Appl Phys A (2014) 114:741–752DOI 10.1007/s00339-013-7878-x

Laboratory tests addressed to realize customized restorationprocedures of underwater archaeological ceramic finds

Cristina M. Belfiore · Mauro F. La Russa · Luciana Randazzo · Giuseppe Montana ·Antonino Pezzino · Silvestro A. Ruffolo · Piergiorgio Aloise

Received: 5 June 2013 / Accepted: 18 July 2013 / Published online: 22 August 2013© Springer-Verlag Berlin Heidelberg 2013

Abstract The present contribution is part of a biennial re-search project funded by the Italian Ministry of Education,Universities and Research (MIUR). This study, currently inprogress, deals with innovative experimental approaches ap-plied to the chemical, mineralogical and physical transfor-mations occurring during the prolonged permanence of ar-chaeological ceramic finds in seawater environments as wellas to restoration and conservation issues of the same under-water artifacts. The experimental approach used in this re-search consisted in the manufacture of ceramic test-pieces(briquettes) and their successive placing in underwater envi-ronment.

This work aims at assessing how textural and compo-sitional parameters along with firing temperatures of a ce-ramic paste could affect total porosity, pore size distributionand kinetics of capillary water absorption. A further purposeis concerned with the study of the potential modification ofporosity and pore size distribution in the same ceramic bod-ies after immersion in seawater (3 and 6 months). The studywas carried out using a multi-technique approach includinginvestigations through polarized light microscopy (PLM),

C.M. Belfiore · M.F. La Russa (B) · S.A. Ruffolo · P. AloiseDipartimento di Biologia, Ecologia e Scienze della Terra(DiBEST), Università della Calabria, Via Pietro Bucci,87036 Arcavacata di Rende, CS, Italye-mail: mlarussa@unical.itFax: +39-0984-493689

C.M. Belfiore · A. PezzinoDipartimento di Scienze Biologiche,Geologiche e Ambientali—Sezione di Scienze della Terra,Università di Catania, Corso Italia 57, 95129 Catania, Italy

L. Randazzo · G. MontanaDipartimento di Scienze della Terra e del Mare (DiSTeM),Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy

capillary water absorption test, mercury intrusion porosime-try (MIP) analyses and theoretical calculation of salt crys-tallization pressures.

The obtained results show a different behavior of thesix experimental pastes as a function of their compositionaland textural features as well as pore size distribution andfiring temperatures. These parameters, in various extents,have been found significantly influencing the kinetics of wa-ter absorption and the susceptibility of ceramic artifacts tosalt crystallization processes, with important implications onconservation procedures needed before the musealization.

1 Introduction

In the last decades, the archaeological research carried outwith scientific methods has extended its field of action tothe underwater heritage, through the experimentation of newtechniques aiming to the conservation of submerged arti-facts [1–3]. The state of preservation of underwater findsvaries mainly according to the intrinsic properties of mate-rials and environmental conditions, these latter influencingthe marine micro- and macro-organisms that colonize thesurfaces [3]. The knowledge of such variables is thereforeessential to understand the alteration and degradation pro-cesses that affect archaeological artifacts during the burialstage, when modifications of their mineralogical and chem-ical composition and physical-mechanical properties oc-cur.

Effects of burial alteration and contamination on ancientceramics were studied by several authors [4–14]. However,most of papers in the existing literature refer to subaerialburial environments, whereas only few studies concern theunderwater ones [15–19]. In seawater, archaeological ce-ramics are affected by several physical, chemical and bio-logical processes, mainly depending on the prevailing en-

742 C.M. Belfiore et al.

Fig. 1 Overview of the marineenvironment of Capo Granitola(Mazara del Vallo, Sicily) andparticular of the two differentopen seabed settings where theexperimental ceramic test-pieceswere placed for experiments

vironmental conditions (e.g. degree of salinity and acidity,temperature and thermal variations, type and concentrationof soluble salts, marine currents or bulk flow of water anderoding fluid movements, microorganisms, fauna and flora)and the characteristics of the underlying substrate (type andhardness of the sediment or rock and associated organisms)[20–22]. Soluble salts (sodium, magnesium, calcium, potas-sium and strontium cations along with chloride, sulfate, bro-mide and bicarbonate anions) contained in seawater havea prominent role in the degradation processes affecting ce-ramics being absorbed through the pores of artifacts. Theselatter, after a prolonged permanence in the salty underwa-ter environment, are taken out to the subaerial environmentwhich is characterized by totally different physico-chemicalconditions. Here, fluctuations of relative humidity and tem-perature mobilize the absorbed soluble salts, thus modify-ing the compositional and textural features of artifacts, andeventually the salt crystallization process accelerates weath-ering endangering durability of ceramics [23, 24]. A deepknowledge of this topics, including a quantification of suchprocesses affecting the underwater artifacts and the indi-viduation of specific intervention procedures, is still miss-ing.

