. The laboratory β dose for our experiment was 5818.8 mGy and the TL sensitivity of the aliquots did not saturate after three laboratory β doses in this work. Thus, in this research, three laboratory β doses were applied to the aliquots. The three-point data (S_i, ΔS_i) were (S_N↓, S_N+β − S_N↓), (S_N+β↓, S_N+2β − S_N+β↓), and (S_N+2β↓, S_N+3β − S_N+2β↓), respectively, and a linear regression yielded the intercept a and slope b. The paleodose P was calculated as follows:

$$P=-B\mathit{ln}(1-{\displaystyle \frac{S_N-S_0}{S_{\mathrm{\infty }}-S_0}})-{\beta }_0$$

(1)

$$B=-{\displaystyle \frac{\beta }{\mathit{ln}(1+b)}}$$

(2)

$$S_{\mathrm{\infty }}=-{\displaystyle \frac{a}{b}}$$

(3)

P = paleodose; a = intercept; b = slope

3. Results and Discussion

3.1. TL Glow Curve and Data

The TL glow curve is presented in Figure 3 and Figure 4. The TL sensitivity, paleodose, and age of all aliquots tested in this experiment are shown in Table S1, with the annual dose rate based on an average value of (4.9 ± 0.9) (17%, ±1σ) mGy/a for ancient Chinese porcelain [25]. The results show a high degree of consistency among the aliquots and a good agreement between the TL age and the historical age of the specimens, confirming the reliability of the experiment.

The modern replica has a low paleodose, and the data only reflected porcelain from less than 100 years ago, which was only useful for verifying the authenticity of ancient ceramics, not for determining a precise luminescence age [25]. We obtained the corresponding artificial radiation doses by irradiating aliquots of the modern replica for 1939.6 mGy, 3879.2 mGy, and 5818.8 mGy, respectively. The results were similar to those of the ancient samples, showing that we successfully simulated the paleodose in this test. Similarly, this manuscript successfully eliminated the natural irradiation dose of Sample JJ and subsequently determined the paleodose consistent with the natural irradiation dose through artificial irradiation (Table S1).

3.2. Contrasts Between Ancient Porcelains and Artificially Irradiated Replicas

In this study, the equivalent paleodose and irradiation time of the artificial irradiation replica exhibited a linear relationship (Figure 5). Therefore, forgers could manipulate the TL pre-dose dating of the porcelain by applying precise and timed irradiation with a known dose, thus increasing the TL age and producing completely inaccurate results. It is difficult to distinguish ancient porcelains from modern imitations only from TL ages measured by the pre-dose method. However, there are still some subtle differences in TL characteristics between ancient samples and artificially irradiated counterfeits.

Firstly, as shown in Figure 3 and Table S1, the ancient samples have a much lower TL peak sensitivity than the artificially irradiated samples. TL sensitivity depends on various factors, such as the quartz content and lattice defects in the porcelain, and the thermal or light exposure history of the samples. The radiation-induced increase in the 110 °C TL peak sensitivity of quartz can be attributed to a higher number of recombination centers activated by irradiation heating [12]. The dependence of the sensitivity on the excitation dose follows, in particular at high doses where the sensitivity approaches saturation [17]. Thus, the TL sensitivity of the crystals increases when they are irradiated again after a large dose of irradiation. Ancient porcelains were mostly made from local clay materials [35], whereas modern counterfeits used pure mineral materials with more quartz crystals. The TL sensitivity increased with the amount of quartz added. The porcelains from different periods and kilns in ancient China had different chemical compositions due to the differences in formulas, which could be used to determine the source and age [36,37,38,39]. However, some handicraftsmen now use the same materials as ancient formulas, making compositional analysis useless for dating. In this case, counterfeits and ancient porcelains should have the same TL sensitivity, since they have the same composition. With the exception of exceedingly rare cases, artificially irradiated samples generally exhibit enhanced TL sensitivity characteristics. In fact, ancient porcelain may be subjected to certain factors that lead to cumulative dose attenuation over such a prolonged period of time.

Secondly, the ancient samples have a lower smoothness in the TL glow curve compared to the artificially irradiated samples (Figure 3). The smoothness of the TL glow curve depends mainly on the size of the quartz particles and the impurity content in the sample. The ancient raw materials were processed roughly, and mineral particles of various sizes were visible in the body under optical microscope observations [35], while modern imitations were ball-milled to make the raw materials particles smaller and more uniform, resulting in a more even light emission during the TL heating process and a smoother TL glow curve. Moreover, modern imitations had fewer impurities, which reduced the disturbance of the TL glow curve. It should be noted that our previous tests also revealed a few ancient porcelains exhibiting a high TL glow curve smoothness. These particular samples, typically characterized by their delicate white appearance, belong to the category of high-quality porcelain crafted with refined raw materials and advanced processing techniques. Assessing whether such samples have undergone artificial irradiation solely based on the smoothness of the TL glow curve poses considerable challenges.

