Ethanol does double duty. It is a fixing agent as well as the most common dehydrant.
The most convenient anhydrous fluid available at reasonable cost, and for most technologists the dehydrant of choice, is ethanol, a common industrial chemical manufactured in bulk. It is available almost completely water free as absolute ethanol, although the slightly less expensive 96% ethanol may be used for the initial few changes.
We tend not to think of ethanol as a toxic chemical but it certainly is. Ingesting relatively small amounts of pure ethanol can kill. It should be remembered that there are two sources of industrial ethanol. One source is from fermented sugars, often from maize, which is then concentrated by distillation. However, the maximum purity obtained by distillation is about 96%, and to obtain absolute ethanol requires the addition of benzene to the ethanol before re-distilling, traces of which remain in the absolute ethanol. Benzene is a known carcinogen. The other source of ethanol is chemical manufacturing from natural gas or oil sources, with trace amounts of who knows what.
Although a fixative in its own right, ethanol does not refix tissues that have been properly fixed and stabilized with other agents. It does, however, complete fixation if the proteins were not properly fixed initially. Its fixation characteristics are not as desirable as some other fixatives and it is preferable, by far, to avoid the completion of fixation by the dehydrating ethanol, and ensure that fixation is complete before beginning dehydration. When this is done, ethanol is a fairly tolerant dehydrant, and tissues may be left in it for some time, even days, without damage.
