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ABSTRACT The overarching challenge of this research is setting up a procedure to select the most appropriate fraction from complex, heterogeneous materials such as historic mortars in case

of radiocarbon dating. At present, in the international community, there is not a unique and fully accepted way of mortar sample preparation to systematically obtain accurate results. With

this contribution, we propose a strategy for selecting suitable mortar samples for radiocarbon dating of anthropogenic calcite in binder or lump. A four-step procedure is proposed: (I) good

sampling strategies along with architectural and historical surveys; (II) mineralogical, petrographic, and chemical characterization of mortars to evaluate the feasibility of sample dating;

(III) a non-destructive multi-analytical characterization of binder-rich portions to avoid geogenic calcite contamination; (IV) carbonate micro-sample preparation and accelerator mass

spectrometer (AMS) measurements. The most innovative feature of the overall procedure relies on the fact that, in case of positive validation in step III, exactly the same material is

treated and measured in step IV. The paper aims to apply this procedure to the ancient mortar of the Florentine historical building (Trebbio Castle), selecting micro-samples suitable for

dating in natural hydraulic mortars. The discussion of the mortar dating results with the historical-archaeological hypotheses provided significant insights into the construction history of

the building. SIMILAR CONTENT BEING VIEWED BY OTHERS INTEGRATED MULTI-ANALYTICAL SCREENING APPROACH FOR RELIABLE RADIOCARBON DATING OF ANCIENT MORTARS Article Open access 28 February 2022 AN

ANALYTICAL STUDY OF THE TABIA MORTAR FROM LIN YIMU’S TOMB, ZHEJIANG, CHINA Article Open access 17 October 2024 AN INNOVATIVE APPROACH FOR PROVENANCING ANCIENT WHITE MARBLES: THE

CONTRIBUTION OF X-RAY DIFFRACTION TO DISENTANGLING THE ORIGINS OF GÖKTEPE AND CARRARA MARBLES Article Open access 16 November 2021 INTRODUCTION Radiocarbon dating is one of the most widely

used dating techniques in the field of archaeology and Cultural Heritage1. This technique is used to typically date organic finds (such as charcoal, wood, bone, or textiles) but also

inorganic materials2, such as carbonate compounds, i.e., lead white3,4. Mortar is an artificial product which has been prepared and used by humans since ancient times, mainly consisting of a

binder, some aggregates and possible additives. In mortar and plaster samples, plant remains, such as charcoal, vegetal, and straw fragments, are the most dated fraction with radiocarbon

method (14C), as reported in literature5,6,7. Other approach concerns the dating with Optically Stimulated Luminescence (OSL) of quartz and feldspar aggregates8,9,10. In addition, among

possible applications to inorganic carbon-based materials, the use of 14C method for dating ancient mortars was proposed as early as in the 1960s, applying the method to the inorganic

binder11,12,13. In mortars, the inorganic radiocarbon-datable component is calcite, which is formed by the reaction of calcium hydroxide with atmospheric CO2 during the setting of the

material (the so-called anthropogenic calcite). Air-hardening mortars are the most suitable for dating because they set and harden incorporating atmospheric CO2. However, since mortars are

heterogeneous materials, other sources of C, which may contaminate the 14C concentration, can be present in the mortar samples. Contaminations can be due to the presence of: * unburned

carbonate of stone used for the production of lime and carbonate aggregate present in the mixture (geogenic calcite). These two sources make the sample older than expected; *

(re)crystallized secondary calcium carbonates and products of delayed hardening (so-called secondary calcite). Secondary calcite forms after the initial hardening of the mortar, thus,

causing an apparent rejuvenation of the sample. Moreover, the type of binder of the mortar sample may not be quite ideal, as there are historical mortars with not totally air lime binder.

Selection of the datable fraction and elimination of potential contamination is a challenge for the international radiocarbon community14,15,16,17. Despite numerous efforts as evidenced by

the extensive literature in the field carried out by the scientific community, an analytical workflow for characterization and dating of inorganic fraction of mortars has not been

established. This paper aims at discussing our sample selection procedure for radiocarbon dating of historic mortars, from the preliminary comprehensive characterization of the material to

the sample preparation for the 14C-AMS measurement. Archaeological and historical survey coupling with accurate sampling, and then in-depth minero-petrographic and chemical characterization

of the mortars are the first two steps (Step I and II), respectively. Separation of binder from the aggregate coupled with characterization of the separated carbonate fractions is mandatory

(Step III). Proactive identification of the origin of calcite allows for the reduction of the possible contamination risk, thus obtaining accurate 14C measurement by AMS (Step IV). As far as

Step II is concerned, a multi-analytical characterization procedure of the mortar fragments, i.e. optical and electron microscopy (OM, SEM–EDS), X-ray diffraction on powders (XRPD),

thermogravimetric analysis (TGA), infrared spectroscopy (FTIR) was designed. In Step III, a non-destructive, original approach capable of identifying the origin of calcite (geogenic and

anthropogenic calcite) was explored using XRPD, OM-cathodoluminescence (OM-CL), ATR-FTIR, micro-Raman. A new experimental set-up for the collection of CO2 evolving from the selected calcite

was installed, by integrating an acidification reactor into our so-called Lilliput graphitization reactors, which are optimized for microgram-sized samples18,19. The graphitization line is

used to obtain graphite samples whose residual 14C abundance is measured by AMS. The procedure was validated in architectural contexts, such as the Tuscany historical building (Trebbio

