Zao Xin Tu

The drug is the clay floor at the centre of a stove hearth, fired in place over years of cooking fires. Its chemistry is therefore that of a low-fired aluminosilicate ceramic. The silicate fraction is inert and contributes surface area rather than any dissolved species.

The alumina and silicate together make the drug a fired clay, which is what gives it an adsorbent, mucosa-coating action of the same general kind as the pharmaceutical clays. The nearest thing to controlled evidence for that class is smectite: a Cochrane review of children with acute infectious diarrhoea found low-certainty evidence that smectite added to rehydration may shorten diarrhoea by about a day and may raise cure rate at day three. That is a purified, standardised clay, not hearth earth, and the read-across is a mechanism analogy rather than evidence for this drug.

Practically the only quality marker the tradition offers. Lei Gong's Treatise on Preparing Drugs insists the article must be the earth of the stove face that has taken fire for ten years or more until it has set like a red stone with yellow inside, and warns explicitly against using ordinary earth from under the stove. Colour is standing in for degree of firing.

Minor mineral fraction. In practice these are the soluble part of the drug and the reason it has been described as replacing some electrolytes as well as acting as an adsorbent.

This is the contamination question that has to be stated. The classical drug assumes a rammed-clay stove floor fired by wood and straw, and modern stove linings are not that: they are refractory brick, castable refractory, ceramic fibre or sheet metal, and the fuel is as likely to be coal or gas. Chinese sources dispensing the drug now warn specifically against passing off ash or earth from coal stoves, gas ranges or industrial furnace slag, on the grounds that these carry heavy metals. The generic risk from eating earth is well documented: geophagic clays analysed in Ghana carried arsenic and lead above WHO and FAO limits with a calculable carcinogenic risk from the arsenic, and ingested clays also bind nutrients and co-administered drugs in the gut.

First record of the drug, entered as zao zhong huang tu, yellow earth from within the stove, and placed in the lower class. Tao Hongjing explains the alternative name Fu Long Gan, crouching dragon's liver, as a euphemism adopted out of respect for the stove god.

Source of Huang Tu Tang, Yellow Earth Decoction, in which the drug is combined with aconite, atractylodes and rehmannia for blood in the stool, vomiting of blood, nosebleed and uterine flooding arising from cold deficiency of spleen qi. This remains the drug's defining formula.

The identity warning that matters most today: do not mistakenly use earth from under the stove. The genuine article is earth from inside the stove face that has taken fire for ten years or more until it has congealed like a red stone with yellow inside, eight-sided in form, to be ground fine and purified by levigation. The tradition itself treats the degree of firing, not merely the location, as what makes the drug.

Collects the earlier names for the drug, including fu xia tu, fu yue xia tu, zao zhong tu and zao nei huang tu, and fixes its character as acrid and slightly warm, entering the spleen and stomach channels, to warm the centre, dry damp, stop vomiting and stop bleeding.