Wood. Wood can last a long time in wet burials. The ink may have been soot-based.
That’s fascinating. Believe it or not for the same structural strength a wood beam outperforms a steel beam in a fire. I’ve been in burned out buildings where the structural integrity of wood beams, particularly the older cuts, pre-1920s, is incredible given the amount of burn.
Steel will twist and deform. Wood is an incredible creation.
Perhaps it was iron gall ink?