Radiometric dating of a building by stimulated luminescence is currently based on the chronology obtained through Thermoluminescence (TL) and/or Optically Stimulated Luminescence (OSL) dating of bricks (Goedicke
Furthermore, Urbanová and Guibert (2017), with an interesting discussion and analysis about all the aspects of the sample preparation through specificity of the measurement protocol and data evaluation to the age estimation, highlight the importance of the material characterization before the SG-OSL analysis.
In the case of young buildings and/or with chronologically close construction phases, dating techniques, stratigraphic and historical assessments should be joined with other studies providing useful information on the composition and manufacture of the building materials, namely defining classes of materials and their specific uses or uses through time in the same structure.
In this context the present paper concerns the results of a multidisciplinary study regarding the chronology of the
Based on the stratigraphic analysis of the building and considering chronological issues raised, regarding the construction sequence, the materials and sampling points were selected.
Located in Coimbra (Portugal), on the left bank of the Mondego River, the
During the building rehabilitation project, a large scale Archaeology operation was implemented. Since all the building’s plasters had previously been removed, it was possible to observe and record a vast amount of stratigraphic information related to its construction process. Several observations are consistent with a phased construction, and allowed a preliminary framework for the buildings construction history. Thus, based on stratigraphic analysis, eight different construction phases were defined (
Concerning the convent’s construction throughout the Modern Age, besides documental information regarding the outset of the endeavour (1602), we have no evidence (documental, or other) allowing precise chronological assignment to the six different proposed construction phases. Furthermore, stratigraphic information solely was not enough to integrate several built elements within the overall sequence of the convent’s construction process. Hence, in order to test the possibility for dating different phases proposed and to surpass some defaults in the stratigraphic information, an initial program of dating and material characterization was designed.
The study included sampling material from built elements which (
Details of the samples studied in this work with ID number, material and sampling point description. height above planking levelSampling point (floor 2) Sample Material Architectural element Height CSLF2 Brick Arc (east) 350 CSLF3 Mortar CSLF6 Brick Arc (south) 80 CSLF7 Mortar CSLF10 Brick Arc (west) 235 CSLF11 Mortar CSLF12 Mortar Wall (west) 145 CSLF14 Mortar Wall (north) 150
After a macroscopic description the mortar samples were divided into two parts: one part for luminescence dating and the other part for XRD measurements. The difficulty to extract pure coarse grain quartz fraction of sufficient quantity from sand used as inert in lime mortar has led the authors to study the luminescent emission from polymineral fine grain phase “enriched in quartz” through HF etching procedures during preparation phase of the samples (Prasad, 2000; Mauz and Lang, 2004; Stella
For brick samples the polymineral fine grain fraction was obtained by PH3DRA standard procedure (Gueli
TL for bricks, OSL and IRSL (InfraRed Stimulated Luminescence) for mortars were performed using semi-automated Risø readers (TL-DA-10 and TL-DA-15) with EMI 9235QA photomultipliers (Bøtter-Jensen, 1997; Bøtter-Jensen
Artificial luminescence signals were induced by two different 90Sr-90Y calibrated beta sources integrated in the Risø systems delivering, respectively, 6 Gy/min in TL-DA-15 model used for mortars and 1.32 Gy/min in TL-DA-10 model used for bricks. 241Am calibrated alpha source delivering 2.7 Gy/min was used to evaluate the luminescence efficiency coefficient
For six aliquots of each sample ED measurements and recovery test as a function of preheating temperature (160–260°C) were made (Choi
