Lea Formula 7 is a salted collodion that comes either premixed or as a two part. It is a classic formula developed by M. Carry Lea that can be used for tintypes, ambrotypes, and glass negatives. It’s an ammonium and cadmium based formula that ages quickly, so it is best to use the mixed collodion within 4 to 6 weeks. This can be prolonged with cool dark storage. When using the two-part, it will ripen and be ready to use within hours. This way you can mix only as much as you will use.