<snip> When I took each radiator off, I empied what was left inside of
it out into
a bucket.  The most amazing black slime came out... and it stank like
heating oil, or some other nasty petroleum product.

Now that got me to thinking... what is in a radiator anyway??  In a
Vanagon
you could put just plain old tap water, but you would get tons of rust
and
corrosion fast, not to mention mineral deposits.
<snip>

I have a 17 room Victorian style house built in 1903 that still uses
cast iron radiators.

Water, in a closed system like these radiators have, looses it's oxygen
and turns black.  That's good.  The blacker the better.  Without oxygen
there's no rust.

My system is hooked up to city water and, through a check valve and
regulator, water is added as necessary to keep the system full.  In that
the system is in use during the winter .... there is no antifreeze.
There has never been a water softener attached.

Just flush everything out .... refill with water .... bleed .... check
for leaks (sounds rather Vanagon like, doesn't it?) .... put it in
service and wait for the water to turn black again.

Mike