Küfeki Stone

Küfeki stone is a natural building stone belonging to the limestone group, characterized by a fossiliferous and porous structure. In the literature, it is also referred to as lumasheli limestone, maktrali limestone, and Bakırköy stone. This type of stone, formed in marine environments and containing abundant quantities of oyster shells and other marine mollusk fragments, has a high carbonate content. When first extracted from the quarry, Küfeki stone exhibits a soft and easily workable nature, but through chemical and physical processes triggered by exposure to the atmosphere, it hardens and gains strength over time. Due to this property, it is regarded as one of the durable building stones capable of maintaining its integrity for approximately 2,000 to 2,500 years.

^{Küfeki Stone (}Anadolu Agency⁾

Küfeki stone is a fossiliferous limestone formed by the lithification of Upper Miocene marine sediments. It is widely found in units belonging to the Bakırköy Formation, located between Davutpaşa and Küçükçekmece on the Thracian side of Istanbul. The stone contains abundant quantities of oyster fossils and possesses a porous, calcitic texture. It exhibits a fine-grained, compact rock structure in light beige and white tones.

The primary chemical component of Küfeki stone is calcium carbonate (CaCO₃). Due to its high carbonate content, it exhibits a rapid effervescence reaction upon contact with acids. Its porous structure allows it to be easily worked immediately after quarrying. Upon exposure to atmospheric carbon dioxide, secondary hydration processes occur, leading to gradual hardening and increased strength over time.

One of the most distinctive characteristics of Küfeki stone is its hardening behavior over time, known as weathering. Experimental studies have shown that before weathering, the stone exhibits high water absorption rates and low mechanical strength values. Measurements indicate that the water absorption rate of raw Küfeki stone in boiling water is 10.06%, while its weight-based water absorption is 15.33%. These values demonstrate that the stone is unsuitable for use as a natural building material in its unweathered state.^【1】

Over time, the porosity of the stone decreases, reducing its permeability to water and gases. Consequently, tensile, compressive, and shear strengths increase. Once the weathering process is complete, Küfeki stone acquires a more resistant structure and demonstrates long-term durability against environmental influences.

Long-term observations of the physico-mechanical behavior of Küfeki stone under atmospheric conditions have revealed that water drainage and moisture effects are decisive factors influencing its properties. After a three-year observation period, samples stored in environments allowing water drainage showed increased weight-based water absorption and porosity values, while sonic velocity and uniaxial compressive strength values decreased. These findings indicate that the application site and environmental conditions significantly affect the performance of Küfeki stone.^【2】

Küfeki stone has been widely used throughout the Roman, Byzantine, and Ottoman periods in the Marmara and Thrace regions. It was preferred as a primary building material in walls, aqueducts, mosques, caravanserais, baths, madrasas, and palaces in Istanbul and Thrace. Particularly during the Ottoman period, it is known that Küfeki stone was extensively employed in the works of Mimar Sinan in Thrace and Istanbul. Its workability and durability provided significant architectural advantages in monumental structures such as the Selimiye Mosque.

^{Processing of Küfeki Stone (}Anadolu Agency⁾

Küfeki stone has been quarried from sites around Bakırköy, Zeytinburnu, Sefaköy, Yeşilköy, Şirinevler, Merter, Haznedar Farm, and Küçükçekmece on the Thracian side of Istanbul. Additionally, quarries in Süloğlu district of Edirne and Pınarhisar district of Kırklareli have been significant production areas both historically and in the present day. These quarries were typically operated in locations where layers occur nearly horizontally and outcrop along valley slopes.

Today, Küfeki stone is preferred in the restoration of historical structures due to its compatibility with original materials. Within the framework of restoration ethics, it is essential to use stones with physical and mechanical properties similar to those of the original building material. Therefore, the physico-mechanical properties of Küfeki stone extracted from Süloğlu and Pınarhisar quarries have been determined through experimental studies and evaluated comparatively. The resulting data demonstrate that the use of scientific criteria in stone selection significantly contributes to the success of restoration practices.

^{Restoration Using Küfeki Stone (}Anadolu Agency⁾