Castle in Florentine surroundings). ANALYTICAL PROCEDURE STEP I: SAMPLING AND THE ISSUE OF CHRONOLOGY QUESTIONS Accurate dating of mortar in masonry requires a comprehensive approach

involving collaboration between experts in mortar analysis, archaeologists and architects who understand wall stratigraphy20,21. Precise sampling is crucial and begins with well-defined

research questions related to chronology, along with documentation and historical research and analysis of the masonry22 (Fig. 1, step I). For example, if the aim is to determine the age of

construction, it is important to avoid areas of repair or renovation. However, if the focus is on determining the period in which the building was in use, these repaired or renovated areas

may be of greater importance. Mortar between stone blocks is likely to be original if the stone block overlies the mortar; on the other hand, if the mortar protrudes beyond the stone block,

this indicates a later intervention. Care should be taken when selecting samples from the ground, i.e. from collapsed ruins, as these may have been transported or weathered, thus being their

association with the event to be dated not accurate. Mismatches in the dating results can arise due to different factors, such as mortar constituents (the type of binder and aggregate) or

environmental factors (the state of preservation, which may be due to e.g. recrystallization and delayed hardening). For instance, bedding mortars or core mortars are generally less altered

over time than plaster and are less exposed to the external environmental parameters23. To minimize secondary carbon sources, sampling sites should be carefully selected, favoring areas that

are less exposed to weathering over exterior surfaces20,21. Analyzing samples from greater depths and intermediate heights helps to mitigate the influence of ambient water, which can

introduce younger samples through rainfall or surface water or obvious aging effects from dissolved geological carbonates in groundwater and soil moisture. As the slaked lime (Ca(OH)2)

absorbs CO2 from the atmosphere, the setting and hardening process starts from the surface and progresses inwards. Delayed hardening in the inner parts of thick walls can lead to

inaccuracies in dating results24. Optimal samples should be taken at a depth close to the wall surface, deep enough to avoid problems with the surface, but not too deep to have problems with

delayed hardening. If carbonate aggregates are present, careful sampling is essential to limit dispersion. In summary, careful sampling and consideration of various factors are crucial for

successful 14C dating results in mortar. These considerations and methods contribute to the robustness and reliability of mortar dating in archaeological investigations. The evaluation of

the degree of carbonation of the mortar with the phenolphthalein test is the first mandatory characterization step. Phenolphthalein indicates the presence of calcium hydroxide in the mortar.

A sample that is not fully carbonated must be excluded for 14C dating. The test can be carried out in situ on the masonry or in the laboratory on a sample. STEP II: ANALYTICAL PROCEDURE TO

CHARACTERIZE MORTARS FOR DATING To characterize mortars and select those materials that can be suitable for dating purposes, it is essential to determine the composition of all the

constituents of the mixture, their relative amounts (binder/aggregate ratio—B/A), the nature of binder and aggregates, the constituents within the binder, as well as the degree of

carbonation. In fact, the information we can get from different analytical techniques can give us hints about the manufacturing process of the materials. In particular, it can suggest us

whether the basic conditions for applying radiocarbon dating are respected and can support us to choose the best approach to select the most appropriate fraction to be dated. For example,

the aforementioned B/A ratio allows us to understand how much material we have to sample to get enough mass at the end of the selection procedure. For a comprehensive characterization,

several investigations must be performed, each useful in reconstructing the overall picture and providing key information to select or exclude material for dating (Fig. 1, step II). The

complementarity of multiple investigations is crucial for an accurate and full understanding of the material. Indeed, the investigations make it possible to determine the relative chronology

of different construction phases within a building or site25,26. Here following the summarized description of the analytical techniques proposed for characterization. OM Thin-section

observation of mortar under an OM in transmitted light provides essential insights into the nature of binder, aggregate and lumps19,27. For the binder, OM provides information on the texture

(micritic, microsparitic, sparitic), the mineralogical composition, the birefringence colors, the structure and the interactions with the aggregate. Moreover, OM allows us to classify the

binder as: air lime, natural hydraulic lime, air lime with addition of pozzolanic materials (i.e. _cocciopesto_, volcanic ash and clay minerals) and modern hydraulic binder28,29. The

description of the aggregate is crucial for the evaluation of contamination sources, taking into account mineralogical composition, particle size distribution, amount of binder with respect

to aggregate (B/A) ratio, macroporosity and alteration products. Petrography is also beneficial for the identification of lumps and organic fragments in the mortar. OM observation make it

possible to recognize the type of lumps and distinguish between residues of stones used to make binders and binder residues. The observation of lumps with OM allows us to recognize their

types and origins, achieving information on the rock used to produce lime as well as suggestions on production technologies27. Petrographic observation contributes to assess the uniformity

of the binder and to identify zones of different crystallinity due to partial recrystallization by circulating water. Sources of contamination, such as recrystallization of calcite and

carbonate aggregates, can lead to exclude samples from 14C dating21. Modern binders should be eliminated from dating, since the dating principle is not applicable to these types of binders.

