Terracotta, a type of earthenware, is a fired clay material used for millennia, from ancient storage vessels to modern garden planters. Terracotta does not hold water like non-porous materials such as glass or plastic. Instead, its fundamental structure causes it to absorb and subsequently release moisture into the surrounding environment. Understanding this property is crucial for its various applications.
The Composition and Porosity of Terracotta
The unique water dynamics of terracotta are an outcome of its raw material and the specific firing process it undergoes. Terracotta is made from iron-rich clay, which gives it the characteristic reddish-orange hue after firing. Unlike stoneware or porcelain, terracotta is fired at relatively low temperatures (typically 1700°F to 2100°F). This low heat is not sufficient to trigger full vitrification, the process where the silica components within the clay melt and fuse into a dense, non-porous mass. Since the clay does not fully vitrify, organic matter burns out, leaving a network of microscopic, interconnected channels and air pockets. This structure results in high open porosity, making the material highly permeable to water.
How Water Moves Through Unsealed Terracotta
The porous structure of unsealed terracotta governs a constant, two-part interaction with water: absorption and release. When water is introduced to the interior, the material draws the liquid into the tiny, interconnected channels through a process known as capillary action. This force allows the water to wick up and laterally through the clay body, effectively saturating the entire wall of the vessel.
Water molecules migrate from the interior contents to the exterior surface, where the liquid changes phase into a gas, resulting in evaporation. This phase change requires energy, which is drawn from the surrounding environment and the remaining water inside the vessel. This energy transfer is called evaporative cooling, which keeps water stored in traditional terracotta pots noticeably cooler than the ambient temperature.
In gardening, unsealed terracotta planters constantly wick moisture from the soil. This can lead to the soil drying out more quickly than in plastic containers, necessitating more frequent watering. The constant movement of moisture and air through the material is beneficial for specific plants, such as succulents and cacti, which thrive in well-aerated soil that dries rapidly.
Changing Water Retention Through Sealing and Glazing
To modify terracotta’s naturally high porosity and increase its ability to retain water, various sealing methods are employed. For temporary waterproofing, unsealed terracotta is sometimes soaked in water before use, which saturates the pores and slows the initial rate of water absorption from the contents.
Glazing
Glazing is the most permanent and effective method, involving applying a silica-based coating to the clay surface. The piece is fired a second time to melt the glaze, forming a smooth, impervious layer of glass that completely seals the pores. A fully glazed terracotta item becomes waterproof and non-porous, preventing both water absorption and the evaporative cooling effect.
Chemical Sealants
For items needing protection from staining or freeze-thaw damage while retaining some porosity, chemical sealants are applied after firing. These penetrating sealers, such as silicones or synthetic acrylics, block surface pores to reduce water absorption without forming a thick, visible layer. Commercial sealants significantly lower the material’s effective porosity, which is important for outdoor pieces in cold climates to prevent the destructive expansion of frozen water inside the pores.