Keywords: Atterberg limits Plasticity Ceramic Clay minerals Morocco Pottery This paper investigates the m ain parameters influencing the plastic behavior of clays used for traditional ceramics production . For this, t wenty -six clayey pastes were selected from twelve traditional ceramic plants around the city of Marrakech (Morocco). According to the lithology, six different ty pes of materials are used as raw material in the ceramic industry of this region. Emphasis is placed on the impact of the characteristics of these clayey materials upon the plastic behavior of these clays . The pastes were characterized through their consis tency using the Atterberg limits. It has been concluded that t he gain size, the mineralogical and the clay mineral composition and content, the effect of diagenesis and the presence of talc -pyrophyllite association play the most important role in the contr ol of the plasticity behavior. 1. Introduction Plasticity is one of the most important parameters for manufacturing clayey ceramic products . It allows to ceramic paste to b e easily workable during shaping process (Idrissi et al., 2021a) . Several factors have been suggested in literature to be responsible for the plasticity behavior of soils and sediments . The knowledge of these factors leads to the optimization of the clay deposits use d in local or regional ceramic industries (Idrissi et al., 2014, 2021b; Idrissi, 2017; El Ouahabi et al., 2019) . The plasticity level of an hydrated soil is mainly influenced by the clay mineralogy of the material, its particles distribution , the diagenesis level, the organic matter and the presence of amorphous clay -size materials (Hajjaji et al., 2010; Daoudi et al., 2014, 2015; Idrissi et al., 2014) . This latter influences differently depending on its content and on the curing period in presence of water (Husein Malkawi et al., 1999) . Generally, identifying the respective in fluences of all these parameters on the plasticity behaviour of the sedimentary material is difficult. Research in this area has been traditionally focused on the influence of clay minerals on the plasticity properties of soil materials (Bain, 1971; Schmitz et al., 2004; Hajjaji et al., 2010; Idrissi et al., 2015) . Dumbleton and West (1966) studied the links between clay contents and the Atterberg consistency limits of natural soils around the world, trying to determine the contribution of the clay mineral to the engineering properties of soil. Kaolinite, chlorite and illite a re considered as clays of low to moderate plasticity (Aiban, 2006) Smectite clay rich soils are described in engineering as the most active and responsible for swelling problems , and are among the highest plastic clays (Low, 1980; Sridharan and Choudhury, 2002) . Fibrous clays (palygorskite and sep iolite) also manifest very high capacity for sorption (adsorption and absorption) (Galan, 1996) and then influence highly the plasticity behavior of soils . Their plasticity limit is even higher than that of smectite (Daoudi et al., 2015) . The other clay species manifest low to moderate plasticity behavior . The region of Marrakech, subject of this study is known for its geological variability (Idrissi et al., 2016) which consequently generates clay deposits of variable nature and then of variable plasticity. To adjust the properties of unfired ceramic body, some regional ceramic workshops mix different clayey materials. They generally mix coarser materials like schistose grains with finer soil to obtain the desired workability. 2 Previous studies have discussed the link between the technological choices and other properties (Rye, 1981; Müller et al., 2013, 2014) . In th is study we will demonstrate that the technological choices for actual and archaeological ceramic materials have also a close link with the plasticity of the ceramic pastes . In this study, t he contribution of each parameter on the plasticity behaviour of the ceramic pastes will be pointed. This study aims to present the plasticity behavior of several clay used in Marrakech ceramic industry and to evaluate the main parameters influencing it such as lithology, grain -size, and mineralogy. 2. Studied area From a geomorphological point of view, th e region of Marrakech comprises the north flank of High Atlas, the massifs of Jbilet and Rehamna, and the Haouz and Bahira plains. Briefly, on the geological side, the studied area displays a large sort of facies with ages allocated from Precambrian to Quaternary. In summary, twelve sites of traditional ceramics occur within a spread of 10 to 80 km round the city of Marrakech. Bouchane and Oulad Dlim sites are respectively located near the Paleozoic Rehamna and Jbilet massi fs. Whereas Saada, Agafay, Tamsloht and Mzouda sites are located in the Haouz plain. The six other sites (Tamazouzt, Talatast, Ait Boussaid, Ourika, Amezmiz and Anougal) are located in the north flank of the High Atlas Mountain. A detailed description of t he studied area and the studied materials can be found in Idrissi et al. (20 18). 