Dave Krall wrote in 141010 MMD:

> I have no idea what this material is, but is there something that
> will dissolve it without damaging the windings of the transformer?

If the transformer potting compound seems to be refined bitumen
("petroleum pitch"), try heating it in an oven to 110-130 F. (43-54 C.).

Then take it outdoors and soak it in high flash point mineral spirits
(white spirit), also called "type 3 hydrotreated heavy naphtha".  The
flash point of grade "T3" is specified as greater than 55 deg. C.  Wear
eye protection, too!

> ...  does the transformer have to be potted again inside the box
> or could it be mounted inside with silicone rubber?

The potting compound, whether it's bitumen or epoxy, serves to (1)
transfer heat to the case, and (2) reduce audible hum due to loose
windings and magnetostriction of the laminated core.  Neutral grade
silicone rubber doesn't seem likely to transfer heat as well as the
bitumen would, although it would help to keep out moisture.

However, if the transformer runs cool under load and the hum level
is tolerable, then you can omit the potting.

Robbie Rhodes
Etiwanda, Calif.

Ref.: http://en.wikipedia.org/wiki/White_spirit 
      http://www.vintage-radio.net/forum/showthread.php?t=37381