From October through June the best method of raising cream is to set the pans for 12 hours in the milk "closet" and then for 5 more hours on a stove or furnace having embers in it.  You want the milk to get hot, but not scalded. Return it to a cool place.  When it becomes cold, take off the cream and drain well until clear of milk.

This method produces much more cream and at least a quarter more butter from the same quantity of milk.  It is also very quick - ten minutes churning is sufficient.  

Care is needed to make sure the milk doesn't get too hot - if it gets close to scalding, the cream will have little tiny flakes in it.

FYI - The name Devonshire is from Devonshire, England where it is called "clotting butter"!