This contribution is part of a biennial research project,currently in progress, funded by the Italian Ministry of Ed-ucation, Universities and Research (MIUR), focused on in-novative experimental approaches applied to the chemical,mineralogical and physical transformations occurring dur-ing the prolonged contact of archaeological ceramic arti-facts with seawater environments [25]. The experimental ap-proach used in this research consisted in the manufactureof ceramic test-pieces purposely manufactured to reproducecompositional and textural characteristics generally showed

by archaeological transport amphorae (the ceramic class byfar most represented between the underwater archaeologicalfinds). The experimental briquettes, produced by using dif-ferent raw materials (clays and sands) with specific mixingand tempering procedures, were then placed in the marineenvironment of Capo Granitola (Mazara del Vallo, Sicily) atapproximately 4 meters of depth (Fig. 1). In particular, twodifferent open seabed environments were chosen for exper-iments, characterized by oxidizing and reducing conditions,respectively.

The present paper is concerned with a specific aspect ofthis wide issue, aiming at evaluating how firing tempera-tures and textural (e.g. size distribution and packing) andcompositional (of both clay and temper) parameters of aceramic paste can influence total porosity, pore size distri-bution and kinetics of water absorption by capillarity, be-ing these parameters considered as crucial to determine themechanisms of underwater degradation of ceramic artifacts.Furthermore, the modifications of total porosity and poresize distribution in the same ceramic bodies after immer-sion in seawater (3 and 6 months) was also investigated.A multi-technique approach was used including polarizedlight microscopy (PLM), capillary water absorption test,mercury intrusion porosimetry (MIP). In addition, the cal-culation of theoretical salt crystallization pressures, accord-ing to the model proposed by Fitzner and Snethlage [26],was also performed since the susceptibility of ceramics tosalt crystallization is also strictly connected to strength, thislatter representing the resistance of materials to crystalliza-tion pressure [27]. As a consequence, from this study usefulindications can be obtained for planning suitable proceduresof cleaning, consolidation and protection of archaeologicalceramics before their musealization.

Laboratory tests addressed to realize customized restoration procedures of underwater archaeological 743

2 Experimental details

Experimental ceramic briquettes (length = 11 cm; width =6 cm; height = 0.8 cm) (Fig. 2), purposely manufacturedin order to reproduce consistent compositional and textu-ral characteristics, were prepared and fired under reasonableand strictly controlled temperature and atmosphere in orderto control the development of size and distribution of theporosity, as well. Specifically, the experiment is being car-ried out upon briquettes of six different types of paste ob-tained by controlling the following parameters:

1. Inclusion or tempering composition:(a) Volcanic sand (V)(b) Silico-calcareous sand (S)

2. Packing of inclusions or tempering:(a) Class of packing: medium-high, approx. 25 vol.%(b) Class of packing: medium-low, approx. 5 vol.%

3. Clay paste composition:(a) Calcareous clay with approx. 8 % CaO content (C)(b) Non-calcareous clay with approx. 1 % CaO content

(NC)4. Firing temperature:

(a) T = 800 ◦C, time of permanence under T max. =3 hours (low-grade sintering)

Fig. 2 Just manufactured experimental ceramic briquettes of the sixdifferent paste types before firing

(b) T = 950 ◦C, time of permanence under T max. =3 hours (high-grade sintering).

The resulting types of ceramic pastes are listed in Table 1.Selection of raw materials (sands and clays potentially

used for pottery making activities in antiquity) was madeon the basis of both geological and ethnographic groundsobtained by previous studies [28–32].

The silico-calcareous sand was sampled from fluvial-marine deposits (Middle Pleistocene) cropping out in thesurrounding of the village of Misterbianco at the northernedge of the Plain of Catania. It is mainly composed of quartzgrains, calcareous bivalve shells and gastropods, indicativeof a shallow marine environment [28–30]. The volcanicsand (V) was artificially obtained by crushing and sievinga basaltic rock collected from a recent stone quarry locatedat the slope of the Mount Etna volcano, near the villageof Nicolosi (around 16 km far from Catania). This basaltshows a porphyric texture, with Porphyritic Index rangingbetween 20 and 30. Phenocrysts are represented by plagio-clase and augite in similar proportions, followed by subordi-nate quantities of olivine and titanomagnetite. The ground-mass is composed by microlites of the same minerals [31].