Particular attention should be paid to magnesium binders30. The 14C dating outcomes may be affected by the presence of much younger 14C, due to the properties of minerals produced upon

carbonation (such as magnesite and nesquehonite). XRPD XRPD analysis of bulk samples includes the mineralogical composition of both the binder and the aggregate, which can be integrated with

the identified phases in thin sections. Single lumps and binder-rich portions can be also analyzed. All these data yield crucial information, revealing whether the mortar is non-carbonated

(portlandite), if the sample contains magnesium lime (brucite, hydromagnesite, magnesite), or if the binder exhibits hydraulic properties31 (tobermorite, hydrogarnet), or if secondary

reactions occurred which lead to the formation of new phases (gypsum, hydrotalcite, hydrocalumite). The presence of these latter two phases in mortar binders strongly influences the

radiocarbon dating of lime mortars, because of their high (CO3)2– anion capture capability32,33. The presence of gypsum indicates that the binder has altered, suggesting an open system and

therefore a context subject to contamination from the external environment34. SEM-EDS Observations under the optical microscope can be further enhanced and supplemented by SEM–EDS which

combines microscopy and X-ray spectroscopy to obtain detailed information on the morphology and elemental composition of mortar constituents. Semi-quantitative elemental analysis is useful

for: (1) estimating the provenance of raw material through the analysis of residues of stones used for lime production; (2) obtaining information on the hydraulic index (HI)35 and the

overall composition of the binder, including the possible presence of Ca and Mg based binder, and of silico-aluminates ferriferous phases; (3) evaluating changes in elemental composition

within reaction rims areas; (4) characterizing lumps, especially if they have a heterogeneous texture; (5) achieving micro-chemical information about the aggregate and providing hypothesis

on its provenance. TGA TGA is used in the analysis of historical mortars for evaluating hydraulic behavior; it involves subjecting a sample to controlled temperature changes while measuring

its mass as a function of temperature. TGA serves for characterization of binder materials (air binder, hydraulic binder, gypsum, etc.)36,37. Moreover, the TGA results can be integrated with

the HI value calculated from punctual micro-chemical analyses carried out with SEM-EDS38. STEP III: SELECTION AND CHARACTERIZATION OF THE POWDER FOR THE SCREENING OF CACO3 ORIGIN Upon

assessing that the sample exhibits datable characteristics, as a consequence of all the analyses performed in Step II, the following process involves the selection and further

characterization of the carbonate fraction. The binder calcite has the same chemical composition as burned carbonate rocks or carbonate aggregates, but different textural, isotopic

signatures and mechanical properties. A mechanical separation of binder-rich bulk and lump was performed, starting from a selection under stereomicroscope. For bulk samples, a portion

enriched with binder and lumps is separated, then sieved to 63 µm and lightly crushed. Our approach aims at finding non-destructive techniques able to determine the origin of the calcite in

the powder samples selected for dating (Fig. 1, step III). Non-destructive techniques allow the preservation of the sample mass so that the same sample can be subjected to several analytical

procedures and treated for 14C analysis (Fig. 1, step IV). The different origin of carbonates (geogenic and anthropogenic) can be detected by the different distortions in the lattice

structure within small crystallites. In principle, different types of calcite interact with electromagnetic radiation in a way that depends on the atomic arrangement. FTIR and Raman

spectroscopies can be used to identify short-range order at the molecular level. In addition, CL analysis, which is conventionally used to assess the origin of calcite, in our approach is

combined with ATR-FTIR and micro-Raman. The most important advantage in this non-destructive approach is that the exactly same powder is analyzed in OM-CL, ATR-FTIR and micro-Raman; and if

the sample is mainly constituted by anthropogenic calcite, it is used for step IV. OM-CL CL is a petrographic technique which represents an additional way of examining thin sections or

powder samples of carbonate specimens39. The phenomenon of CL of mortars has been discussed since 1997 and has been used in numerous studies to evaluate the origin of carbonates13,40,41.

Different densities and distribution of atomic defects in the calcite crystal structure serve as markers to identify the origin of calcite. Considering this principle, geogenic calcite and

anthropogenic calcite may have different luminescence intensities due to the different formation process. The phenomenon can be easily observed with petrographic microscopes equipped for CL

analysis (OM-CL), this instrumentation is relatively inexpensive and easy to use. For the non-destructive analysis of powders, we used OM-CL. The disadvantage of this technique lies in the

resulting color hues, especially when multiple emissions of the same powder result in a composite hue. Typically, a qualitative analysis was performed, just attributing “hues” to the

different observed colors (see for example tile red, dull purple, brown, dark brown, grey, dull grey and black). In such a framework, interpretation of data could be influenced by the

operator him/herself20,42,43. This problem can be solved by combining several analytical techniques to obtain a validated and unambiguous result. FTIR In the context of mortar dating,

spectroscopy has already been used to distinguish the origins of calcite. As demonstrated in previous studies44,45,46 conventional Fourier transform infrared spectroscopy in transmission

mode with KBr pellets can be employed for rapid sample analysis, using the heights of v2 and v4 bands. In order to use non-destructive analysis and preserve the sample for further analysis

and dating, ATR-FTIR was tested on samples with known composition and origin to establish whether this mode could lead to the same results as the FTIR technique with KBr pellet47. Since it

