Storage and use are very dangerous and require special precautions especially when mixing. The 19 th -century practitioners of photogravure introduced ferric chloride as an etchant. Because of its unique ability to seep through a permeable gelatin resist very gradually, ferric chloride was found to be ideal for the variable depth of etching required in photogravure. It is this variable depth of etch, controlled by the variable thickness of the gelatin resist, that makes possible the variety of tones in photogravure prints. The traditional acids generated excessive heat, bubbles, and were prone to foul-biting.
Ferric chloride gives a more controllable etch, plus it is safer than the traditional acids.
This forms acidic solutions so should be handled accordingly, but does not have the dangers of handling concentrated acids. Ferric chloride solution might cause minor skin irritation from prolonged contact. Ferric chloride is actually a salt, not an acid. Ferric chloride is a sort of warm rust color when fresh and gradually turns dull brownish-green after much use. I usually top up used solutions with fresh ferric chloride and discard the excess rather than discarding the whole thing.