If you get the formula for an unknown potion currently in your inventory, it will change to what it is. If you make a potion that comes up as unknown, drink it and if its a legitimate potion such as something like wolverine or perfume, the recipe will show up.
I had trouble with Argentia too. The first time I put it on the sword the formula did not appear. I assumed I made a mistake in making it and went back and made more than one, carefully. This time, when I oiled the sword the remaining oils in my inventory got their proper names and I had a formula. Since then, I have always made 2-3 of the "unknown" whatever it is I am making so that when I use one, it forces the game to deal with the rest of them I am carrying.