Fu Shi

Pumice is a quenched volcanic melt, so its silica is glassy rather than crystalline. It is essentially insoluble in gastric fluid, releases no ions of consequence and has no measured systemic pharmacology; the traditional action is attributed to the stone's saltiness, coldness and lightness rather than to anything it delivers. Being amorphous, it is not respirable crystalline silica, but grinding the stone in a pharmacy still generates fine dust that should be handled with local exhaust and a mask.

The aluminium here is locked into a glass network, not present as a soluble salt, and it should not be equated with the aluminium hydroxide of antacids. No absorption study exists for pumice taken by mouth, so the aluminium exposure it represents is unquantified rather than known to be zero.

Present as network modifiers in the glass rather than as free salts. They are the reason the composition of pumice is a property of the volcano it came from, so that two batches of the same nominal drug can differ measurably in bulk chemistry.

Responsible for the grey to buff colour of the stone. Pharmacologically inert at these levels.

The vesicles give the stone its low bulk density and its abrasive, high-surface-area texture. Floating on water is the identifying characteristic of the drug in the classical descriptions and is the shared feature that lets pumice and the bryozoan skeleton be used under one name despite unrelated chemistry.

Volcanic glass carries whatever trace element inventory its parent magma had, and sea-collected material additionally carries adherent marine residues. No published survey of lead, cadmium, arsenic or mercury in medicinal pumice was located. Since Chinese mineral and animal drugs as a class show the highest heavy metal exceedance rates, the honest position is that the contaminant load of this drug is unmeasured, not that it is low.

Earliest account of how the floating stone forms, before it was listed as a named drug in its own right.

Records fu shi, the floating stone, water-ground for screens obscuring the eye, and gives it as balanced in nature and non-toxic, quenching thirst, treating the stranguries and countering the poison of wild animals.

Describes fu shi as congealed from water foam, white in colour and light in body, salty and cold in flavour and qi; it enters the Lung, removes phlegm-heat from the upper burner, stops cough and softens hardness, clears the upper source of water and treats the stranguries. This is the entry that anchors the modern indication.

The floating stone is to be avoided in cough of deficiency-cold type, and excessive use is held to damage qi and blood. Internal dose is given as 3 to 5 qian in decoction or worked into pills and powders; externally it is ground to powder or used as an eye wash.