## 3 Material and Methods
### Analytical Techniques  
The mineralogical analyses were carried out at the laboratory AGEs of the University of Liege, Belgium, according to the methodology of [PERSON] and [PERSON] [32]. The mineralogy was determined by X-ray powder diffraction on bulk samples and on oriented clay samples at room temperature using a classical powder diffractometer Bruker Advance 8 brand type, equipped with Cu radiation (K\(\alpha\)Cu \(\frac{1}{4}\) 1.54056 A) at an intensity of 40 mA and a voltage of 40 kV. It was configured with a step size of 0.013\({}^{\circ}\) (20) for a measurement time of 30 s and data were collected over the interval of 2\({}^{\circ}\)-70\({}^{\circ}\) (20). A mineral phase identification was performed using X'Pert HighScore Plus software associated with PDF-2 2007 release software. Prior to analysis, the analyzed samples were ground and sieved to 80 \(\upmu\)m. The <2 \(\upmu\)m fraction was taken from the suspension after a settling time was calculated according to Stocke's law and it was then placed on a glass slide, meaning the XRD patterns were thus recorded between 2\({}^{\circ}\) and 30\({}^{\circ}\)2\(\upmu\) using the same step size and time per step parameters. These oriented aggregates were subjected to three successive treatments: air drying, glycolation and heating at 500 \({}^{\circ}\)C for 4 h, in order to confirm the type of clay mineral phases. Qualitative and semi-quantitative estimates (\(\pm\)5-10%) were referred to as the measurements of the maximum intensity of X-ray models according to [PERSON] [33]. The infrared spectra were carried out at the Dschang University laboratory in Cameroon. Diffuse reflectance infrared spectra were recorded between 4000 cm\({}^{-1}\) and 500 cm\({}^{-1}\) using a Thermo Scientific Nicolet iD7 FTIR spectrophotometer. The spectrum resolution was 4 cm\({}^{-1}\) and the spectra were obtained by the accumulation of 200 scans. The geochemical characterization consisted of determining the major element concentrations using the X-ray fluorescence spectrometer (XRF). This analysis was performed on the raw materials using a Niton XL3t980 hXRF analyzer (X-ray tube: 50 kV; anode: silver; silicon detector: 8 mm). The analysis provides raw data in the form of a spectrum with the specific fluorescence energy (in keV) on the x-axis and the photon number (in CPS) on the y-axis. The instrument was calibrated and configured in mining/mineral mode. The materials studied were crushed and sieved to 80 \(\upmu\)m and were then analyzed to obtain the base oxides contained in the materials. The predominance of the oxides was determined by using the triangular diagram SiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\)-Fe\({}_{2}\)O\({}_{3}\). The evaluation of the level of chemical alterations of the analyzed clays was carried out by the ternary diagram of the evaluation of the chemical alteration index [34]. Weakly altered materials are those with values between  
Figure 3: Photos of the different facies analyzed in the eastern part of the Douala sub-basin: (**a**) dark gray clay (Y1C1, NiC1); (**b**) light gray clay (J1C1, NiC2); (**c**) purple clay (J1C2); (**d**) multicolored clay (Y1C3, J1C3, NiC3); and (**e**) yellow clay (Y1C2, NiC4). 50 and 60%, moderately altered materials are those with values between 60 and 80% and highly altered materials are those with values > 80%. The certainty of the clayey materials in ceramics has been demonstrated using the Fiori and Fabbri diagram [35]. The Chemical Index of Alteration (CIA) was calculated as follows:  
\[\text{CIA}=(\text{Al}_{2}\text{O}_{3}/(\text{Al}_{2}\text{O}_{3}+\text{CaO}+ \text{Na}_{2}\text{O}+\text{K}_{2}\text{O}))\times 100 \tag{1}\]  
The physicochemical properties were determined using various methods at Mission de Promotion des Materiaux Locaux (MIPROMALO) in Yaounde, Cameroon. The particle size distribution was determined by a wet sieving method for fractions greater than 80 \(\upmu\)m. Particles that were less than 80 \(\upmu\)m were analyzed using sedimentation according to the French standard NF P18-056 procedure.