Any fast Google will tell you that the serpent cross means “fixing the volatile”, but that doesn’t really tell you what it means so let’s unpack it a little. First, volatile in general means “changeable”. In specifically alchemical terms it often refers to mercury, which is certainly one of the most changeable elements alchemists worked with. Thus, in laboratory terms, “fixing the volatile” can mean rendering mercury by heat or other reaction to separate it into a stable form, or applying mercury as a solvent to a mineral. In more broadly alchemical symbolic terms, it means a lot more.
Several of the substances produced by mercury reactions are red. The color red usually stands for the active principle, in alchemy. This may, from one direction or another, help explain why Mercury, in code terms, often stands for Spirit. Spirit, active, animal, mercury and red are all symbolically associated alchemical terms.
Some mercuric substances, such as calomel (mercury chloride), have also been used medicinally. Notably, mercury was also one of the treatments for syphilis, which, if it was used in antiquity, may help explain its significance in alchemy as a purifying agent. Efficacy against such a feared disease would certainly make the reputation of any substance.
We find another connection to healing in the derivation of the symbol itself, which appears to come from one of the biblical tales of Moses, in which he nails a bronze serpent to a cross and displays it to relieve sufferers of snakebite and/or plague depending on your version.
So “fixing the volatile” generally means stabilizing the active principle, something which can separate harmful and beneficial elements from each other or even transform the harmful (pure active, too active) into the beneficial (balanced active).
The symbol that Izumi, Ed and Al wear, however, is not the crucified serpent alone. It is the crucified serpent winged and crowned. As we can see in many manuscripts, wings are used to mark progress or advancement of an alchemical solution toward perfection. Crowns mark the final stage of a spirit or solution: perfection, completion, ascension.
If we interpret snake = Mercury = spirit, which is a common symbol chain, then the symbol can suggest that the final “rendering” of the spirit, by death or enlightenment, will produce the pure, perfected, incorruptible spirit that, in alchemical terms, tends to go along with an incorruptible body. In this reading, the symbol indicates immortality, the standard promise of the philosopher’s stone.
One somewhat less classic way to read the whole symbol is as a statement that when the volatile is fixed into a stable, presumably healing, form, that is perfection. That to reach a stage that is beneficial is either a) as close to perfection as we can come or b) the only perfection we truly need to seek. My own feeling is that this would be Izumi’s interpretation. Certainly the fact that she wears this symbol on her body, which we have no indication Dante does, suggests that it means something significant to her. Another possibility, of course, is that it is a reminder to herself of own hubris in attempting human transmutation. Another possibility, of course, is that it is a reminder to herself of own hubris in attempting human transmutation. It seems likely, after all, she knew that symbol was associated with that transmutation in some way.
Image courtesy and comments from Jung Hearted