As regards the two clayey raw materials, the first onebelongs to the Terravecchia Formation (Upper Tortonian-Lower Messinian) and is represented by an illitic clayey-silt, with a medium-low content of calcareous microfaunaand fine/very fine detritic quartz grains. The second clayeyraw material, belonging to the Argille Varicolori Formation(Cretaceous-Eocene), can be classified as a silty clay onceagain characterized by abundant illite and illite–smectitemixed layers. Detritic quartz grains have been found to con-stitute the coarse silty fraction and the finer sand grainsscarcely occurring [32].

Ceramic briquettes were placed in specifically designedand man-made Ertacetal® (acetal copolymer) stamped covershields, and immersed in two open underwater environmentscorrespondingly characterized by monitored oxidizing or re-ducing conditions (Fig. 3). The experimental apparatus ishosted at the Italian CNR-IAMC dockside (Granitola, north-western Sicily). Briquettes are going to be periodically col-lected within four steps (3–6–12–18 months). In this work,

Table 1 List of the six ceramicpastes realized for experimentswith the indication of firingtemperatures, amount of temperand composition of clays andtemper

Typology T (◦C) Packing(vol.%)

Temper Clay

Paste I 800 ◦C 5 % Volcanic sand (V) Non-calcareous clay (NC)

Paste II 800 ◦C 25 % Volcanic sand (V) Non-calcareous clay (NC)

Paste III 800 ◦C 5 % Silico-calcareous (S) Calcareous clay (C)

Paste IV 800 ◦C 25 % Silico-calcareous (S) Calcareous clay (C)

Paste V 950 ◦C 5 % Silico-calcareous (S) Calcareous clay (C)

Paste VI 950 ◦C 25 % Silico-calcareous (S) Calcareous clay (C)

744 C.M. Belfiore et al.

Fig. 3 Ceramic briquettes in Ertacetal® stamped cover shields justplaced in the seabed of Capo Granitola, at about 4 meters of depth

the first results relative to recuperations at 3 and 6 monthsare reported.

Weekly monitoring of the most important physico-chemical parameters of seawater in the two environmentswas carried out. Measurements were performed by meansof a Handheld Multiparameter Instrument YSI 556 MPS(Multiprobe System). In particular, temperature, pH, con-ductivity, salinity, total dissolved solid (TDS), Redox po-tential (ORP) were measured. It is worth noting that exceptfor ORP, all other physico-chemical parameters do not showsignificant variations between the two monitored environ-ments. As expected, lower ORP values were registered inthe open seabed reducing environment. Results of physico-chemical monitoring are reported in Table 2.

3 Analytical methods

In order to characterize the ceramic briquettes several an-alytical techniques were used. Polarized light microscopy(PLM) was performed on thin sections of the six ceramicpastes using a Zeiss Axiolab microscope, in order to deeplyinvestigate compositional and textural features. Descrip-tions were made following the scheme proposed by Whit-

bread [33], which is widely used for archaeological ceram-ics.

The parameters associated with fluid uptake and trans-port inside the pores were determined through the capillarywater absorption test [34]. According to recommendations,dried samples were weighed and separately placed in ves-sels containing a pile of filter papers immersed in water upto half of their depth, with one face of the sample lyingon the paper. Absorption occurred only through the base,by vertical suction. The containing vessels were sealed andkept at a constant temperature of 20 ◦C. At regular inter-vals, samples were extracted and weighed, after spongingwith a damp cloth to eliminate excess water. The amountof water absorbed per surface unit (Qi ) is expressed as:Qi = (mt −m0)/S, where S is the area of the sample, whilem0 and mt are the sample weights measured at times t and 0,respectively.

The distribution of pore access size and its evolution asa function of the time of immersion in seawater, as wellas the pore volume were determined by mercury intrusionporosimetry (MIP). Freshly cut ceramic chips of ca. 2 cm3

were oven dried for 24 h at 110 ◦C and then analyzed usinga porosimeter Micromeritics Autopore IV, with maximumpressure of 400,000 kPa. In addition, in order to appraisethe susceptibility to soluble salt decay, theoretical crystal-lization pressures pertaining to the experimental briquetteswere calculated starting from the porosimetric data [35].They represent the maximum pressures that could be ex-erted by salt crystallization within the pore network. Theequation for calculating the crystallization pressure was de-veloped from a thermodynamic model proposed by Well-man and Wilson [36, 37]. The model predicts that crys-tallization will originate in larger pores and that porousmaterials with larger capillaries connected to smaller onesare the most susceptible to damage [37]. The pressure thatbuilds up between two such connected pores, once crys-tallization takes place, is given by the following equationproposed by Everett [38], by assuming a spherical geom-etry: �P = 2γ (1/r − 1/R), where �P = crystallizationpressure (dyne/cm2), γ = crystal-solution surface tension(dyne/cm), r = smaller pore radius (cm) and R = largerpore radius. A surface tension of 80 dyne/cm was used, assuggested by Fitzner and Snethlage [26], which correspondsto the surface tension of halite crystals, supposed to be suit-able for our system. For the calculation, the pores <10 mi-crons were grouped according to radii into seven classesin the following ranges: class 1, 0.0037 < r < 0.01 µm;class 2, 0.01 < r < 0.05 µm; class 3, 0.05 < r < 0.1 µm;class 4, 0.1 < r < 0.5 µm; class 5, 0.5 < r < 1 µm; class 6,1 < r < 5 µm; and class 7, 5 < r < 10 µm. The median radiiof pore classes 1 to 7 are: 0.00685, 0.03, 0.075, 0.3, 0.75, 3,and 7.5 µm, respectively. To calculate the effective pressurethat can arise in the porous media (natural stones, ceramics,

