wrote
Are you sure it is asphalt tar? Older transformers I have worked on
were potted in coal tar. Asphalt tar will cold flow and will really
warm flow and make a mess when your transformer gets warm. Coal tar
will not flow until it gets to a rather high temperature. It also
doesn't dissolve in anything very easily. I am not sure if this makes
any difference to you, but you may be able to Goggle a better solvent
for coal tar.

Well, I'm not entirely too sure, but it melts at about 250F-300F, and smells
like the stuff the roofers use to seal cracks with. .

Jon