3. Material The studied clay manifests various nature. Globally, we distinguish six forms of raw materials: Paleozoic schist, Quaternary soil, Mio -Pliocene and Triassic argillite and colluviums, washing sludges resulting from gravel pits and fine rivers sediments. In order to adjust the properties of both, the unfired ceramic body and the corr esponding final product, the regional ceramic plants mix different clayey materials. Thus, t he soil layers above the schist are generally mixed to them to improve the plasticity and to react as a binding agent. However, colluvium contains naturally, a si gnificant quantity of plastic clay. Large choice of clays is present in Mzouda site, the potters try to mix the plastic raw material with the non -plastic one. The argillites show various degree of diagenesis from one site to another and this affect their pla sticity degree (Idrissi et al., 2018) . In total, 26 pastes have been studied, they are used by the potters to manufacture different kind of product s. The Table 1 resumes the details of each sample. The abbreviation used for samples nomenclature is based on the source area of each raw material. For the Saada site particularly, potters use the silty clays provided f rom different sources: Zaraba Canal (Zar), Wadi N’Fis (Nfs) and Wadi Tensift (Ten). Table 1 : Nature and sources of pastes used to manufacture various products . Samples Area Lithology Added materials Color OD Oulad Dlim Schist Soil Brown Bo1 Bouchane Green Bo2 Nfs Saada Silty clays - Gray Ten Zar Ag1 Agafay Soil Ag2 Tms Tamsloht Mz1 Mzouda Schist Sand Yellow Mz2 Argillite Red Mz3 Colluvium Orange Mz4 Argillite Silt Red Mz5 Schist Gray Mz6 - Orange Tmz1 Tamazouzt Washing mud - Gray Tmz2 Ta Talatast Colluvium - Red AB Ait Boussaid Argillite - Brown Our1 Ourika Red Our2 Our3 Schist - Green Our4 Colluvium Silty clays Brown Our5 Argillite Colluvium Red Am Amezmiz Schist Soil Brown An Anougal - Orange 3 4. Methods The used methods for X -ray diffraction, X -ray fluorescence spectroscopy and particles size distribution are described in Idrissi et al., (2018) . The Atterberg limits was carried out on the fraction of the material passing through the 425µ sieve. The required weight is about 150 g of the undersize of this sieve. Liquid limit The measurement of the liquid limit was executed by a Casagrande apparatus based on the method of Atterberg. The operation consists in spreading a liquid paste in a container so as to obtain a layer of about 1 cm thick in the middle. The paste is grooved in the cup so as to divide it into two equal parts sep arated by a V -shaped groove having at bottom of the cup exactly 2 mm wide. The cup is then dropped onto the base by actuating the crank at the rate of 2 strokes per second until the groove is closed over a length of 10 mm. We count the number of blows requ ired to cause the groove in the clay bed (Méthode de Mesure, 1981) . The number of shocks required for closing is noted and about 10 g are sampled in the area where the groove h as closed to measure the water content of the paste . The number of shocks must be between 15 and 35 for the measurement to be valid. The operation is repeated on the same paste after retracing the groove to check if the number of strokes remains the same, otherwise it is necessary to knead the paste again on the cup and repeat the operation. It is necessary to cumulate at least three tests (number of shocks with the corresponding water contents) in order to draw up a curve which makes it possible to deduce the liquidity limit which is the water content corresponding to 25 shocks. Plastic l imit The plastic limit is determined by rolling a small ball of semi -soft paste (which does not stick to the fingers) to form a clay coil 10 -15 cm in length and 3 mm in diameter. After obtaining the clay coil, we put it back into a ball and repeat the coiling clay procedure. The operation is repeated until the paste breaks during winding before reaching 3mm in diameter. At this time, the coil clay is weighed on a small be aker of known mass to calculate the wet mass (Mw). The sample is placed in an oven for 24 hours at 105 °C in order to deduce the dry mass (M d). The water content (%W) is calculated relative to the dry mass with the equation 1: %𝑾= (𝑴𝒘 −𝑴𝒅) ×𝟏𝟎𝟎 𝑴𝒅 Eq. 1 The same operation is repeated on another coil clay. The difference between the two water content values should not be more than 1%. And we take the arithmetic mean of these two values as the plastic limit. Plasticity index The plasticity index was measured through the Atterberg limits using the equation 2 : 𝑷𝑰=𝑳𝑳−𝑷𝑳 Eq. 2 Where: PI is the plasticity index, LL is the liquid limit and PL is the plastic limit. 