Which reagent is used for fixing and removing excess gold chloride in Gomori's reticulin method?

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In Gomori's reticulin method, after silver impregnation and development, the image is toned with gold chloride to intensify contrast and stabilize the metal deposit. The fixing step with sodium thiosulphate then dissolves unbound gold chloride and removes any residual unreduced silver, effectively locking the image in place and reducing background staining. This fixing action is essential to prevent diffusion and to produce crisp, well-defined reticular fibers. Ferric ammonium sulfate serves to sensitize tissue for silver uptake, and formalin acts as the reducer during development, but the step that removes excess metal and fixes the image is done with sodium thiosulphate.