A chalk used to sketch up frescos. The red brownish chalk was also used as the preliminary drawing. It is named after the Turkish city Sinop. In this city you could purchase the red-ochre pigment composed of iron-oxides burried in clay called sinople.
Sinopia was widely used by Italian artists in the 15th century. In Florence in particular it was an ingredient in tempura and oil painting. The pigment useful for painting flesh colours.