After the SAR sequence, the same regeneration dose of the first point of beta irradiation is given again to check whether the sensitivity corrected OSL (LR/TR)/(L1/T1) is reproducible by the recycling ratio R. This value, moreover, identifies the presence of a possible systematic error in the interpolation of Ln/Tn onto the dose–response curves. On the same aliquots, the recovery test was performed. The cycle of SAR modified protocol was repeated 7 times using increasing regeneration doses from 0.5 to 7.5 Gy. For ED calculation, data are acceptable if the recovery test and recycling ratio are within ±10% of unity and recuperation test near to zero (Murray and Wintle, 2003). Being young samples, whose OSL signal is not dominated by the fast component, the integral signal of the first 0.4 s was used as OSL intensity, after subtraction of the background calculated from the following 0.4 s, for minimize the contribution from the medium and slow OSL signal components (Ballarini
Plateau temperature range and preheating (Ph) temperature choose for ED evaluation.Sample Plateau ED range (°C) Ph temperature (°C) CSLF3 180–220 200 CSLF7 180–240 210 CSLF11 180–220 200 CSLF12 180–220 200 CSLF14 180–240 210
For each sample passing no significant departure from normality was found. So, the frequency distributions were related with Gaussian fits (with standard deviations) and Mean ED values were obtained (
Number of aliquots (n), luminescence efficiency coefficient (k-value), Equivalent Dose (ED) Value and relative standard deviations (SD) obtained, respectively, by radial plot (ED Central Value, see Fig. 2) and from ED frequency distributions (Mean ED) for mortar samples. from Radial Plot from ED frequency distributionsSample Number of aliquots ( k-value ED Central Value Mean ED CSFL3 39 0.125 ± 0.010 1.17 0.10 1.18 0.18 CSFL7 47 0.161 ± 0.012 2.03 0.21 2.02 0.26 CSFL11 41 0.134 ± 0.010 1.13 0.11 1.12 0.17 CSFL12 38 0.148 ± 0.010 2.41 0.15 2.42 0.22 CSFL14 44 0.157 ± 0.010 2.61 0.18 2.63 0.25
The ED for age calculation was obtained by radial plots analysis (Galbraith
The ED values for bricks were determined using TL measurements and especially the added dose method on the polymineral fine grain phase (Aitken, 1985; Guibert
Dating of bricks: k-value, Qβ e qβ values obtained by TL measurements.Sample k-value Qβ (Gy) qβ (Gy) CSLF2 0.259 ± 0.011 2.09 ± 0.19 0.13 ± 0.02 CSLF6 0.232 ± 0.018 2.87 ± 0.23 0.05 ± 0.01 CSLF10 0.220 ± 0.010 3.18 ± 0.28 0.09 ± 0.02
U, Th and K contents (
Radioactive measurements: Sample name, sample material with U, Th, K contents obtained by high gamma spectrometry and 238U/226Ra ratio.Sample Material 238U/226Ra U (ppm) Th (ppm) K (%) CSFL2 1.03 ± 0.11 4.54 ± 0.22 23.26 ± 1.49 2.46 ± 0.07 CSFL3 0.97 ± 0.08 3.76 ± 0.18 16.57 ± 1.07 0.90 ± 0.03 CSFL6 1.02 ± 0.12 3.26 ± 0.16 18.00 ± 1.15 2.00 ± 0.06 CSFL7 0.97 ± 0.09 3.91 ± 0.19 15.70 ± 1.00 1.51 ± 0.05 CSFL10 0.98 ± 0.10 4.30 ± 0.21 17.75 ± 1.14 2.33 ± 0.07 CSFL11 1.05 ± 0.08 3.40 ± 0.16 16.30 ± 1.04 1.10 ± 0.03 CSFL12 0.99 ± 0.08 3.52 ± 0.06 18.48 ± 0.49 2.37 ± 0.07 CSFL14 0.97 ± 0.10 3.70 ± 0.06 19.80 ± 0.31 2.27 ± 0.07
The dose contributions of the sample were corrected on the basis of porosity factor (W) experimentally measured and of saturation factor (F) estimated on the basis of sampling point (height, inside or outside) (Aitken, 1985). In this particular case an F value of 0.3 ± 0.2 was considered. The annual environmental dose rate was measured using TL dosimeters (GR200A) enclosed in capsules placed
Contributions to the annual dose rate: Sample name, sample material, porosity factor W, alpha and beta internal dose rates (Dαint and Dβint) and environmental component (Denv).Sample Material W Dαint(mGy/a) Dβint(mGy/a) Denv(mGy/a) CSFL2 0.16 ± 0.01 29.65 ± 1.25 3.22 ± 0.08 1.22 ± 0.05 CSFL3 0.21 ± 0.02 22.71 ± 0.93 1.71 ± 0.04 CSFL6 0.15 ± 0.01 22.24 ± 0.95 2.53 ± 0.06 1.37 ± 0.05 CSFL7 0.22 ± 0.02 22.36 ± 0.90 2.18 ± 0.05 CSFL10 0.17 ± 0.01 24.95 ± 1.01 2.94 ± 0.07 1.57 ± 0.05 CSFL11 0.27 ± 0.02 21.38 ± 0.89 1.80 ± 0.05 CSFL12 0.25 ± 0.02 23.31 ± 0.39 2.87 ± 0.06 1.75 ± 0.05 CSFL14 0.23 ± 0.02 24.78 ± 0.28 2.86 ± 0.05 1.65 ± 0.05
ED values obtained on the two FG brick and mortar samples were entered in the following age equation:
where
