Often used for sweet breads (like Panettone), here's a technique I had seen Beesham Soogrim try. And more recently, baking writer Sam Fromartz brought it up. You place a firm sourdough in a container of water. Once risen, it is ready to use. It's known as a "lievito madre" in Italy, where sometimes they wrap and tie it in cloth. Regardless, the results were good.