A sodium or ammonium thiosulfate is the solution that fixes an image onto the photographic paper.
This is a chemical that converts a silver halide into soluble or complex silver salts that dissolve in the fixer.
In the imaging process, the film does loses its original silver halide milkiness overlaying the image and becomes clear.
Hence, she places the photograph into the solution to fix the ammonium thiosulfate image onto the paper.
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