Dating of samples: Sample name, sample material, technique, method with ED, annual dose rate, the individual dating results (referred to the year of the TL measurements) and the corresponding calendar dates obtained for both bricks and mortars.Sample Material Technique Method ED (Gy) Annual dose rate (mGy/a) Age/2016 (a) Age (AD) CSFL2 TL Added dose 2.22 ± 0.19 11.43 ± 0.85 194 ± 22 1822 ±22 CSFL3 OSL SAR 1.17 ± 0.10 5.41 ± 0.37 216 ± 24 1800 ±24 CSFL6 TL Added dose 2.92 ± 0.23 8.62 ± 0.66 339 ± 37 1677 ± 37 CSFL7 OSL SAR 2.03 ± 0.21 6.67 ± 0.47 304 ± 38 1712 ± 38 CSFL10 TL Added dose 3.27 ± 0.28 9.44 ± 0.63 346 ± 38 1670 ± 38 CSFL11 OSL SAR 1.13 ± 0.11 5.76 ± 0.37 196 ± 23 1820 ± 23 CSFL12 OSL SAR 2.41 ± 0.15 7.47 ± 0.44 322 ± 28 1694 ± 28 CSFL14 OSL SAR 2.61 ± 0.18 7.81 ± 0.48 344 ± 31 1672 ± 31
Mineralogical characterization was based on X-ray diffraction (XRD) performed with a Philips X’Pert Pro PW3710 diffractometer equipped with a CuKα radiation, operating at 40 kV and 20 mA. Non-oriented aggregates of pulverized bulk material were measured between 2 and 60° using a step of 0.02° 2θ and 1.0 s scanning time in each step.
The macroscopic description of the mortars was performed in the laboratory after sampling. The mineralogical characterization of the mortars by XRD was performed later.
Contact spectrophotometry was used for colorimetric characterization of the mortars. The measurements were made with a portable Konica-Minolta CM2600D instrument, equipped with an integrating sphere in the geometry d/8° after the usual procedures for black and white adjustment, selecting an area of 11 mm diameter. Such measures are carried out directly
After the qualitative interpretation of all the mineral constituents and taking into account the limitations imposed by the method, a semi-quantitative estimation was carried out in order to facilitate the mineralogical interpretation of the results. However it must be kept in mind that the percentages obtained should be only relative indicators of the concentration of the minerals and not absolute values since the associated error is high.
Semi-quantitative analyses were performed by measuring the areas of the peaks corresponding to the typical diffraction maxima spacings of each mineral divided by the respective reflector powers (Schultz, 1964; Moore and Reynolds, 1997; Thorez, 1976; Dias, 1998). The whole occurring phyllosilicates (total percentage) were determined by considering the main peak area at Bragg’s law interatomic spacing d=4.48 Å peak.
All the mineral associations of the analyzed mortar samples present, as essential minerals, a large proportion of quartz followed by calcite and K-feldspars, sometimes with similar values for both these minerals (
XRD semi-quantitative mineralogical composition of mortars (%) and aggregate/binder ratio (Qz – quartz; K-felds – potassium feldspars; Na-plag – sodium plagioclases; Cal – calcite; Dolo – dolomite; Phyl – phyllossilicates; * – traces).Sample Qz K-felds Na-plag Cal Dolo Phyl Aggregate (A) Binder (B) A/B Ratio CSFL3 65 21 1 13 * * 86 13 6.62 CSFL7 65 5 * 26 2 2 70 26 2.69 CSFL11 71 14 1 13 1 * 85 13 6.54 CSFL12 59 12 10 18 1 * 71 18 3.94 CSFL14 66 11 1 21 1 * 77 21 3.67
An initial macroscopic observation showed that mortar samples consist mainly of quartz as aggregate and carbonates as cement (binder). In all the analyzed samples the sum of quartz and K-feldspars reaches a mean above 75%. Since some carbonate aggregates (dolostone, dolomitic limestone and limestone fragments) were identified by macroscopic observation, the carbonate composition of the bulk mortar evaluated through XRD corresponds both to the aggregate and the lime paste from the binder, without discrimination. However those carbonate compounds should not influence the binder/aggregate ratio, since they are in a very low occurrence. Furthermore, in what concerns to the binder, this technique only provides the detection of crystalline phases resulting from carbonation and hardening of the original burned limestone that have produced the lime paste (Sanjurjo-Sánchez
The quartz/K-feldspars ratio is always higher than three. Sample CSFL7 is different by having quartz reaching thirteen times the proportion of K-feldspars, higher proportions of carbonates and noteworthy quantities of phyllosilicates.
