Crystals form when atoms or molecules join together in a repeating pattern, and they build faster when they have a surface to start on. Here, laundry bluing supplies the tiny particles that give crystals something to grow on, while table salt provides the building material. Add ammonia to speed evaporation, and within hours feathery crystals sprout across the sponges as dissolved particles lock into that repeating structure.

When the liquid in the mixture turns to gas and moves into the air, the salt dissolved in it has nowhere to go. It joins together in a repeating pattern and forms feathery crystals across the sponges. The ammonia speeds this process so the crystals form faster. The bluing supplies tiny particles for the crystals to grow on, and the table salt provides the building material.