Investigation on the Role of Porosity in the Structure of Historical Ceramics

Structural characterization of historical materials, especially clay-based compositions, as well as technical and archaeological aspects, has a significant role in the diagnosis of the deterioration process. One of the most important parameters in the structure of clay-base ceramics (such as pottery, brick, and tile ware) is porosity that includes all open and closed pores in the matrix of porous material and it is formed through primary (during the preparation of raw materials) or secondary processes (during firing process and/or burial condition).  The main function of open porosity is its role as moisture or any electrolyte transfer channel that can launch or continue many chemical reactions. This function has consequences such as ions transfer, phases dissolution, hydration, and crystallization of soluble salts. Therefore, take note of the percentage of the object’s porosity, especially in the excavation process, is highly essential. In this paper, according to the analytical point of view, through porosimetric methods such as mercury intrusion porosimetry (MIP) and immersing method (based on ASTM C20-92), the most important effects of the presence of porosity in the ceramic matrix were investigated. These parameters are a) relation between firing temperature, percentage of porosity, volume, and radius of pores; b) relation between porosity and moisture transfer through the matrix; and c) relation between porosity and salt crystallization pressure. Without pay attention to the object’s condition like types and amounts of pores, the possibility of increased permeability and diffusion of humidity, moisture shock immediately after excavation can occur as a result of cleaning or desalination process as well as salt crystallization pressure, capillary condensation, and further chemical reactions and physical erosions.