5. Results and Discussion Plastic behaviour Plasticity is mainly influenced by the mineralogy of the material, but also by the sand, silt and clay fraction. These latter exhibit a wide range of engineering characteristics. To make terms more useful in engineering practice, the Casagrande’s plasticit y chart classifies clayey and silty soils according to their behaviour (Holtz and Kovacs, 1981) . The results for consistency limits (i.e., plastic limit [PL], liquid limit [LL]) of the studied samples, and the plasticity indexes (PI) are shown in Table 2. The studied samples are plotted in various area s on the ‘Liquid Limit - Plasticity Index ’ Chart ( Figure 1), meaning that these pastes tend to have different engineering behavior . In fact, Holtz and Kovacs (1981) consider that soils in which fine grains do control the e ngineering behavior are classified according to their plasticity characteristics. Plasticity, swelling capacity and organic matter content are taking a part in this classification (Figure 1). The A -line generally separates the more claylike materials from those that are silty and also the organics from the inorganics. Almost all studied samples are above the A-line, but they have low to high values of plasticity and swel ling (32% < LL < 55%) as indicated in the top of the diagram . The Plasticity index (PI) indicates the presence of two main groups of pastes. The first group is characterized by PI values ranging between 15 and 20. This group comprises principally the past es composed of schist, washing muds, silty clays and soil. In this group, we can see that the plasticity index and the liquid limit of schist are the lowest (LL ≤ 37%). A low liquid limit means that the content of retained water is low, so the paste is les s plastic and more stable. For the second group, the PI ranges between 21 and 32. This group comprises principally the pastes composed of colluvium and argillite. All pastes made by colluviums are characterized by high PI (25 -27). 4 The result in Figure 2 reveals that almost all investigated pastes are plotted inside the optimal moulding region, except Ten, Nfs and Our5 that are located inside the acceptabl e area, while the other are located outside both the acceptable and optimum moulding regions. Furthermore, most sample could exhibit low shrinkage due to the fact that their plastic limits are relatively low and consequently imply less colloidal and interg ranular water responsible of shrinkage. Table 2: Particle size distribution and consistency limits (%) of the studied samples. Lithology Samples Consistency limits (%) Plasticity indexes (PI) Particle size distribution (%) Liquid limit (LL) Plastic limit (PL) Clay fraction (<2μm) Silt fraction (2–20μm) Sand fraction (>20μm) Schist Bo1 34 19 15 18 26 56 Bo2 35 19 16 20 27 53 OD 36 21 15 25 23 52 Am 37 19 18 19 25 56 An 36 19 17 18 22 60 Our3 35 18 17 23 12 65 Mz1 34 17 16 20 26 54 Mz5 33 15 18 19 39 42 Mz6 32 16 16 30 27 43 Wadi sediment Nfs 48 32 16 34 34 32 Ten 47 30 17 40 53 7 Zar 48 24 24 62 33 5 Soil Ag1 38 21 16 50 35 15 Ag2 39 21 16 51 28 21 Tms 45 25 20 34 43 23 Argillite Mz2 34 17 17 38 17 45 Mz4 37 19 18 40 35 25 AB 40 22 18 27 12 61 Our1 52 22 30 66 25 9 Our2 40 19 21 48 30 22 Our5 55 23 32 41 31 28 Colluvium Mz3 43 18 26 25 34 41 Ta 49 24 25 26 26 48 Our4 46 19 27 36 27 37 Washing sludges Tmz1 39 23 16 60 22 18 Tmz2 39 24 15 64 24 12 Figure 1: Plotted samples on Casagrande Chart (Casagrande , 1948) [after Howard (1977) and Dakshanam urthy and Raman (1973) ]. CL: Clay of Low liquid limit. ML: Silt of Low liquid limit. 5 Figure 2: Studied pastes workability through the Atterberg limits (Bain and Highly, 1978) . Impact of particle size distribution on the plasticity The particle size distribution results of the studied samples are listed in Table 2. They show wide variation, having clay fraction (<2 μm) from 18 to 66%, silt fraction from 12 to 53% and sand fraction from 5 to 65%. In order to understand the influence o f particle size distribution on plasticity, we first projected the studied pastes on the Shepard classification diagram (1954) following the relation between sand, silt, and clay (Figure 3). The samples generally plot in the sand -silt-clay, silty sand, silty clay and clayey sand fields. Schist samples have relatively more sandy composition, whereas washing muds, silty clays and soils are relatively more clayey. Samples composed of argillite are characterized by wide variation in particle size and are plotted in various fields on Shepard's diagram. The Shepard’s classification does not consider the type of sediment and its origin. The Unified Soil Classification System deve loped by Casagrande (1948) considers that