How to Freeze Water So the Ice Is Porous

Fill a container with fresh, pure water, and add salt to it. The typical salinity of seawater is about 1.2 ounces of dissolved salt per quarter gallon of water. Make your water slightly saltier than this by adding about 2.5 ounces for every quarter gallon.

Mix the salt thoroughly with the water until it has completely dissolved. Place the container in a freezer with a temperature at least as cold as 17 degrees Fahrenheit. Allow the water to freeze for at least a couple of hours or longer if you have a gallon or more.

Remove the bucket from the freezer when the water has thoroughly frozen. Observe that the ice already is significantly less dense than regular frozen-solid water, noticeable by the ice's whiter color and lack of transparency. You can make it more porous still.

Drill several small holes in the bottom of your container, and place the container with ice still in it in a cold environment -- not sub-zero temperatures but also not in the sun. A cold refrigerator is ideal. As the ice froze, the newly forming ice absorbed water from the salt crystals and left them behind. Because this process is uneven, an increasingly briny solution of highly saline water was trapped in scattered tiny pockets inside your ice. Leaving the ice in a cold place allows the brine to drain out of it slowly (the reason for the holes in the bottom of the bucket) and leave gaps of air that increase the porosity.