Laboratory tests addressed to realize customized restoration procedures of underwater archaeological 745

Table 2 Results of open seabed environment monitoring

Site of measurement Date Temperature(◦C)

pH TDS(g/L)

Salinity(ppt)

ORP (mV) Conductivity(mS/cm)

Oxidizing openseabed environment

16/03/12 16.81 8.20 36.64 37.51 230.64 47.55

28/03/12 15.68 8.14 35.88 36.62 169.42 45.38

05/04/12 17.33 8.08 37.07 38.01 78.35 48.70

12/04/12 15.66 8.03 36.20 36.98 204.23 45.75

19/04/12 15.90 8.02 35.65 36.36 178.99 45.31

26/04/12 17.62 8.12 35.63 36.36 51.52 47.09

04/05/12 17.14 8.07 36.40 37.24 85.56 47.59

11/05/12 18.13 8.15 36.19 37.02 133.91 48.36

18/05/12 16.79 8.11 36.34 37.17 100.61 47.14

25/05/12 16.18 8.14 36.73 37.60 124.33 46.98

31/05/12 18.64 8.14 36.77 37.68 83.63 49.69

08/06/12 21.49 8.09 36.23 37.05 137.73 51.99

14/06/12 19.95 8.11 36.56 37.44 73.04 50.81

20/06/12 20.52 8.16 36.65 37.55 79.99 51.56

28/06/12 17.76 8.09 36.23 37.05 86.79 48.03

05/07/12 20.65 8.08 36.18 37.00 145.30 51.03

12/07/12 23.88 7.99 36.32 37.12 113.56 54.69

17/07/12 18.83 8.08 36.45 37.31 119.85 49.46

25/07/12 20.83 8.03 36.35 37.19 93.87 51.47

02/08/12 20.21 8.12 36.33 37.18 97.57 50.78

10/08/12 24.22 8.07 36.28 37.07 99.45 54.99

30/08/12 23.16 8.10 36.19 36.98 87.57 53.72

Reducing openseabed environment

28/03/12 16.00 8.09 36.98 37.88 −59.36 47.11

05/04/12 17.91 7.98 37.17 38.08 −23.90 49.37

12/04/12 16.77 8.01 35.74 36.48 205.23 46.34

19/04/12 16.10 8.01 36.14 36.93 64.67 46.15

26/04/12 17.98 8.08 35.94 36.72 29.90 47.87

04/05/12 18.04 8.11 36.36 37.21 71.91 48.51

11/05/12 18.26 8.10 36.25 37.08 −15.21 48.59

18/05/12 17.05 8.09 36.75 37.64 −12.78 47.96

25/05/12 16.51 8.11 36.77 37.65 −12.00 47.40

31/05/12 19.08 8.09 36.83 37.75 −15.00 50.25

08/06/12 21.66 7.87 36.66 37.54 −16.28 52.80

14/06/12 20.63 7.87 36.71 37.62 −92.57 51.77

20/06/12 20.54 7.81 36.68 37.58 −58.61 51.63

28/06/12 17.84 7.73 36.39 37.23 −87.85 48.33

05/07/12 20.61 7.80 36.44 37.30 6.79 51.35

12/07/12 24.18 7.95 36.46 37.28 15.75 55.21

17/07/12 19.15 8.02 36.20 37.02 12.45 49.47

25/07/12 20.82 7.98 36.38 37.23 −43.14 51.50

02/08/12 20.49 7.85 36.37 37.22 −57.76 51.13

10/08/12 25.22 8.09 36.35 37.12 21.19 56.16

30/08/12 23.05 8.00 36.28 37.09 −9.57 53.73

746 C.M. Belfiore et al.

Fig. 4 Photomicrographsrepresentative of the sixexperimental ceramic pastes:I (a), II (b), III (c), IV (d), V (e),VI (f). Images were taken atcross-polarized light, mag. 2,5×

etc.), it is necessary to consider the volume percent of thepores of each class Vr that should be related to the volumepercent of coarse pores VR . The multiplication of the factorVr /VR for the theoretical pressure of each class permits thecalculation of the effective pressure value, which is the sumof the effective pressure of each pore class.

