Science Writing · Chemistry · Festivals

Made to Return: How Durga Idols Are Built to Go Back to the River

A Durga idol begins as riverbank clay and goes back into the water after the festival. A chemist's look at why the old way of making it works so well.

In Patna, you can tell Durga Puja is near weeks before it arrives. Bamboo scaffolding goes up along Boring Road and around Dak Bungalow crossing, and the traffic starts taking odd detours. When I was growing up, Puja meant the pandals on Govind Mitra Road and Arya Kumar Road. Crowds packed shoulder to shoulder till well past midnight, drums from one lane to the next, and that moment, after a long wait in the queue, of finally standing in front of the goddess.

Long before the lights go up, she is being made in small workshops and roadside sheds around the city. Walk past one in late summer and you see her at every stage at once. A bare frame of bamboo and straw in one corner. A grey clay figure drying in another. A finished face waiting for paint. And a few days after Dashami, many of these same idols make their way, with music and prayer, to the Ganga ghats.

As a chemist, I now look at those sheds a little differently. The idol is clay from a riverbank, and at the end of the festival it goes back into water. Almost every step in between makes sense once you look closely at the materials. The kumors, the potter-artisans, whether in Kumartuli in Kolkata or in a shed in Patna, have worked all this out over generations, mostly without ever writing it down.

Bamboo, straw and jute

The work starts with a frame, the kathamo, made of bamboo and wood. In many workshops the frame itself is worshipped before anything else is done, often months ahead of the Puja.

Over this frame go bundles of straw, tied tight with jute rope, until there is a rough body, arms and head. Bamboo, straw and jute are all plant fibres. Chemically they are mostly cellulose, with some hemicellulose and lignin.

Cellulose is simply glucose units joined into long chains. The chains lie side by side and stick to each other through hydrogen bonds, which is why a stalk of straw weighs almost nothing yet is hard to pull apart along its length. Lignin is the stiff, glue-like material that holds the fibres together. So the skeleton is strong, cheap, and made entirely of things that grew out of the ground.

Two clays

Artisans don't use just any mud. Two kinds are common. Entel mati is the sticky one. It goes on first, often mixed with chopped straw or rice husk, to build up the body. Bele mati has more sand and silt in it and is used later, for the finer layers that give the idol its smooth skin.

Why is clay sticky at all? Clay isn't just very fine soil. It is made of particular minerals, kaolinite, illite and montmorillonite among them. These are aluminosilicates, built from thin sheets of silicon, aluminium and oxygen stacked like the pages of a book. A single clay particle is a tiny flat plate, usually less than two micrometres across.

The faces of these plates carry a slight negative charge, and water molecules and dissolved ions crowd around them. In wet clay the plates can slide over each other on thin films of water and still cling together. That is what lets an artisan press out a nose, a fingertip or the fold of a sari and have it stay put. Montmorillonite goes a step further: water gets in between its layers and the clay swells.

Sand grains are far bigger and behave nothing like this. Mix in sand, as in bele mati, and the clay gets less sticky but also shrinks less as it dries. So when an artisan decides which clay goes where, the decision is really about particle size and water content, even if nobody in the workshop would put it that way.

There is one more thing I like here. River silt is soil the river has carried down from upstream and dropped along its banks. The idol is made of what the river brought.

Drying without cracks

Wet clay holds a lot of water. As it dries, the surface loses water first and the plates there pull closer together. The outside shrinks while the inside is still damp, and that mismatch is what cracks it.

Artisans deal with this in several ways. The straw and husk in the first layer work as reinforcement, spreading the stress so small cracks don't run, the same trick used in mud houses for thousands of years. The idol is dried slowly, in shade, so moisture leaves evenly. Clay goes on in thin layers over several days rather than one thick coat that would trap a wet core under a dry crust. Joints and cracks are covered with strips of cloth dipped in clay, and the fine bele mati smooths over the rest.

Then there is the step they don't take. The idol is dried in air and sun, but it is never fired in a kiln.

Heat clay past about 500 to 600 °C and the minerals lose the water locked into their crystal structure. Past about 900 °C the particles begin to fuse. Now you have terracotta, and there is no going back. A fired pot does not turn into mud when you put it in water.

Sun-dried clay hasn't crossed that line. Its plates are held mostly by weak forces and a little leftover water. Put it in water and the water slips back between the plates, the clay softens, and it becomes mud again. From the very first layer, the idol has been made so that it can go back.

Colours

Once the clay is dry, it gets a base coat, then colour, and last of all the eyes. Painting the eyes, chokkhudaan, is done with special care and usually comes near the very end.

Older Indian palettes leaned on minerals and plants. White was chalk or white clay (calcium carbonate, kaolin). Reds and browns were ochres, coloured by iron oxides like haematite. Black was soot, which is almost pure carbon. Blue was indigo. To make the colour stay, painters mixed in natural binders such as plant gum or tamarind seed paste, whose long sugar chains dry into a film that holds the pigment grains in place.

What I find nice about these materials is that iron oxide, chalk and carbon are already present in ordinary soil and river sediment. Put them back in the river and they are simply home.

Today many artisans use synthetic paints. They are brighter and last longer, and that is what a lot of buyers want. But the chemistry changes. Enamel and oil paints are built on synthetic resins that don't wash out easily, and some bright pigments have historically contained lead, chromium or cadmium. That is why pollution control boards and many puja committees now ask for water-based, lead-free colours, and more artisans have started offering idols painted only with natural colours.

Plaster of Paris

In some places, mostly for other festivals, idols are cast in plaster of Paris (PoP), and it behaves very differently.

PoP is made by heating gypsum to around 150 °C, which drives out part of its water:

Add water to the powder and the reaction runs the other way. Gypsum crystallises again as a mass of tiny interlocking needles, and the paste sets hard in minutes. That is why it is so handy. It can be poured into moulds, it picks up fine detail, it is light, and many identical idols can be made quickly.

The difference from clay lies in how the solid holds together. Clay is plates loosely held by weak, reversible forces. Set plaster is a locked crystal network. Gypsum does dissolve in water, but only about two grams per litre, and slowly. So a large plaster figure can sit intact in a pond or river for a long time, while a clay idol softens within hours or days.

None of this has anything to do with anyone's devotion. It is just how the materials behave, and it is why many states and city bodies encourage clay idols, or set up special tanks for immersing PoP ones.

Back to the river

On Vijaya Dashami, after the last aarti, the idol is carried to the water with drums and singing, and people tell each other she will come again next year. Devotees believe the goddess returns to her home in Kailash, and the image that held her for these days goes back to the elements.

The chemistry follows the same path. In the water, the dried clay soaks up moisture between its plates and loses its strength. The fine particles spread out and settle as sediment, much like the silt the river leaves every monsoon. Straw, jute and bamboo, being cellulose and lignin, are slowly broken down by microbes. Chalk, ochre and soot settle back among the minerals they came from. Many cities now pull the bamboo frames out soon after immersion, so the river is left only with what it can take.

Seen this way, the old method suits its ending very well. Unfired clay, plant fibre, earth colours, plant gums. Nearly everything in a traditional idol is something the river and the soil can take back, whether that was planned or simply learnt over many years.

The artisans in those Patna sheds don't talk about aluminosilicates or hydrogen bonds, and they don't need to. Their hands know when the clay is too wet, when a crack is about to spread, and which soil will hold a fine line. That knowledge gets tested every year, one Puja at a time.

Next year the scaffolding will go up on Boring Road again, and somewhere in a shed nearby, a new idol will be taking shape from the river's clay.