One heaping cup of sifted flour.
Put the potatoes into a large bowl or tray and mash them to powder with a potato beetle, or a wooden spoon. While still hot, mix in the sugar and butter, beating all to a lumpless cream.
Add a few spoonfuls at a time, the potato-water alternately with the flour by the handful, beating the batter smooth as you go on until all of the liquid and flour has gone in. Beat hard one minute before pouring in the yeast. In hot weather, it is well to stir into the yeast a bit of soda no larger than a grain of corn already wet up in a teaspoonful of boiling water.
Now whip up the batter with a wooden spoon for another minute, and the sponge is made.
Throw a cloth over the bowl and set by for five or six hours to rise. If you intend to bake in the forenoon, make the sponge at bedtime. If in the afternoon, early in the morning.
When the sponge is light sift a quart and a cup of flour into a bowl or tray with two teaspoonfuls of salt. Into a hollow, like a crater in the middle of the flour, empty your sponge-bowl, and work the flour down into it. Wash out the bowl with a little lukewarm water and add this to the dough. If it should prove too soft, work in, cautiously, a little more flour. If too stiff, warm water, a spoonful at a time until you can handle the paste easily. The danger is in getting it too stiff.
Now, knead and set for risings first and second, as you have already been instructed. This sponge will be found especially useful in making
Graham Bread.
One quart of Graham flour, one cup of white flour.