Fixer
A solution that dissolves the undeveloped silver halide, making the image permanent and no longer light-sensitive.
Measured in
minutes
In depth
Without fixing, the remaining halide would darken on exposure to light and destroy the image. Fixer works by forming soluble complexes that wash away, and it becomes exhausted as silver accumulates in it. Under-fixing leaves a milky residue that will stain over time, which is why clearing time is tested rather than guessed.
Examples in practice
Testing fixer by timing how long a film leader takes to clear, then fixing for twice that, prevents under-fixing.