Membrane is a unique element. It exists as thin sheets. They exhibit fluid-like behaviour, despite being almost two-dimensional. If a membrane is thickened by a spell, it quickly becomes unstable and will revert back to being between 5 and 10 nanometres thick.
In its pure state, a membrane allows for the passage of most humours, salts and aethers through it undisturbed. Soap can be used to make membrane-based binds which are more selective.