Second, the iron oxide film coating is removed in an oxalic acid bath. And last, the oxalic acid is washed off the crystal with fresh water. Washing the clay off the crystal is not always as easy as it sounds, especially with clusters. Points are fairly easy to rinse with a garden hose. I do not recommend doing it in your kitchen sink because the clay can plug up your sink traps and plumbers are more expensive than your crystal.
Most clusters have small cracks and crevices filled with clay.
A pressure washer works best to flush out the clay. You can let the clusters dry in the sun for a couple days after washing to shrink the clay and pressure wash again. This process may have to be repeated several times. The cleaner the crystal the more effective the acid bath. Trapped clay prevents the oxalic acid from removing the iron film in that spot.