has been shown that differences in grinding degree affect peak widths and relative heights of carbonate archaeological materials34,48, samples with same preparation procedures were analyzed

to replicate the typical pre-treatment that might be carried out on unknown samples for dating purposes. The distinct trend lines highlight the systematic differences in v2 versus v4 peak

heights in ATR-FTIR mode for calcites formed through various processes. Two trend lines were created (geogenic and anthropogenic calcites), which can help to determine the origins of unknown

samples, offering preliminary insights into their formation. The ability to discern calcite origins through the ATR technique is particularly advantageous in the field of mortar dating, as

powdered samples can be collected and reused for dating if they contain anthropogenic calcite. MICRO-RAMAN Micro-Raman spectroscopy is a valuable tool for the characterization of mortars,

enabling high lateral resolution analysis of the mineral phases of aggregates and binder components49,50. So far, some studies have demonstrated that micro-Raman spectroscopy can be

successfully used to estimate the content of cations (Mg2+, Fe2+ and Mn2+) in carbonates, as the vibrational frequencies of the translational (T) and librational (L) modes of carbonates are

significantly related to their cation composition51,52. Raman spectroscopy has been used to investigate variations in atomic bonding in biogenic calcite crystals and to distinguish the

degree of crystallinity of calcium carbonate in biological materials by assessing the frequencies and width of the v1 and v4 bands53. Raman analysis of CaCO3 polymorphs in54 found that the

amorphous calcium carbonate exhibits a broad peak in the lattice mode region (below 400 cm−1) and that the most prominent band associated with the carbonate ion at around 1085 cm−1, which

appears as broader and significantly less intense than usual, slightly shifts towards lower wavenumbers. We carried out a study to determine the origin of calcite using micro-Raman

spectroscopy. The potential to distinguish between geogenic and anthropogenic calcite using micro-Raman spectroscopy was established for the first time by the authors55. Raman spectroscopy

and statistical methods have shown that the anthropogenic calcite samples exhibit a broadening of the L, v1 and v4 bands (calculated from FWHMs) compared to geogenic calcite samples.

Structural disorder within the calcite crystals or the presence of low crystalline order is reflected in relatively broad FWHM values and wavenumber shifts. The wider and shifted toward

lower wavenumber is the spectral band, the lower the crystallinity within the mineral. The influencing parameters (including band position, band intensity, the area covered by the bands and

the FWHM values of L, v4 and v1) for distinguishing the origins of calcite were successfully identified and they can be used to determine the origin of calcite in unknown samples intended

for dating. The potential of micro-Raman on distinguishing different calcite domains was also confirmed by Toffolo et al.56. In this paper, the micro-Raman analyses were performed on

petrographic thin sections in archaeological lime samples. STEP IV: CARBONATE MICRO-SAMPLE PREPARATION AND AMS MEASUREMENTS The limited sample material due to the high possible level of

heterogeneity of the mortars, the sample loss during the characterization step and the highly selective pre-treatment process, motivates us to use the micro-sample 14C preparation. In this

framework, the so-called Lilliput graphitization line at the LABEC laboratory in Florence, one of the laboratories of CHNet, the INFN network for Cultural Heritage, was integrated with a

reaction chamber designed for the extraction of CO2 from carbonates (Fig. 1, step IV). The Lilliput line is particularly useful in the case of mortar treatment, because it allows managing

samples as small as only 50 µg of carbon, well below the limit of the “traditional” larger samples of about 700 µg18,57. Such small processed masses provided the possibility to investigate

the feasibility of dating even individual lumps of binder in mortar samples. Typical processing masses for mortar samples are:—approx. 2.5 mg in the case of lump; —approx. 5 mg in the case

of bulk mortar. ACID DISSOLUTION AND LILLIPUT GRAPHITIZATION REACTORS The extraction of C from the selected inorganic fraction of the mortar is carried out by acid dissolution. The carbonate

sample, mechanically separated and previously characterized with non-destructive techniques, is treated with H3PO4 in the acidification line. For bulk samples, 2 evolving CO2 fractions are

usually collected per sample: the first in a few seconds (0–10/30 s) and the second thereafter (10/30–60 s). The selected shortened reaction time is intended to avoid the risk of geological

contamination, at least in the first fraction, as contaminants may still be present despite mechanical separation. In the case of lump samples, a fraction from the first few seconds of the

reaction is collected without the risk of contaminants reacting with the acid. The CO2 extracted from the acidification line is then cryogenically transferred into the graphitization chamber

using liquid nitrogen. The amount of CO2 collected is monitored by pressure measurements. Typically, about 100 mbar of CO2 is collected for each sample; this pressure basically corresponds

to about 50 µg of graphite at the end of the reaction given the inner volume of the Lilliput reaction chambers. The graphitization reaction occurs on small copper inserts previously prepared

with Fe catalyser pressed on them and is triggered at 600 °C in presence of H2 excess; the reaction produces water, which is trapped within the cold finger. After the graphitization

process, the copper inserts with the graphite deposited on them are mounted in specially modified aluminum holders that fit into the ion source of the accelerator to measure the radiocarbon

concentration. RESULTS AND DISCUSSION APPLICATION OF THE PROCEDURE ON HISTORICAL BUILDING: TREBBIO CASTLE STEP I The analytical procedure for dating was applied to mortar samples from the

