Foolproof Kohakutou - Japanese Rock Candy Kohakatou (Makes 1 1/2 trays of rock candy) 2 cups water 15 grams agar agar (Thai Brand) 4 cups sugar flavoring, optional food coloring, optional Directions Bring water and agar to a boil. Once agar is dissolved, add in the sugar. Place a candy thermometer in the mix and cook mixture over medium heat until mixture reaches 230 degrees Fahrenheit. Pour into a greased 13x9-inch dish. Let mixture set for at least 4 hours. Cut or rip the set agar apart and place on parchment. Let candies sit for 3-4 days until a sugar crust develops on all sides. Candy is then ready for eating or packaging.