During fixing, undeveloped non-image silver is dissolved by thiosulfate without damaging the metallic silver image. The ﬁrst ﬁxing bath does most of the work but becomes increasingly contaminated by the soluble silver thiosulfate and its complexes. Soon, the entire chain of complex chemical reactions cannot be completed successfully, and the capacity limit of the ﬁrst ﬁxing bath is reached. A fresh second bath ensures that all remaining silver halides and silver thiosulfate complexes are dissolved. An intermediate rinse is optional, but it protects the second bath from contamination. Fixing time must be long enough to render all residual silver halides soluble, but not so long as to allow the ﬁxer and its by-products to permeate the paper ﬁbers; the former being far more important than the latter.
So, one-bath fixing works, but it has some disadvantages: longer fixing times, fixing never as complete as two-bath fixing, residual fixer penetration deep into the fibers, leading to longer and not as effective print washing.