<<<Can anyone suggest a method to keep sourdough starter warm (i.e., 70 -
85 degrees) in a house that remains in the low 60's for the winter?>>>
This really isn't as important as some would have you believe. Sourdough
critters are really pretty resilient, and even a 60 degree kitchen should
be fine for building up a starter, especially one that already has a
culture built in. You'll just have to keep a closer eye on it for the first
few feedings to find out what its rising time is (count on it taking 1.5-2
times as long as the instructions indicate, but even this is by no means
definite), after which you can count on it behaving the same in perpetuity.
Another popular solution is to put it under an overturned styrofoam cooler
with a 15-watt lightbulb running in the top, but I'd only advise this if
you have some reason for wanting to speed up the process. Many sourdough
enthusiasts actually seek out retarded rises, for the improved flavor they
impart.
Omar