In response to Mr. Heyworth, who reported in Digest 970706 of finding
electrical leakage in the insulating fill on the back of ceramic-based
sockets in his piano:

This hard, white stuff may well be "Sauereisen Cement", a sort of air-
drying ceramic patching material, made here in Pittsburgh.  Made of
powdered refractory and potassium silicate, a water-setting cement
material, it is (according to the catalog) slightly hygroscopic.

It might check okay with the megger in the dry winter; in any case the
leakage would be small, as you say -- not of the magnitude that would
have worried people in early days.  It was once used to fill electric
irons, bases of light bulbs, and other electrical gear with ceramic
foundations.

The black coating used on electrical fittings up until the 50's instead
of galvanizing, is, by code, a compound (trade named "Minerallac" by one
vendor) made from coal-tar bottoms, which is high in carbon.  It was not
meant to be non-conductive, and must not be by your finding, but rather
it was used to protect the steel or iron from corrosion.

An interesting aside: some player piano tubing, although made from
polymer, is so high in carbon content that it is conductive to a degree.
I tried it once as an insulating sleeve, but the circuit didn't work till
I found the bug, which naturally was the last thing I tried.  Rubber
tubing was sometimes used as a clip pad or bend relief for flexible
wiring in a piano, but it can't be relied upon as an insulator in such
an application.

Richard Vance

 [ Thanks for your research, Richard.  I enjoy learning about the old
 [ chemistry along with the old technology !  By the way, how should
 [ one translate "Sauereisen"?  "Sour Rising"? ... Sounds like a 
 [ good name for bread dough!  -- Robbie