walls of the tower of the Trebbio Castle, one of the most important and significant examples of aristocratic villas owned by the Medici in the area around Florence (Mugello)58 (Fig. 2). The

building was investigated through the building archaeological approach, which identifies the stratigraphic units of the building, and then associated with written or other sources, allowing

the formulation of hypotheses about the construction phases of the masonry59. Based on this methodology, four main construction phases from the thirteenth century to the first decades of the

seventeenth century, were identified: * Phase 1 (before 14th cent.): the presence of a square tower is documented. The tower was partially rebuilt and its original structure is only visible

in the lower part. * Phase 2 (14th cent.): addition of storeys to the pre-existing tower with improved masonry and crenellated walls, characterized by different building construction

techniques than the previous phase. * Phase 3 (1420–1433): addition of new storeys to the tower and complete renovation of the upper structure, including corbels and wider walkways,

attributed to Michelozzo (as reported written source in60). * Phase 4 (post 1433): modern and contemporary restorations, including mortar sealing, reconstruction and rectification of

structural problems to restore a late medieval appearance. Some restoration works were performed following the numerous historical earthquakes that affected the Mugello area58. The

chronology of the building phases is the result of a combination of stratigraphic studies, written sources and the chronotypological abacus of Mugello masonries58. The archaeological reading

of the masonry and the resulting hypotheses about the construction phases formed the basis for the selection of the mortar sampling points. A total of 27 bedding mortar and plaster samples

were collected using a hammer and chisel (Supplementary Table 1). STEP II The comprehensive minero-petrographic characterization was performed on all mortar samples (Supplementary Table 2).

Mineralogical composition analysis of bulk mortars by XRPD revealed the presence of calcite, quartz, feldspar (plagioclase and k-feldspar), lizardite and micas. Calcite can be referred to

the binder, lime lumps or fragments of aggregate; quartz, feldspars, lizardite and micas can be related to the aggregate. Gypsum was recorded only in TC19 and 24, probably due to the

sulphation phenomena of the binder34, Supplementary Fig. 1. From the petrographic observation, these mortars are made of natural hydraulic binder, obtained by firing of marly limestone

(Alberese limestone, Monte Morello Formation), diffusely employed in Florentine area61. The aggregate exhibits a heterogeneous composition, utilizing sandy sediments from local watercourses.

Finer sands (< 400 µm) predominantly consist of single crystals of quartz, feldspars, spathic calcite, while coarser fractions contain fragments of arenaceous rocks, serpentinites, and

Alberese limestone. Rare fragments of cocciopesto were also found. Since the raw materials used are the same and the production technologies are similar, no minero-petrographic criteria were

identified to differentiate samples belonging to different construction stages. Within the same construction stages, different characteristics are found in the mix-design (i.e., B/A, grain

size distribution). For radiocarbon application we focused on the binder aspect, aggregate composition and the presence of lump. So, the binder in the plaster samples (TC1-11, Supplementary

Fig. 2a, b) has undergone some chemical alteration due to the dissolution and slow recrystallization of calcite by the circulating moisture in the masonry. This process can develop in

specific areas of the samples, e.g. in pores and along fractures (referred to as secondary calcite) or change the entire texture of the binder (referred to as partially binder

recrystallization). This prevented us from selecting these samples for dating, causing an apparent rejuvenation of the sample62. As for the bedding mortars, the binder is better preserved.

Care in the preparation of the mixtures is evident, considering careful selection of aggregates and a consistently high binder content. However, even these samples exhibit characteristics

that allow the selection of only certain samples for dating: samples superficially collected, those with non-homogeneous carbonation processes (heterogeneous texture ranges from

microsparitic to sparitic in TC12, 13, 14, 15, 16, 18, 22, 24, 25, Supplementary Fig. 2c, d), or binder recrystallization (as in TC12, 13, 14, 15, 16, 17, 18, 19, 21, 22, 24, 25), those

showing gypsum in XRPD (as in TC19, 24), and those with almost exclusively carbonate aggregate (TC20, 23; Supplementary Fig. 2e) have been excluded. Heterogeneous texture can be due to

delayed carbonation processes or binder dissolution-recrystallization63, in fact, in most samples the two features are combined. Within bedding mortars, we focused on two samples that could

provide key insights into the historical attribution of construction phases and with mineralogical-petrographic characteristics which are more suitable for dating. These samples come from

the crenellated masonry, TC26, and the infill masonry, TC27 (see in Fig. 2c). The samples show complete carbonation through phenolphthalein test. On initial macroscopic examination, sample