materials in which silts and clays do not affect the engineering performance are classified according to their grain size characteristi cs. The studied samples were previously plotted on the McManus Diagram (2018) to show their degree of permeability and porosity. It has been noticed in that previous study that most of the studied samples have a low porosity and permeability which allow them to have good workability due to the good cohesion between particles. And e ven the plastic properties of coarse pastes could be improved by better crushing process. The results obtained in Figure 4 show that there is no obvious relationship bet ween the clay fraction content of the studied sample and their plasticity behavior; samples with approximately the same clay fraction content can display very different plasticity indexes and reversely which is normal because the correlation coefficien t is very low (COR[X,Y] = 0.25) . Furthermore, the Shepard diagram shows that the pastes based on schist and colluviums have relatively the same grain size composition; they are characterized by coarser materials, however, while the former displays the lowest p lasticity, the second have medium to high plasticity. the mineralogical data are crucial to explain these behaviors. Impact of mineralogical composition on the plasticity Whole rock composition The mineral composition of the bulk -samples consists mainly of clay minerals, quartz, feldspar, hematite and carbonate (calcite and dolomite). Quartz and feldspar are ubiquitous with a proportion varying between 20 to 55 wt. % and 5 to 35 wt. % respectively. Hematite is present in minor amounts in most of th e samples (0 -10 wt. %). The total carbonate (calcite + dolomite) ranges between 0 and 15 wt. %. The clay minerals content ranges between 25 and 60 wt. %. During the shaping phase, clay minerals act as plastic and binder components , while other silicates an d carbonates as skeletal components. Clays composition and plasticity The mineralogical data of whole -rock are not sufficient alone to explain the plasticity behavior and the clay minerals composition is necessary. In fact, this latter is fundamental to th e understanding of engineering behavior of fine -grained soils. The clay minerals of the studied paste in the fraction < 2 µm are diversified. They are made by a mixture of illite (10-40%) , kaolinite (5-15%) , smectite, mixed layers (<10%) , vermiculite and chlorite (<5%), p yrophyllite -talc association ( 5-10%) is present in the samples Our1, Our2, Our4, Our5 and Tms. Fibrous clay is only present in one paste (Ta), with a content of 18%. 6 Clay fraction -total clay duality The fraction < 2 µm could include clay minerals and very fine non -clay minerals. Total clay designates all clay minerals even if their size exceeds 2 µm. The Figure 5 depicts the clays mineralogy and the clay fraction rate of the whole of studied pastes, with an indication of their lithological nature and of their CF/TC ratio. This figure includes the mineralogical parameter of clays in order t o explain the decorrelation highlighted previously between the grain -size and the plasticity. In fact, the pastes with plasticity indexes ≥ to 24 are composed of significant content of smectite, fibrous clay and “smectite + talc - pyrophyllite” association. The content of smectite could not have great impact if the clay fraction is very low (<20 %). More discussion about the difference between the total clay and the clay fraction is to find in Idrissi et al. (2018) . Figure 3: Classification of studied clay raw materials based on the sand -silt-clay ratios (Shepard, 1954) . Figure 4: The plasticity indices of the samples studied as a function of the percentage of the fraction <2 microns (COR[X, Y]=0.25). 7 Figure 5: Total clay mineralogy (Obtained by XRD) and clay fraction rate (Obtained by Laser Diffraction Analyzer ) of studied pastes with an indication of their lithological nature and their clay fraction / total clay ratio (CF/TC). 8 Daoudi et al. (2010) show that the burial diagenesis is the main factor controlling the genesis of the clay assemblages of the sediment ary series of the Lower Mesozoic of the Western High Atlas. The effects of burial on these clay assemblages are expressed as the downward increase in the proportions and size of the illite crystallites. In addition, depending on their granulometry, mineral ogical and chemical composition, argillites represent the most heterogeneous materials studied. They belong to different sedimentary series which have undergone varying degrees of diagenesis and this has affected their degree of plasticity. The plasticity indices of the argillites are distributed between two extreme poles: clay of low plasticity and clay of high plasticity ( Figure 5) and their “clay fract ion/total clay (CF/TC)” ratios