Horasan mortar

Horasan mortar (horasan harcı) is a traditional mortar made by mixing slaked lime with crushed or ground fired clay, usually brick and roof tile. The fired clay reacts with the lime and gives the mortar a pozzolanic character, so it can set not only by contact with air but also in damp conditions, and it resists water better. The brick dust gives it a distinctive pinkish-red colour. It was used over a long span, from Roman and Byzantine buildings through the Seljuk and Ottoman periods.

Where it was used

Horasan mortar was chosen above all where water and moisture were constant companions:

• in water structures such as hammams, cisterns, aqueducts and fountains,
• as bedding mortar in masonry walls and thick joints,
• in floor and roof substrates, renders and screeds.

In some monumental buildings the thick horasan joints between brick courses are also interpreted as a flexible component that helps the wall dissipate energy during earthquakes.

Horasan mortar in restoration

Before repair, samples of the original mortar are analysed in the laboratory to identify binder, aggregate and additives. The new mortar is formulated close to the original and never harder than it. The guiding principle is that the new mortar should be the weakest link in the wall: stress and moisture should pass through the mortar first, sparing the stone and brick. Cement mortar reverses that balance. Being harder and less permeable, it pushes moisture and salts into the brick, moves differently from the original materials and is hard to remove later. The pink tone of horasan is also useful evidence for capturing a building’s original surface colour in 3D restitution models and visualization.