Dissolve the 2 packages of dry yeast in the 1 cup lukewarm water and set aside.
In a very large mixing bowl, place the 1 1/2 cups of brown sugar, 1 cup Karo syrup, 4 heaping T shortening and 3 T of salt. Mix well.
Add 5 cups of lukewarm water and the yeast and water mixture. Mix well. It will be messy at this point until you begin to add the flour.
Beat in the 6 cups of rye flour and mix until smooth. Scraping sides of bowl often. Add enough white flour one cup at a time until the mixture is easy to handle. Never mix until the dough is too hard. Rye bread is sticky. When it can be handled, turn out onto a well-floured board. Use lots of flour as it is very sticky. Knead well while adding enough white flour until mixture can be lifted and placed into a large well-greased bowl. Brush with melted shortening. Cover with warm dampened cloth. (I cover it with plastic wrap sprayed with cooking spray and it works well.) Let stand over night.
In the morning shape dough into well greased tins. Let rise 1 1/2 to 2 hours. Prick all around with a fork and pass your hand over the tops - dip hand in cold water first (honestly I'm afraid to not do this because who knows what might happen!!!!). Put in pre-heated oven immediately.
Bake 1 hour and 10 minutes very slowly in a 300º oven. Remove from tins and cool on racks, slightly great tops while they are still warm.
The lukewarm water does need to be lukewarm. If water is too hot the yeast will not work. I can only tell you that this recipe takes practice, so do not be surprised if the first try does not turn out like Mom's!
Mom always used round tins. Loaf pans do work also, honestly!!!!!