Thirdly, the coefficient of determination (R²) for the linear regression analysis of TL sensitivity three-point data (S_i, ΔS_i) differs between ancient samples and artificially irradiated samples. As shown in Figure 6, modern replica aliquots and artificially irradiated replica aliquots have slightly higher R² values than ancient samples, implying a more linear relationship in the regression fit for the TL sensitivity three-point data of replicas, using more points may result in slightly different outcomes. The main difference between natural and artificial irradiation of porcelains is that the former involves long-term exposure to low doses, while the latter involves short-term exposure to high doses. Natural irradiation accumulates interference from various uncontrollable factors such as heating, light exposure, and natural decay over time [33]. Artificial irradiation samples, on the other hand, have minimal influence on these factors because of the relatively short time from irradiation to TL testing. Consequently, artificially irradiated samples exhibit a better linear regression fit for the three-point data. In this study, we classified test aliquots with an R² value greater than 0.97 as modern replicas and those with an R² value below 0.97 as ancient porcelains. Hence, if encountering porcelain exhibiting exceptionally high R² values in the TL pre-dose result, special attention should be paid to determine whether it is a replica.

The above discussions are about the differences between naturally irradiated samples and artificially irradiated samples in this research. However, in practical work, determining whether porcelain has undergone artificial irradiation cannot solely rely on paleodose data by the TL pre-dose method. It requires a comprehensive judgment based on TL age, typological characteristics, and chemical compositions of the sample [35,37]. Thus, the establishment of an integrated discrimination model is a key goal of future work.

3.3. A Methodology for the Identification of Imitations

In general, modern imitations exhibit distinct differences in their pre-dose dating curves and data compared to ancient ceramics due to variations in raw materials and recipes. However, it is noteworthy that some forgers utilize authentic raw materials and recipes entirely. Moreover, in our previous TL pre-dose dating studies, a very limited number of ancient porcelains have demonstrated similarly excellent pre-dose activation effects as modern imitations. In such cases, the three criteria mentioned above may become invalid. Additionally, we acknowledge that these criteria possess a certain degree of subjectivity and advocate for further evidence to substantiate our assertions.

Therefore, we opted for further experimental validation using a porcelain shard, Sample JJ, with a clearly documented production date. The conducted test demonstrated that the TL pre-dose age of the sample aligns with its production period. Subsequently, the sample underwent high-temperature annealing to completely eliminate any accumulated natural irradiation signal and was then artificially irradiated with a dose equivalent to the natural irradiation paleodose. Figure 4A,B illustrate the pre-dose sensitivity curves for natural irradiation and artificial irradiation at the same dose for the same sample. The comparison indicates a significantly lower TL intensity and smoother curve for the naturally irradiated sample compared to the artificially irradiated one, with a noticeably smaller R² value (Figure 6). The observed disparities between natural and artificial irradiation within the same sample underscore the feasibility of these criteria for assessment.

Based on the aforementioned analysis, we propose a methodology for identifying artificial irradiation. The complete procedural framework can be referred to as depicted in Figure 2, which involves employing pre-dose dating to obtain paleodose and annealing undated grains to eliminate natural irradiation signals, followed by applying an artificial irradiation dose equivalent to the measured paleodose, and finally comparing the results of natural irradiation and artificial irradiation. In the case of ancient porcelains, there will be a significant difference between natural irradiation and artificially irradiation, while modern imitations would exhibit minimal discrepancies.

In addition, the challenging issue is how to detect real age if an ancient porcelain has been artificially irradiated. For instance, a 300-year-old piece of porcelain, after receiving some artificial irradiation, receives a 600-year-old TL pre-dose age. Its TL glow curve and data are almost identical to the 600-year-old porcelain. This is a difficult problem for TL dating of porcelain and requires more in-depth research.

4. Conclusions

In this paper, we selected several pieces of ancient Chinese porcelain from different periods and modern porcelain irradiated with varying artificial doses. Using the TL pre-dose method, we determined the paleodoses and compared their TL characteristics. We observed a significant discrepancy in TL sensitivity between ancient and artificially irradiated samples, with the peak TL sensitivity in ancient samples being considerably lower. Secondly, the TL glow curve of ancient samples exhibited less smoothness compared to the artificially irradiated ones. Additionally, linear regression analysis indicated a more favorable fit for the TL sensitivity three-point data of the artificially irradiated samples. Consequently, discerning between natural and artificial irradiation can be feasible in TL pre-dose dating of porcelain when considering these aspects.

Simultaneously, we propose a method to identify artificial irradiation imitations by comparing the TL pre-dose dating results. This comparison assesses the same porcelain’s natural cumulative irradiation against its post-annealing and subsequent artificial irradiation. In such cases, ancient porcelains will exhibit significant differences, while modern imitations show minimal variances.