TC26 appears to have a compact mortar with few fractures and a hazel coloration. Millimetre-sized lumps of varying coloration, from white to yellowish, are visible. The mortar sample TC27 is

compact and has a hazel color. Millimetre-sized lumps of yellowish to white hues can also be observed. The main mineralogical and petrographic characteristics of these two samples are

listed in Table 1. Petrographic observations of TC26 and TC27 reveal the presence of a binder with homogeneous structure and micritic texture with small dark inclusions. Lumps are present,

referring to both unmixed binder and unburned limestone. The aggregate exhibits a heterogeneous composition and bimodal grain size distribution, consisting of abundant carbonate rock

fragments (limestone and calcarenites), sandstone, serpentinite and crystal of quartz, feldspars, and calcite (Supplementary Fig. 3). TC26 and TC27 differ in: the B/A, 1/3 and 1/2,

respectively, and for the coarser aggregate grain size in sample TC27 (0.7–1 mm). SEM–EDS analyses on the binder showed significant variability in SiO2 and CaO content, along with the

presence of different types of lumps, confirming the use of Alberese limestone. A comprehensive study of lumps, combining OM, OM-CL, and SEM–EDS analyses in the same area, revealed that also

the texture of lumps is heterogeneous (Fig. 3). They exhibit a similar texture to the binder in OM, appearing brick-red in CL, and SEM analysis indicates a CaO and SiO2 + Al2O3 + Fe2O3

composition comparable to that of the binder. SEM–EDS analysis of the thin sections indicates that only small amounts of Mg are present (less than 1.8%). To gain further insight into the

binder composition, SEM–EDS microanalyses were carried out on polished thin sections of both binder and lime lumps. The micro-chemical composition of lime lumps and binder is reported in

Table 3. In addition, the hydraulicity index (HI) was calculated using Boynton's formula35 (Table 2). TC26 exhibits an HI of 0.16 ± 0.05, TC27 shows an HI of 0.20 ± 0.08, classifying as

weakly hydraulic. HI results are compared with TGA analyses performed on 3 portions of binder-rich mortar per sample. The hydraulic water (%) originating from the hydraulic components

varies between 7.02% and 8.89%, while the CO2 decomposition from air lime binder is between 27.0% and 31.9%. The results of SEM–EDS are in agreement with those of TGA and show that the

mortars have slightly hydraulic behavior (Supplementary Fig. 4). STEP III Being carbonate aggregates abundant, to avoid possible contamination, we decided to focus on lumps for dating. Four

lumps were selected for TC26 samples (labelled as TC26L1, L2, L3, and L4), and four for samples TC27 (labelled as TC27L1, L2, L3 and L4) (Fig. 4a). XRPD analyses were conducted on the

powdered lump samples after sieving to determine mineralogical composition (Table 3); OM-CL, ATR-FTIR and micro-Raman analyses (Fig. 4b–d) were performed to assess the origin of the calcite.

XRPD analysis on lumps showed that the primary component is calcite, as expected. TC26L1, TC26L2, TC26L4, TC27L1 and TC27L4 exhibit red-brown luminescence, which is consistent with their

position on the anthropogenic calcite trend in ATR-FTIR, classifying them as pyrogenic carbonate. However, TC26L3, TC27L2, and TC27L3 exhibit orange CL and geogenic trends in ATR-FTIR,

confirming that these lumps consist of geogenic calcite. ATR-FTIR detected a broad band centered at 1080 cm–1, attributable to νas(Si–O–Si) and ascribed to amorphous silicates64, likely

originating from the calcination of stone rich in silicate components (e.g., clay minerals) (Supplementary Fig. 4). Micro-Raman analyses, conducted on TC26L1, TC26L2, TC26L4, TC27L1, TC27L4,

show a Raman shift of L and v1 bands toward lower wavenumber, along with their higher FWHM values, which is also observed for the v4 band (Table 3). The micro-Raman results definitively

confirm the data collected by other techniques. The observed values are typical of anthropogenic calcite. STEP IV Based on the result of the characterization process discussed in the

previous sections, lumps TC26L1, TC26L2, TC26L4 and TC27L1 were chosen to be suitable for dating. The reaction times, along with the masses of the graphitized samples, are listed in Table 4.

The reaction time of 30 s was chosen since, having assessed the anthropogenic origin of the calcite, the risk of contaminants reacting with the acid and the sample mass were low. The AMS

results are reported in Table 4. When samples belonging to the same fragment or construction phase have consistent radiocarbon concentration between each other, a weighted average can be

calculated to obtain a more precise result. Indeed, the lumps TC26L1, TC26L2, and TC26L4 from the same mortar portion exhibit consistent radiocarbon concentrations. The results of the

weighted average of the three radiocarbon concentrations and the corresponding conventional radiocarbon age are also reported in Table 4. The calibrated age for samples TC26L1 + TC26L2 + 

TC26L4 results from the measured conventional radiocarbon age (Fig. 5). Although the calibrated age of the TC26 lump samples spans two of the phases identified in the archaeological analysis

of the tower (phases 2 and 3), the characteristics of the masonry where the sample was taken suggest an interpretation of the 14C results as more likely within the middle of the fourteenth

century (phase 2). The dating results for sample TC27L1 are presented in Table 4. Given the conventional radiocarbon age measured (Fig. 5), sample TC27L1 is considered modern. A discrepancy

can be observed between the assumed chronology and the measurement of the radiocarbon concentration. Based on the historical-archaeological hypothesis, it is assumed that the masonry dates

from the middle of the fifteenth century (phase 3). However, this discrepancy could be related to the extensive joint sealing of the upper part of the tower. The combination of the

calibrated age and the historical information; allow us to formulate a specific interpretative hypothesis and attribute this operation to the restoration season following the seismic events

that affected the Mugello area between the mid-15th and mid-seventeenth centuries58. After this intense and destructive earthquake period, intensive restoration and reconstruction activities

were carried out on all the Medici properties in the area (e.g. Cafaggiolo, the Fortezza di San Martino, the town of Scarperia). Sample TC27, being associable to the modern phase, could

have intercepted one of these activities. The comparison between the radiocarbon dating and the archaeological results offered two new interpretations. On the one hand, the 14C dating for

sample TC26 yielded a range of dates that included the fourteenth and fifteenth centuries, while the chronotypology of the masonry of Mugello allows us to shift the focus to the first one.

