Marble millstone, around 1 metre large in diameter, that comes from Hagia Sofia's Alms House is on public display near ticket desks in front of Hagia Sofia museum, Istanbul.
Small information tag dates stone back to Ottoman Period (1743).
It is not evident how the millstone was used (arguably by horse power) and what for (most likely to prepare flour).
Hagia Sophia Almshouse was a charity built by Sultan Mahmud I (1730-1754) in 1743 at the northeast of Hagia Sophia for distributing food to poor and orphan people.
There are repair epigraphs over the doors of almshouse regarding to repairs carried out in different dates.
The big ceremony door of the almshouse at the Bab-i Humayun direction is one of the best examples of baroque style in Istanbul.
There is an epigraph dated 1155 on this door written by Calligrapher Besir.
sources: ayasofyamuzesi.gov.tr, own visit and pictures