Irish moss, contrary to its name, comes from red algae, Chondrus crispus. Harvesters gather red algae off the coasts of Maine and Ireland. This bright seaweed lays on rocks to dry and is then known as Irish moss. Irish moss creates the basis for the food thickener carrageenan, according to the University of Hawaii. Rehydrating the Irish moss allows the cook to add this thickener to dairy products for a creamier consistency.
Place the Irish moss into a colander and rinse it under running water in a sink until the water emerges clear from the bottom of the colander.
Add the rinsed Irish moss to a 1-quart container and pour 1 quart of water over the Irish moss. Cover the container.
Leave the moss to sit for three days. Remove the Irish moss daily during the soaking time and rinse it again in the colander. Discard the original soaking water from the covered container and replace with a fresh quart of water when you return the Irish moss to soak in the container.
Use the Irish moss as directed per your recipe's instructions after the three days of soaking.