TC27, on the other hand, it was the 14C dating that provided new interpretative approaches to the historical-archaeological data and made it possible to identify specific restoration

interventions carried out in the modern period on a masonry that in the written source dates to the early fifteenth century60. CONCLUSION The proposed multistep procedure is demonstrated to

be a successful approach for selecting a suitable mortar sample for radiocarbon dating. Multi-analytical approach (OM, SEM–EDS, XRPD, TGA) has been proven to permit the selection of the most

suitable mortar to be dated. Non-destructive analyses (XRPD, OM-CL, ATR-FTIR and Micro-Raman) of selected specific mortar portions, such as binder-rich or lump samples (mechanical

separation), enable the characterization of the calcite origin (differentiation between anthropogenic and geogenic calcite) with the perspective of reusing the samples for following analyses

and dating. The acidification line to extract CO2 was coupled to the so-called Lilliput graphitization line, for dealing with small carbonate samples (2.5 mg lump and 5.0 mg bulk). Our

procedure has been successfully applied to single lumps collected from Florentine natural hydraulic mortars to date Trebbio Castle construction phases. The new proposed multi-analytical

procedure allowed us to discard most of the samples, identifying problems that could affect their correct dating (step II, III). Consequently, the preparation and the following AMS

measurements (step IV) focused on just those micro-samples that had been found suitable as the result of characterization in step III, exploiting the same material. The results obtained from

the comparison of the mortars with the historical-archaeological hypothesis provided relevant insights into the construction history of the building. This paper aims at emphasizing that it

is only through a well-structured analytical procedure that it is possible to select suitable samples and approach for the dating of traditional historical mortars. METHODS CARBONATION TEST

The phenolphthalein test (standardized by UNI EN 14630, 2007) is carried out using a 1% solution of phenolphthalein in ethyl alcohol. Applied to the surface of the freshly cut sample.

OPTICAL MICROSCOPE The Axioscope A.1 Zeiss transmitted light polarizing optical microscope, connected to a digital video camera, allowed for the acquisition of sample images in thin

sections, which were processed using AxioVision software. The acquired images were further analyzed to obtain information on the morphological and morphometric characteristics of the samples

using the ImageJ program. X-RAY POWDER DIFFRACTION (XRPD) The mineralogical composition was analyzed using a Philips X’Pert PRO X-ray powder diffractometer (XRPD) with a Cu anticathode

(wavelength λ = 1.54 Å). The instrument operated at a current intensity of 30 mA and a voltage of 40 kV. The 2θ range explored was between 3 and 70° with a step size of 0.02° and a total 

time per pattern of 16 min 27 s. XRPD analyses were conducted on both powder bulk samples and specific lumps. SCANNING ELECTRON MICROSCOPY WITH ENERGY-DISPERSIVE X-RAY SPECTROSCOPY (SEM–EDS)

The ZEISS EVO MA 15 SEM–EDS with a tungsten filament and an energy-dispersive X-ray spectroscopy (EDS) analytical system, Oxford Ultimax 40 (with a resolution of 127 eV @5.9 keV and an area

of 40 mm2), was utilized for semi-quantitative microchemical and morphological analyses. These analyses were conducted on thin sections (prepared after carbon-metallized pretreatment) taken

from both the binder and lumps areas, as well as from powder samples. The operational settings were as follows: an acceleration potential of 15 kV, a beam current of 500 pA, a working

distance of 9–8.5 mm, a live time of 20 s to achieve an acquisition rate of at least 600,000 counts using Co standard, a process time of 4 for point analyses, and a pixel dwell time of 500 

µs for map acquisition with a resolution of 1024 × 768 pixels. The microanalysis employed the Aztec 5.0 SP1 software, implementing the XPP matrix correction scheme. This process utilized

purchased standard elements for calculations, enabling “standard-less” quantitative analysis. Constant analytical conditions, such as filament emission, were monitored through numerous

analyses of a Co metallic standard. THERMO-GRAVIMETRIC ANALYSIS (TGA) Thermogravimetric analyses (TGA) were carried out on historical mortar samples using a Perkin Elmer Pyris 6 system and

Netzsch TG 209 F3 Tarsus. Fragments from each sample were mechanically broken down using a porcelain pestle, and the portion passing through a sieve with 63 µm openings (ISO R 565 Series)

was selected as a binder-rich specimen. About 5 mg of the sample was used for TGA, and the analysis was conducted within the temperature range of 110–1000 °C. The samples were dried using

silica gel as a desiccant at room temperature for a minimum of one week. The TGA experiments were performed in open alumina crucibles, with a heating rate of 10 °C min−1, and a nitrogen gas

flow of 30 mL min−1. CATHODOLUMINESCENCE (OM-CL) Optical microscope cathodoluminescence (OM-CL) analysis was conducted using the CL8100 MK5 model by Cambridge Image Technology Ltd., coupled

with a Leica DM2700P polarization optical microscope. The microscope is equipped with a high-sensitivity 12 MP Leica Flexcam C1 camera and dedicated LAS X software, enabling the acquisition

of digital images in various formats. FOURIER TRANSFORM INFRARED SPECTROSCOPY (ATR-FTIR) FTIR spectra were collected with a portable Bruker Optics ALPHA FT-IR Spectrometer equipped with SiC

