Titebond II I believe. It's been put in other, smaller bottles though so I am not 100% certain
Interesting...where does the black colour come from?
Fantastic work to watch and its wonderful that you have documented it so well for the Forum, Thank you for that effort. It may spark a spate of Scratch build necks. (I'm looking at you @Sonic Mountain. You too @Andy40)
FrankenLab Hand crafting guitars, because Death Rays are expensive.