4 Results and discussion

4.1 Petrographic analysis

Thin section observations allowed to investigate composi-tional and textural features of the six experimental ceramicpastes, which are briefly described as follows (for terminol-ogy and abbreviations see Whitbread [33]).

Pastes I–II (non-calcareous clay, volcanic temper, fir-ing temperature 800 ◦C) (Fig. 4a–b). As regards the mi-crostructure, both pastes are medium-grained, with temperranging in size between ∼0.2 and 1 mm. The packing of

temper is 5 % in paste I (corresponding to an open-spaceddistribution) and 25 % in paste II (double-spaced distribu-tion). The grain size distribution is unimodal and the sort-ing good in both cases. Voids are scarce in paste I (wherethey are dominantly represented by irregular- and linear-shaped mesovoids and only rare macrovoids) and commonin paste II (where mainly linear and irregular vughs andvesicles belonging to mesovoids class occur along with sub-ordinate macrovoids). A preferred orientation of both lin-ear voids and groundmass, this latter exhibiting a fluidalaspect, was observed only in paste II. The groundmass inplane polars is homogeneously reddish–orange in paste II,whereas a variable color ranging from gray in the centralpart to red in the intermediate and orange–red in the outer-most one, characterizes the paste I. Micromass is opticallyslightly active. Temper consists of dominant subrounded tosubangular basalt fragments (showing a porhyritic texturewith a microcrystalline to hyalopilitic groundmass and phe-nocrysts of dominant plagioclase and subordinate clinopy-roxene, olivine and oxides) and common single crystals of

Laboratory tests addressed to realize customized restoration procedures of underwater archaeological 747

Fig. 5 Water absorption curvesof the six ceramic paste types.The axes x and y represent thesquare root of time and the Qi

value, respectively

the same volcanic minerals, among which plagioclase is themost abundant. Many dark red and variable sized nodules ofamorphous portions (Acf) can be observed in both pastes.

Pastes III–IV (calcareous clay, silico-calcareous temper,firing temperature 800 ◦C) (Fig. 4c–d). Regarding the mi-crostructure, these two pastes are fine- to medium-grained,with temper reaching 500 µm as maximum size. The graindistribution is double-spaced in paste III (packing 5 %) anddouble- to single-spaced in paste IV (packing 25 %). Thesorting is moderate to good in both cases and the grain sizedistribution is bimodal due to presence of bigger grains be-longing to the temper fraction and smaller ones referred tothe silty fraction naturally occurring in the raw clay. Voidsare scarce and consist of irregular and linear vughs and vesi-cles belonging to both mesovoids and macrovoids classes.No preferred orientations were observed. The groundmass inplane polars has a homogeneous reddish–orange color and alow optical activity. Temper consists of dominant monocrys-talline quartz, frequent micas (muscovite and biotite), com-mon nodules of micritic calcite, scarce feldspars and rarepolycrystalline quartz. The silty fraction of groundmass ismainly constituted by quartz and micas. Dark red and vari-able sized nodules of amorphous portions (Acf) were ob-served also in these paste types, along with microfossils’ re-mains (foraminifera).

Pastes V–VI (calcareous clay, silico-calcareous temper,firing temperature 950 ◦C) (Fig. 4e–f). Compositional andtextural characteristics of these two types are almost iden-tical to those of pastes III and IV which correspondinglyhave the same temper packing but lower firing temperature(V similar to III and VI similar to IV). The only differencesobserved regard the groundmass which is optically inactiveand with a brown color in these high-fired samples. In ad-dition, the grain size of temper in paste V is smallest thanin the others, since it reaches a maximum size of about250–300 µm.

4.2 Capillary water absorption test

The kinetics of water absorption by capillarity is a cru-cial parameter to determine the mechanisms of underwaterdegradation of ceramic artifacts, since water is one of thekey factors for the effectiveness of soluble salts as destruc-tive agents. Water absorption capacity is an indirect measureof connected porosity and is frequently used as indicator ofthe durability of a material [39, 40].