Globar source and a DTGS detector. The powdered samples were analyzed using a Platinum ATR single reflection diamond module collecting 24 scans, in the 4000–400 cm−1 spectral range, with a

resolution of 4 cm−1. The spectra were processed using OPUS 7.2 software (Bruker Optics GmbH, Ettlingen, Germany) and Spectragryph 1.2.15. Instrument was used in the laboratory of ISPC-CNR

(Institute of Heritage Science in Sesto Fiorentino), Italy. MICRO-RAMAN SPECTROSCOPY A Renishaw InVia Raman spectrometer, characterized by high resolution, was utilized in combination with a

Leica DMLM microscope. The experiments involved employing a 785 nm excitation line, a 50 × long working distance objective (NA 0.5), a spectral resolution better than 1 cm−1, and a

theoretical laser spot diameter of 1.9 μm. The laser operated at a power of 80 mW, and each spectrum was acquired over a period of 5 s. Our focus was primarily on the low-to-medium region of

the spectral range, specifically collecting data within the range of 100–1400 cm−1. DATA AVAILABILITY All data generated or analysed during this study are included in this published article

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Central  Google Scholar  Download references ACKNOWLEDGEMENTS The authors would like to thank Arch. Lucrezia Cuniglio, for the support and the collaboration for the collection of samples.

Additionally, appreciation is extended to Laura Chiarantini and Tiziano Catalani for their technical assistance in the SEM-EDS analysis, as well as to Silvia Danise and Elena Pecchioni for

facilitating access to OM-CL instrumentation. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Department of Earth Sciences, University of Florence, 50121, Florence, Italy Sara Calandra, Teresa

Salvatici & Carlo Alberto Garzonio * Institute of Heritage Science, National Research Council of Italy, 50019, Sesto Fiorentino (Florence), Italy Emma Cantisani, Barbara Salvadori &

 Fabio Fratini * Institute of Heritage Science, National Research Council of Italy, 20125, Milan, Italy Claudia Conti * National Institute for Nuclear Physics, Unit of Florence, 50019, Sesto

Fiorentino (Florence), Italy Serena Barone, Lucia Liccioli & Mariaelena Fedi * Department of Physics and Astronomy, University of Florence, 50019, Sesto Fiorentino (Florence), Italy

Serena Barone * Department of Historical Science and Cultural Heritage, University of Siena, 53100, Siena, Italy Andrea Arrighetti Authors * Sara Calandra View author publications You can

also search for this author inPubMed Google Scholar * Emma Cantisani View author publications You can also search for this author inPubMed Google Scholar * Claudia Conti View author

publications You can also search for this author inPubMed Google Scholar * Barbara Salvadori View author publications You can also search for this author inPubMed Google Scholar * Serena

Barone View author publications You can also search for this author inPubMed Google Scholar * Lucia Liccioli View author publications You can also search for this author inPubMed Google

Scholar * Mariaelena Fedi View author publications You can also search for this author inPubMed Google Scholar * Teresa Salvatici View author publications You can also search for this author

inPubMed Google Scholar * Andrea Arrighetti View author publications You can also search for this author inPubMed Google Scholar * Fabio Fratini View author publications You can also search

for this author inPubMed Google Scholar * Carlo Alberto Garzonio View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS S.C., E.C., C.C., M.F.

designed the research; E.C., M.F., C.A.G. supervised the research project; F.F., A.A., T.S. archaeological and architectural research; F.F, E.C. collected and prepared the samples; S.C.,

E.C. OM, XRPD, SEM–EDS characterizations; S.C., C.C. Micro-Raman measurements; S.C., B.S. FTIR measurements; S.B., L.L., S.C. micro-sample preparation; M.F., S.B., L.L. AMS measurements and

data analysis; S.C., E.C., M.F., S.B., L.L., A.A., F.F. archaeometric interpretation of results. All authors collaborated to the writing of the manuscript. CORRESPONDING AUTHOR

Correspondence to Sara Calandra. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing interests. ADDITIONAL INFORMATION PUBLISHER'S NOTE Springer Nature remains

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licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/. Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Calandra, S., Cantisani, E., Conti, C. _et al._ A new

multi-analytical procedure for radiocarbon dating of historical mortars. _Sci Rep_ 14, 19979 (2024). https://doi.org/10.1038/s41598-024-70763-2 Download citation * Received: 18 March 2024 *

Accepted: 21 August 2024 * Published: 28 August 2024 * DOI: https://doi.org/10.1038/s41598-024-70763-2 SHARE THIS ARTICLE Anyone you share the following link with will be able to read this

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KEYWORDS * Historical mortars * Radiocarbon dating * Geogenic and anthropogenic calcites * ATR-FTIR * Micro-Raman * Microsample preparation