are between 0.7 and 1, 7. This suggests that the effect of diagenesis seems to have played a major role in the behavior of the plasticity of these materials. In addition, the observations in scanning electron microscopy show that the Bouchane paste based on shale has illite particles with large sizes (> 20 µm; Figure 6). This explains the low clay fraction / total clay ratio in the case of shale -based pulps ( Figure 5). Figure 6: SEM micrographs of schisto se fabrics from Bouchane showing the large particles of illite (I) with a grain -size > to 5µm. The difference in the plasticity behavior between pastes based on schist (PI ≤ 18) and on colluvium (25 ≤ PI ≤ 27) , despite the similarity expressed by their coar ser grain -size distribution, can be explained by their clay composition. Indeed, in most schist samples, the clay composition is dominated by the illite and the kaolinite which are considered as clays of low plastic ity. Whereas the colluviums are character ized by a high content of some specific clays: fibrous clays [Ta], pyrophyllite -talc association [Our4] , smectite and smectitic mixed layers [Ta, Our4 and Mz3] . The most influential minerals on plasticity It is well known that the smectite s and the fibrous clay s display the highest plasticity behavior of clays; this explains the high plasticity indexes of samples Mz3 (PI = 26) and Ta (PI = 25) . For the third paste made by colluvium (Our4 ) as well as in other pastes made by argillite (Our1 and Our5), and soil (Tms), the clay composition is characterized by low amount of smectite , but coupled with a variable amount of the talc- pyrophyllite association which result from the dismantling of metamorphic rocks of Paleozoic and Triassic (Daoudi and Pot De Vin, 2002) . The t alc has good ceramic properties. It is an ideal complement to ball clay (clay of high plasticity) to create a dryable body that still has excellent w orking properties with a good feeling in the hands of the artisan . Talc and Pyrophyllite are close to the smectite in structure and chemical composition, but behave very differently from smectite (Giese et al., 1991) . The former are hydrophobic (Steenberg and Harris, 1985) while the latter is hydrophilic. Thus, talc and pyrophyllite are not able to intercalate water and swell due to the relatively weak interaction between the surfaces 001 of these two minerals and water molecules (Giese et al., 1991) . Consequently, talc powder has a low plasticity (Norsker, 1990) . But these hydrophobic minerals are known to possess a weak van der Waals bond giving them the property of dispersing easily in water and keep them from clumping (Mukherjee, 2013) . This author explains that with such dispersing minerals (anti caking), other clays are being dispersed and then easily dissolved in water without clumping. Thus, a greater surface area of the clays will be made available to re act with water and to increase the amount of water absorbed by smectite. This explains why the plasticity indexes are the highest in samples with the couple of talc-pyrophyllite and smectite. The highest plasticity indexes are identified in the pastes based on argillites from Ourika site ( pastes Our1 and Our5). This high plasticity allows to artisans of Ourika site to produce the most complicated decoration forms at the region of Marrakech (Fig. 2f). These sample s are chara cterized by: a high rate of clay fraction (66 and 41 % respectively), a high total clay content (41 %) and the presence of the pyrophyllite -talc and smectite association . The plasticity indexes of soils are not very different from those of schist except for the paste of Tamsloht which contains the pyrophyllite -talc association and reaches a PI of 20. The PI of washing muds is still low despite the high rate of clay fraction. This is due to the low content in total clay (<28%). 6. Conclusion This study shows as expected, in most of the studied samples, there is a clear relationship between the plasticity behaviour in one hand and the grain size distribution, clay content and clay mineral composition in the other hand. Fine grain -sized materials as well as those containing smectite and fibrous clays display the most plastic 9 behaviour, whereas the pastes with coarser grain -size associated with the presence of illite display the lowest plasticity. Adding that despite its low plasticity, the talc - pyrophyllite association can display the highest plasticity indexes when coupled with smectite in the same materials. However, in some categories of samples (argillites), the plasticity can be influenced by the effect of diagenesis . 7. Acknowledgments The authors would like to thank the ArGEnCo Laboratory (University of liege) for its support in carrying out the Atterberg limit tests, and especially professor Frédéric Collin . 8.