Frank -
1. Be sure your oven is heated to the correct temperature before you put
your loaves in. It's my theory that the surface is sealed properly that
way (okay, so it's only a theory).
2. Brush loaves with 1/2 tsp. cornstarch and 1/2 c. water (boiled and
cooled slightly) once before putting in the oven and again 10 minutes later.
3. If you have an electric oven, throw a few ice cubes in the bottom of the
oven at the beginning and again when brushing with cornstarch mixture the
second time.
4. Be sure to include some sweetening in your recipe such as sugar,
molasses or malt syrup or powder.
5. If all this fails, your oven is probably too cool. Crank it up a few
degrees above what the recipe calls for.
Good luck!
Sourdough Gloria