Use the paintbrush to spread a layer of glue over the inside and around the edges of each shell.
Sprinkle a generous amount of alum powder over the glue while it’s still wet.
Gently shake out any extra alum powder and let the glue dry completely.
Heat 2 cups of water to just below a boil and stir in about 40 drops of food coloring.
Add ¾ cup alum powder to the water and stir until the powder has completely dissolved.
Let the solution cool for 30 minutes.
Submerge each dry, alum-coated shell in the solution, allowing it to rest on the bottom of the container with the inside of the shell facing up. Leave at least 2 inches of solution above the shells. Let sit for 24 hours.
Use a spoon to carefully remove the seashells from the solution and place them aside to dry completely. Enjoy your creation!
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