Water absorption curves of all six ceramic pastes areshown in Fig. 5, where x and y axes represent the squareroot of time and the Qi value, respectively. It is worth not-ing that pastes I and II, both characterized by non-calcareousclay and volcanic temper and fired at 800 ◦C, absorb thesmallest amounts of water. In particular, paste I with 5 vol.%of temper absorbs water quite more slowly. Conversely,pastes III, IV, V and VI, with calcareous clay and silico-calcareous temper, display a greater water absorption (par-ticularly pastes III and IV which were fired at lower temper-atures) and a similar trend to each other. These data highlightthat type of clay and temper used as well as temper packingand firing temperature (this latter determining the grade ofsintering of the artifacts) are all factors affecting the waterabsorption and determining the capacity of the fluid to bestored and circulate within a ceramic sample, hence favoringits deterioration and reduction of the mechanical strength.Therefore, the capillary transport observed for each pastetype represents an important indicator of the durability ofsamples since the flow mechanism is closely related to thetransportation of ions through the porous material, the evap-oration process and the wetting and drying cycles.

4.3 Porosimetric analysis

Ceramic briquettes also underwent porosimetric analysis be-fore and after immersion in seawater (three and six months)

748 C.M. Belfiore et al.

Fig. 6 Pore size distribution curves [log differential intruded volume (ml/g) vs. pore radius (µm)] obtained by MIP analysis of the six ceramicpastes before and after seawater immersion (3 and 6 months)

in order to investigate modifications of the pore structure af-ter the permanence in the marine environment. The knowl-edge of such data is crucial for conservation procedure, sincethe porosity has long been associated with salt deteriora-tion of porous materials as it affects movement of fluids in-side the material through inflow, capillarity and evaporation[41, 42]. The pore size distribution, in particular, is consid-ered to be a main factor controlling the uptake and transportof fluids within a stone material, so determining the degreeof damage due to salt crystallization attack.

Porosimetric analyses carried out on ceramic briquettesof both oxidizing and reducing open seabed environmentsshowed markedly similar results. For this reason, porosimet-ric data referred to ceramics of the only oxidizing environ-ment are discussed in the following section.

Results obtained by the MIP analysis (Table 3) revealedsome differences in the pore size distribution of the sixceramic paste types before and after immersion (3 and 6

months) (Fig. 6). Such differences resulted to be scarcelynoteworthy in both volcanic tempered pastes (Fig. 6a–b). Inparticular, as regards paste I (V + NC 5 % 800 ◦C), theopen porosity before immersion is about 24 % and the val-ues after 3 and 6 months are very similar (21 and 21.5 %,respectively). In general, the mesoporosity is the pore sizeclass mostly represented (Fig. 6a). In paste II (V + NC 25 %800 ◦C), the value of open porosity in the just manufac-tured briquette is about 23 %, while after 3 and 6 monthsof immersion, the values decrease at about 22 %. Meso-(0.002–0.05 µm) and macro-pores (>0.05 µm), accordingto IUPAC classification [43], are the size classes mostly rep-resented. Even in this case, no noticeable differences in thepore size distribution were observed after the underwaterpermanence (Fig. 6b). On the whole, from a microstruc-tural point of view, the porous network of pastes I and IIdid not undergo any significant modification after contactwith seawater, it is clear that salt crystals only partially fill

Laboratory tests addressed to realize customized restoration procedures of underwater archaeological 749

Table 3 Porosimetric dataobtained by MIP analysis of thesix ceramic paste types, before(TQ) and after three (3M) andsix (6M) months of seawaterimmersion

Porosity(%)

Total intrusionvolume (mL/g)

Median porediameter(volume) µm

Median porediameter(area) µm

Paste I TQ 23.9 0.12 0.04 0.02

3M 21.1 0.11 0.05 0.02

6M 21.5 0.11 0.05 0.02

Paste II TQ 23.1 0.12 0.06 0.01

3M 21.6 0.11 0.06 0.01

6M 21.8 0.11 0.04 0.01

Paste III TQ 33.1 0.19 0.35 0.02

3M 24.2 0.13 0.09 0.01

6M 25.4 0.14 0.02 0.01

Paste IV TQ 29.5 0.16 0.39 0.02

3M 23.7 0.14 0.11 0.01

6M 26.1 0.14 0.03 0.01

Paste V TQ 32.1 0.18 1.32 0.35

3M 27.4 0.15 0.16 0.03

6M 31.2 0.17 0.16 0.01

Paste VI TQ 28.7 0.17 1.44 0.04

3M 29.6 0.16 0.25 0.01

6M 31.9 0.18 0.19 0.01

pores. In particular, if crystals grow inside the pores, thepore entry size is not modified, but the global volume willappear smaller. In both cases, since we assume that therewere no cracks induced by the crystal growth, the pore vol-ume should rise back to its original value after the final dis-solution [44].

