Stir the yeast into 1/2 c warm water & let proof for 10 minutes. Stir in the rest of the water & the biga. Mix until the starter is broken up & well mixed. Add the salt & the flour, 1 c at a time, mixing until the dough comes together. Add more water if necessary to prevent the dough coming away from the bottom of the bowl. Knead on a very lightly floured board until the dough comes together. Set in an oiled bowl, cover with plastic wrap & let rise until doubled, about 1 1/4 to 1 1/2 hours. Divide the dough into 10 equal pieces. Shape each one into a cylinder 8" long & as fat as a thumb by roling it on a lightly floured work surface with the palms of your hands. The dough will be springy & elastic. As you roll each one, cover the others. Set the rolls onto an oiled baking sheet & connect the ends, forming rings, about 4" in diameter. Pat the tops so they are level, cover with a towel & let rise until they are puffy & doubled, about 1 hour. Preheat oven to 400F. Bring a large pot of boiling water to a boil. Put the friselle, two or three at a time, into the boiling water. Turn them over to submerge both sides, but leave them for only 1 or 2 minutes in all. Remove carefully & drain on paper towels. Set the friselle on oiled baking sheets & bake for 20 minutes until golden & toasty. Remove from the oven, let them cool slightly & then cut in half horizontally. Set them cut side up on the baking sheets. Reduce the heat to 350F & return the friselle to the oven for 15 minutes, until crisp & crunchy. Use for bruschetta.