Morphometric and textural observations during the initial macroscopic analysis showed a large proportion of mostly subangular to angular silicate sand in all samples. All samples contain also aggregates of variable dimensions below the class of fine gravel, essentially less than 8 mm. Coupled with the XRD data, these characteristics points to a provenance of the aggregates from raw alluvium materials, probably those occurring in the left bank of the Mondego River, where the
The very scarce presence of phyllosilicates, in particular clay minerals, is remarkable and probably should be explained by the aggregate origin and the technology for the production of mortars. We can assume that raw materials sources were mainly nearby Mondego’s channel deposits comprising a low rate of clay particles, with the above commented exception of sample CSFL7.
The aggregate/binder ratio establishes two groups of samples (
Spectral Reflectance Factor of the mortar samples was measured and the CIELAB1976 space coordinates were used for colour specification. The data acquired were relative to the standard observer 10° and D65 illuminant.
Color data in the CIELAB 1976 space both rectangular L*, a*, b* (respectively Lightness, red-green color component and yellow-blue color component) and cylindrical, C*, h (respectively chroma and hue angle) coordinates with the related uncertainties.Sample L* a* b* C* h CSFL3 66.10 ± 1.67 4.63 ± 0.12 22.73 ± 0.58 23.19 ± 0.82 78.52 ± 2.77 CSLF7 55.67 ± 1.54 8.21 ± 0.23 25.57 ± 0.71 26.86 ± 1.01 72.25 ± 2.72 CSFL14 59.63 ± 1.33 11.45 ± 0.25 32.88 ± 0.73 34.82 ± 1.12 70.83 ± 2.28 CSFL12 56.72 ± 1.60 9.23 ± 0.26 26.69 ± 0.75 28.24 ± 1.08 70.95 ± 2.71 CSFL11 64.25 ± 1.52 3.46 ± 0.08 20.32 ± 0.48 20.62 ± 0.69 80.39 ± 2.70
The
In this specific case this evidence is validated by the good agreement found between colorimetric and XRD results.
Standard statistical analysis and normal distributions obtained for ED measurements of each mortar samples underlined that the used methodology represent an useful approach to obtain indications about the accuracy of the dating results.
The data obtained through mineralogical characterization of mortars identify two different types of materials based on the aggregate/binder ratio. The two groups are confirmed by colorimetric measurements (
Regarding the mineralogical characterization, the methodological approach should have been oriented to the composition and provenance issues if other laboratory tests were available, such as thermal analysis (DTA/TGA) and microscopy (SEM-EDS and/or petrography); chemical analysis; grain size distribution of the aggregates, etc. These complementary methods would make it possible to deeply investigate the composition of the binders and other mortar compounds and to allow a more consistent analysis on materials provenance and composition. Thus, it is only possible to point out the most probable provenance of the aggregates considering only the main characteristics (textural, morphological and mineralogical).
Still, the overall results allowed for a better chronological integration and understanding of stratigraphic data collected, thereby:
compositional and dating differences between samples CSFL11 and CSFL12 are consistent with stratigraphic observations since the wall corresponding to sample CSFL11 was interpreted as being built over the pre-existing wall correspondent to CSFL12. This is an area of the building which integration in the construction sequence has been harder ( the significantly earlier dates obtained for samples CSFL2/CSFL3, allow the integration of the architectural elements from which these were taken within a restructuration of the cells in the late religious occupation of the building, or most likely, already within the industrial occupation; the minor interval between samples CSFL12 and CSFL14 dates, although not questioning the proposed construction sequence, suggests the possibility of a close development between construction phases C5 and C3 ( this last observation can be also be true when looking at the chronology obtained for pair of samples CSFL6/CSFL7, which also confirms the possibility of its integration in phase C5 (
The paper discusses a multidisciplinary approach to the chronological integration of a building with a known complex construction and occupation history.
The results obtained support previous observations regarding the progressive nature of the construction process and the sequence of some building actions, also allowing a first approach to obtain more precise chronological information. It also allowed the integration of some architectural features as later additions to pre-existing construction units.
Furthermore, findings alert to the relevance of using stratigraphic and material analysis information, in order to interpret results from dating analysis, namely difference in date obtained between brick and associated mortars.
We can conclude that the strategic approach is effective for attaining the main purpose of chronological reconstruction of a historical building, namely in the sampling design, selected analytical methods and procedures.