Concerning the four silico-calcareous pastes (III, IV, Vand VI), a somewhat different behavior was observed be-tween them and also within the same type before and af-ter immersion. Specifically, regarding paste III (S + C 5 %800 ◦C) (Fig. 6c), several groups of mesopores are evidentin the just manufactured briquette and its open porosity isabout 33 %. After permanence in seawater, it is clear thatthe porous network undergoes modifications, testified by thevariation on the shape of the curve. In particular, after 3months the changes in the pore size range comprised be-tween 0.5 and 1 µm are noticeable, while after 6 months anincrease of the pore size classes between 0.005 and 0.01 µmand a diminution of those between 0.1 and 0.01 µm are ob-served. In addition, the open porosity decreases during tests,reaching values of about 24 % after 3 months and 25.5 %after 6 months. Probably, such a decrease of porosity is dueto the crystallization of salts that occlude some pore classes,in fact peaks of pores have slightly shrunk and shifted tosmaller values thus indicating that a large part of the poreentries has been slightly obstructed by the salt crystals. Fur-thermore, due to this obstruction of the pore entry by saltcrystals, the whole pore volume is seen with a lower diam-

eter by the instrument, hence the peak will shift to lowervalues.

In paste IV (S + C 25 % 800 ◦C) (Fig. 6d), the behav-ior of briquettes before and after 3 and 6 months of im-mersion is very similar to that observed for paste III, sincethe same variations of the porous network and a comparabledecrease of the open porosity were detected. In particular,the MIP value before immersion was 29.5 %, while after 3and 6 months it diminished to about 24 and 26 %, respec-tively.

Regarding paste V (S + C 5 % 950 ◦C) (Fig. 6e), in thejust manufactured briquette the MIP value is 32 %, whichdecreases to 27.4 % after 3 months of immersion and in-creases to 31.2 % after 6 months (probably due to saltcrystals that growing into the porous network create micro-cracks in the sample). The pore size classes (from 5 to 1 µm)observed in the briquette before immersion shift towardssmaller pores (1–0.01 µm) and the distribution changes frombimodal to polymodal. In this case, the pore size distributionis influenced by the firing temperature that is higher than theprevious pastes. As asserted by Cultrone et al. [45], in fact,the formation of small pores, with a diameter below 1 µm,is promoted by carbonates in the raw clay and by a firingtemperature between 800 and 1000 ◦C. Such pore sizes neg-atively influence the quality of ceramics, as their capacity toabsorb and retain water increases.

Finally, modifications in the porous network of paste VI(S + C 25 % 950 ◦C) are very similar to those observedfor paste V. In this case, the open porosity is 28.7 % before

750 C.M. Belfiore et al.

Table 4 Theoretical Crystallization Pressure (MPa) calculated for the six ceramic paste types

Pore radii range (µm) Paste I Paste II Paste III Paste IV Paste V Paste VI

Class 7 10–5 0.0002 0.0031 0.0003 0.0003 0.0001 0.0019

Class 6 5–1 0.0002 0.0002 0.0001 0.0002 0.0001 0.0002

Class 5 1–0.5 0.0042 0.0434 0.0515 0.0651 0.0987 0.1152

Class 4 0.5–0.1 0.0427 0.2827 0.4861 0.6064 0.7088 0.8426

Class 3 0.1–0.05 0.3241 0.6286 1.1344 1.3671 1.2838 1.5405

Class 2 0.05–0.01 6.9640 7.5869 8.9045 10.5737 8.6749 10.4368

Class 1 0.01–0.0037 27.3898 29.9914 31.0231 36.9115 28.5259 34.3380

Theoretical crystallizationpressure (MPa)

34.7253 38.5363 41.6000 49.5242 39.2923 47.2751

immersion and the porosimetric class is mainly unimodal.After 3 and 6 months the porosity value increases to 29.6and 31.9 %, respectively, even in this case probably due tosalt crystals growth in the porous network that creates micro-cracks in the samples.

Summarizing, the analyzed experimental pastes are char-acterized by an open porosity mostly comprising macrop-ores and mesopores. It can be noticed that significant differ-ences in pore size distribution subsist between the two dif-ferent series of experimental ceramic pastes (non-calcareousand calcareous) as well as within each single series as afunction of the tempering amount (5 or 25 vol.%) and/orfiring temperatures. In particular, the non-calcareous pastetempered with 5 % of volcanic sand (paste I) showed alarger amount (around 90 %) of mesopores with a diame-ter between 0.0037 and 0.05 µm if compared with the 25 %tempered paste (paste II), which resulted to be character-ized by the 69 % of pores falling in the same size class.This result can be interpreted considering the specific com-position of the volcanic temper here used, that was obtainedby mechanical grinding of basaltic lava. Thus, the lava frag-ments, which are visibly characterized by their own porositydue to degassing during their specific formation process, ev-idently contribute to the higher abundance of macropores ofpaste II.

