# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 1 Introduction  
Residual clays, also known as primary clays, are formed in situ through the prolonged chemical weathering of silicate-rich rocks, a process involving leaching of soluble cations such as Ca\({}^{2+}\), Na\({}^{+}\), and K\({}^{+}\), leading to the concentration of more stable minerals such as kaolinite, illite, and smectite in the soil profile [1]. The mineralogical composition and physical properties are heavily influenced by the parent rock type and the prevailing climatic conditions, leading to significant variability even within localized areas. Tropical and subtropical regions, where intense weathering conditions prevail, are where these clays are more common. These clays have been extensively used in the production of red ceramics, including bricks, tiles, and structural components. Their suitability forcertain applications is largely determined by mineralogical composition, particle size distribution, and plasticity. For instance, kaolinite-rich clays are favored for their low shrinkage and good firing properties, while smectite-rich clays offer high plasticity but may pose challenges during drying and firing due to their expansive nature [2]. Several studies have highlighted the potential of residual clays in ceramic production. In Brazil, a study on clayey soils highlighted their suitability for red ceramic fabrication, with certain samples meeting the strength requirements for construction materials [3]. In China, red clays were characterized for use in ceramic manufacturing, emphasizing the importance of mineralogical and chemical analyses in determining their applicability [2]. The use of locally sourced residual clays in ceramic production offers significant economic and environmental benefits. Economically, it reduces dependence on imported raw materials, lowers transportation costs, and supports local industries and employment. Environmentally, it promotes sustainable georesources use by minimizing the carbon footprint associated with material transport and processing [4]. The incorporation of residual clays into ceramic products aligns with circular economy principles, encouraging the efficient use of natural resources and the reduction in waste. The comprehensive characterization of residual clays is crucial for optimizing their use in ceramic applications. Understanding their mineralogical, chemical, and physical properties enables manufacturers to predict their behavior during processing and firing, ensuring product quality and performance. Such studies also facilitate the identification of suitable clay sources, inform blending strategies to achieve desired properties, and contribute to the development of standards and guidelines for raw material selection [5]. Portugal has a rich tradition in ceramic production, with numerous regions exploiting local clay deposits for different applications. Studies in areas like Bustos [6] and Vila Nova da Rainha [7] have characterized local clays, assessing their suitability for ceramics, and have highlighted the importance of mineralogical and chemical analyses in determining their applicability. These investigations underscore the potential of Portuguese residual clays in supporting sustainable ceramic industries and preserving cultural heritage. The study area, Sao Pedro do Corval, in southern Portugal, is renowned for its pottery and ceramic craftsmanship, being one of the biggest centers of traditional pottery in the Iberian Peninsula, with a long-standing artisanal heritage that dates back centuries. The area is characterized by diverse geological formations, including metasodimentary rocks, which give rise to various residual clay deposits. These clays have been the raw material for handmade pottery production across generations in the region. The proximity of the clay sources to the village not only enabled the sustainable development of this craft but also shaped the socio-economic identity of the community.