OK, a point I need to make - sourdough is NOT yeast-free. I know you also made this clear, SRL, but I felt the need to emphasise. We have an alternative lifestyle magazine here in Australia called "Grass Roots".s Not sure if it's still in print... it's very hippie. I remember a lot of misconceptions tis magazine was trying to fix for people, years ago, on sourdough vs conventional yeast bread. There were people who were sending in their own recipes for naturally leavened bread as a yeast-free alternative for people with yeast intolerances.
The "naturally leavened" bread - you mixed up the dough, including sucrose in some form (they tended to use honey) and put it out, uncovered, on the windowsill for a few days. And almost by magic, it would acquire leavening, all by itself!
The magazine editors pointed out that this "natural leavening" was actually wild yeast just blowing in and landing in the dough. So much produce that requires some natural bio-active agent got its start through natural exposure to these agents in the environment. We make wine by pressing grapes, the natural yeasts are on the grape skins already.
Not all natural yeasts are good and will give a good result. So this method won't always work well. And you have to always remember - it's still yeast.
Good luck with this Friendship loaf. I don't bake as much as I used to, but we still bake a fair bit of bread. We used to average about a loaf a day (one kilogram loaf). I also make bread rolls, sweet bread, savoury flavoured bread - all sorts. You can have a lot of fun playing with bread dough.
Marg
I'm going to bake once, save a few cups in the freezer, and pitch the rest. 