On the contrary, relative abundance of sand fragments(temper packing) seems to have only negligible effects onthe pore size distribution, if any, in the case of calcareousclays tempered with silico-calcareous grains (whose poros-ity is certainly lower than that of basaltic ones). Indeed, poresize distribution of pastes III–IV and pastes V–VI are almostidentical, while, some significant differences can be noticedbetween calcareous clay pastes fired at different tempera-tures. These differences are appreciated by comparing poresize distributions of pastes III–V and IV–VI which corre-spondingly have the same temper packing but different firingtemperature. The higher is the firing temperature the loweris the abundance of mesopores with respect to macropores.This effect could be interpreted considering the formation of

Fig. 7 Histograms showing theoretical crystallization pressure (effec-tive pressure of each pore classes) of experimental ceramic pastes

secondary porosity >0.05 µm due to the greater sintering ofgroundmass and the complete decomposition of calcareoustemper.

4.4 Theoretical crystallization pressure

According to the above described procedure, the pressurevalues that can theoretically build up in the experimentalpastes I–VI were calculated (Table 4). They were assignedto the pore size classes until the upper limit of 10 µm underwhich, as well known, capillary activity fully operates. Totalcrystallization pressures are also reported.

In general, values of crystallization pressure appearedto be rather comparable between all the examined exper-imental pastes. This is specially true for pastes charac-terized by common compositional and/or textural features(Fig. 7), that is to say: composition of clayey groundmass(non-calcareous or calcareous), maximum firing tempera-ture (800 or 950 ◦C), temper composition (volcanic or silico-

Laboratory tests addressed to realize customized restoration procedures of underwater archaeological 751

Fig. 8 Diagram showing the relation between open porosity and theo-retical crystallization pressures in the six experimental ceramic pastes

calcareous) and relative abundance of temper (packing 5 or25 vol.%).

Specifically, experimental pastes I and II (both made withnon-calcareous raw clays and fired at 800 ◦C) show values oftheoretical crystallization pressures slightly lower than thoseof experimental pastes III, VI, V and VI (all manufacturedusing calcareous raw clays). This result is in good agree-ment with the relatively lower values of open porosity (seeTable 3) showed by these pastes. As a consequence, it canbe stated that the use of non-calcareous clay and volcanicsand, producing a lower degree of sintering of the ground-mass, could lead to a relatively better resistance to solublesalt decay. Figure 8 shows the relation between open poros-ity and theoretical crystallization pressures in the six exper-imental ceramic pastes. Quite comparable values of openporosity and crystallization pressure are evident for pastesmanufactured using calcareous clays and silico-calcareoussand. From the point of view of theoretical crystallizationpressures, very similar values join together pastes III and Von the one hand and pastes IV and VI on the other hand,two by two grouped by the same abundance of temper anddistinguished only in terms of firing temperature (pastes IIIand IV = 800 ◦C; paste VS and VI = 950 ◦C). The highestcrystallization pressure was found for pastes IV and VI, withvalues of 49.52 and 47.27 MPa, respectively. This differentbehavior seems to be again mainly related to the specificpore size distributions of the ceramic pastes and the devel-opment of secondary open porosity after firing. In terms ofincrease of theoretical crystallization pressures, the behaviorof all calcareous ceramic pastes come across a compromisebetween the influence of firing temperature and the percent-age of added temper (packing).

5 Conclusive remarks

This study demonstrates that the type of raw materials (bothraw clays and temper), as well as the different temper pack-ing and firing temperatures are all key factors affectingporosity, kinetics of capillary water absorption and, conse-quently, the durability of a ceramic artifact. In particular, thedifferent behavior of a ceramic paste to capillary water ab-sorption and salt crystallization processes is related to spe-cific pore size distributions and the development of a sec-ondary open porosity after firing. Specifically, between thetwo different series of ceramic pastes experimentally pro-duced (one with non-calcareous clay and volcanic temperand the other one with calcareous clay and silico-calcareoustemper), the second displays a greater water absorption anda major susceptibility to salt crystallization phenomenonwhich, as is known, represents a main risk for conservationpurposes.

Therefore, the obtained data suggest that a deep knowl-edge of the intrinsic characteristics of a ceramic find recov-ered from underwater excavations is needed in the restora-tion field, in order to optimize the choice of the most suitablerestoration procedures before musealization. The use and ef-fectiveness of specific treatments are, in fact, essential to thefuture conservation of ceramics artifacts.

Acknowledgements This work was financially supported by re-search grants from Italian Ministry of Education, Universities and Re-search, MIUR-PRIN 2009 “Innovative approaches applied to the ex-perimental study of underwater ceramics”.

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