ink

Evidence

"The river Euphrates was reported to be a mosaic of red and black from the blood of Muslims killed and from the ink of countless books thrown into the river."

Remarks

The text provides a historical account of the Mongol invasion of Baghdad in 1258 CE (13th century) and the destruction of its libraries. It is noted that the river Euphrates turned black due to the immense quantity of ink from manuscripts thrown into the water (p. 111). This implies the large-scale production of manuscripts using black ink (likely carbon or iron-gall) in the Abbasid period.