Dichromate formalin mixtures such as Orth’s fluid are unstable and deteriorate, so they must be made just before use. This is conveniently done by keeping a stock solution and adding formalin to it immediately before the tissue is placed into it. Gray recommends thet fixation be carried out in the dark and that the dichromate be washed out with a dilute formalin solution, also in the dark. Presumably, this is because dichromate can be photosensitive under some circumstances.

Time

Generally, one or two day’s fixation for normal slices of tissue should be satisfactory.

Aftertreatment

Wash well with running tap water or dilute formalin in the dark to remove excess dichromate.