"Ingestion: Gastric irritant. Ingestion may be followed by severe pain, vomiting, diarrhea, and collapse. If death does not occur in 24 hours, esophageal perforation may occur, as evidenced by fall in blood pressure and severe pain. A narrowing of the esophagus may occur weeks, months, or years after ingestion, making swallowing difficult." - Material Safety Data Sheet for calcium hydroxide
Brick Lane, a street with so much neon lighting it leaves your eyes buzzing in their sockets. A wash of pungent spice fumes spreads from every open door, and in classic South Asian style, every shopfront is represented by a salesman with eyes tuned to draw in eye-contact like a net. This slim road of Victorian architecture to the east of the City of London is the heart of Britain's Bangladeshi community, and the restaurant strip here has one of the highest densities of curry houses of anywhere outside the subcontinent. Right in the midst of the restaurants is a large specialist supermarket, where the floor to the ceiling is stacked with more variations on spice, herb, vegetable and pulse than a westerner would ever imagine possible... While rooting around this Thursday amongst the rajma and maida I came upon a tiny and rather cheap looking green vial, barely four centimeters tall, labelled paan choon.

Examining the ingredients contained it appeared to be a bottle of one chemical - calcium hydroxide, Ca(OH)2, a compound any chemist should have at least a passing acquaintance with. This compound, one with a rather formidable reputation, is the key ingredient in wall plaster.

Plaster is among the earliest, if not the earliest, high temperature artificial material. It's widely used in the the neolithic settlements of the Near East and Levant that are currently interpreted as the birth of urbanism, such as Anatolia's Catalhoyuk and Palestine's Jericho. At these sites we see large scales of production and sophisticated use, with plaster daubed onto hair or reeds forming the walls of most dwellings. The plaster production seen here is considerably more sophisticated than contemporary pottery (a more durable material with unarguable examples from older sites), and is used as the canvas for a culture of art and mural making that produced the earliest known landscape paintings. To take this artistic potential further, the Ain Ghazal statues of neolithic Jordan (undated, but also from the 7th millenium BCE) are among the earliest human forms made from synthetic materials. Composed of plaster daubed onto a base of reeds, these are surreal looking human shapes. Often with bizarre anatomies (two heads to one body, flat temples), these statues always have penetrating eyes outlined in black paint.

The material is produced by the heating, wetting and recarbonation of calcium compounds, typically starting with Calcium Carbonate (in this piece I'm ignoring its twin, plaster of paris). Initially, this material's life begins as limestone outcrops, limestone being one of geology's most variable rock types. This sedimentary unit can be anything from a fossiliferous and crystalline portland limestone to a chalk (the exotic igneous examples are sufficiently uncommon not to turn up in the history of plaster production), anything will make a good plaster when sufficiently cooked. When quarried, these rocks are then crushed and put in a lime kiln ready for calcination... At this point I'll present you with three formulae:

(1) CaCO3 → CaO + CO2

(2) CaO + H2O → Ca(OH)2

(3) Ca(OH)2 + CO2 → CaCO3 + H20

Calcination, reaction 1, takes place between 800° and 1000° dependant on the redox conditions of the kiln, but once the conditions are right the granular chunks of limestone become a fine white powder of calcium oxide, termed quicklime. This material is considerably smaller than the initial calcium carbonate and shrinks, spinach like, in the lime kiln. The next step in plaster production is exposing this powder to water, resulting in an aggressive exothermic reaction, reaction 2, and the swelling of the material to several times its previous volume. This new compound is calcium hydroxide, termed slaked lime. This is slightly soluable in water, and upon dissolution forms a corrosive alkaline solution capable of producing contact burns and cleaning drains.

The wonder of this material, however, is that if left exposed to a normal atmosphere it will gradually reabsorb its original carbon dioxide and revert to calcium carbonate (reaction 3). This means that it can be worked (and a wet solution of slaked lime is an extremely workable substance, capable of intricate design and holding its shape, as is particularly seen in Islamic design) and then left to slowly harden, a process than can take years to fully recarbonate, but eventually forming an artificial rock as substantial as its original outcrop.

In order to keep this material together during this process, a base is often used (explaining the hair and reeds mentioned above), and in construction a sand grit is added (making it lime mortar) to increase its workability and final strength.

"Ras Al Khaimah: Police on Saturday seized one tonne of illegal 'paan' in what is believed to be the biggest quantity of this illegal substance ever recovered in the emirate's history.

The RAK Police took the seized stimulant to the municipality office to take necessary legal measures against the culprits and conduct more intensive investigations. " - A newspaper report on paan trafficing into Dubai

So, why would this noxious alkaline have a place amongst the cuisine of Bengal? Well, in the long tradition of relatively alcohol free countries, Bengal has a culture of alternative stimulants. Like the khat of the Arabian peninsula, or the coca leaf of Colombia and Peru, the region between Pakistan and New Guinea has a mild stimulant called paan, which is ingested by chewing. Paan is prepared by taking the leaf of the betel vine, adding a little paan choon and some optional spices, and placing in its centre the seed of the areca palm. This mixture is then rolled into a small triangular package and placed in the mouth. Both the nut and the leaf are mild stimulants, and while I've never taken either they are described as giving a combined high comparable to a cigarette or cup of coffee. Notably, they are regarded as a breath freshener in the Indian subcontinent, and as a cleanliness hazard in the UAE.

The stimulant compounds are alkaloids, and the addition of a cheap, abundant and slightly chalky tasting alkaline to the recipe means that the saliva of those chewing the paan rolls maintains the ideal chemical conditions for absorbing these stimulants and maintaining their solubility. Since it's the alkaline content that is the potential health risk, and it's the alkaline content that is the desired component, the greatest health impact from this is a slightly increased bone mass from the calcium. Discussing the paan itself, in India it has a major cultural role and is chewed after meals, and ceremonially at weddings and receptions. The reason for the problems in Dubai come down to the red coloured saliva this produces, and that much like chewing tobacco this is best not swallowed, resuting in a spitting habit that is disapproved of (the primary paan chewers are immigrant workers from the subcontinent). The health risks of this habit cannot be entirely dismissed, with the International Agency For Research On Cancer having classified it (specifically the areca nut) as a source of oral cancer, resulting in failed campaigns in The World's Largest Democracy to reduce its use.

Paan, keeping people eating wall plaster for 4000 years.