What type of sealer is this? Where I studied, in the printroom, there was a large bottle with the label 'sealer' on it (it seems that they transferred the contents of most products they used from the original containers to their own and labelled them accordingly, for safety reasons).


We used it mainly for collagraphs (and sometimes for sealing problematic relief blocks).
It was a farely thick fluid of a light brown colour.
It didn't take long to dry, as far as I remember.
It made the surfaces waterproof.
Once dry it couldn't be remover with turpentine/white spirit, it needed some stronger solvent.
It didn't have toxic vapours, if it had any scent at all it was 
quite faint.


Does anyone know what it was?
Sounds like it could be shellac, possibly suspended in mineral spirits. It might also be damar varnish - but really that is speculation. Why don’t you track down someone from the place and ask them? 

They probably had very large containers that were hard to work with, so they would put them in more “handy” jars,
.