Biyin is a white powder used to draw poisons out from the skin. It is kept by Papa Mert, an apothecary of the undercity, who bade Sidra al-Najjar scour her hands with it after she had gathered samples of moss and fungus that might well prove dangerous; the powder does its work by lifting away whatever contaminant has found its way into the pores.
Papa Mert is at pains to insist upon its use whenever unknown substances are handled, holding it a prudent precaution against toxins.