The Great Wall and Sticky Rice

In a pot as big as a dining table, sticky rice is boiling.

The smell comes first. Anyone who has stood in a home kitchen on the last night of the year, minding a pot of bánh chưng, knows that smell at once. Bánh chưng is a square cake of sticky rice, wrapped in leaves and boiled for hours for the Lunar New Year. The smell is sweet and warm, with the faintly earthy scent of rice grains just cooked through. It clings to clothes and hair, and it stays there all evening. Steam rises in waves, blurring the faces of the people standing close.

Then comes the sound. A wooden paddle sweeps around the bottom of the pot, scraping the sides with a steady, rasping rhythm. The longer the sticky rice porridge cooks, the thicker it gets, and the thicker it gets, the heavier it is to stir. Every so often the person stirring has to switch shoulders. Below, the fire roars. Above, the smoke.

It is a kitchen. But no one will eat this porridge.

Right beside the pot is a pit. In the pit is slaked lime, milky white and thick as paste, still warm from the moment the quicklime met water. Ladle by ladle, the hot porridge is poured into that pit of lime and mixed. It is mixed until the two blend completely into one soft, ivory-white mass.

What has just been made is not food. It is mortar.

In China, people were already mixing sticky rice porridge with lime like this about 1,500 years ago. Sticky rice mortar held brick to brick and stone to stone. It went into tombs, towers and city walls. The city wall of Nanjing, which has stood for about 600 years, is held together by that very porridge. So are the brick sections of the Great Wall that were built under the Ming, the dynasty then ruling China.

It does sound funny. Cooking porridge to build a wall. But the old builders were not joking. Centuries later, when chemists looked at a fragment of old mortar under a microscope, they finally understood why the method worked so well.

Here is the secret. As lime dries, it forms tiny crystals of a substance called calcium carbonate. When lime dries on its own, those crystals are free to grow as they please. They grow into large lumps that sit loosely side by side, with gaps everywhere between them. Rainwater seeps in through those gaps, and the wall begins to flake away in patches.

Sticky rice holds a special starch called amylopectin. The starch in sticky rice is almost all amylopectin, which is why sticky rice is so much softer and stickier than ordinary rice. When the porridge blends into the lime, strands of that starch slip in between the crystals as they form, and keep them from growing.

So the crystals have to stay small. And small things pack tightly. A handful of gravel poured into one jar and a handful of sand poured into the jar beside it show the difference at once. The sand fills every empty space, while the gravel leaves gaps all through the jar.

Sticky rice mortar dries into a mass just as dense. It grips more firmly, bears more weight, and water has a hard time getting through. There is a Ming tomb whose sticky rice mortar is so hard that a modern bulldozer ramming into it had to give up. Many buildings held together with sticky rice mortar have come through earthquakes and are still standing.

It has been called the world's first composite mortar. A single batch holds something taken from the rice field and something taken from the kiln where stone is burned.

That leads to a harder puzzle: how many pots of porridge it would take to glue together a wall that long. To find out, the first step is to measure how long the line really is. And it was while measuring that people found a surprise.

Measuring the Great Wall is not a job for one person with a tape measure. For several years on end, survey teams carried satellite positioning equipment up mountainsides and waded across ravines. They walked along the line of the wall, marking point after point, then pieced the points together into a map.

For the line built by the Ming alone, the final figure was 8,851.8 kilometers. They measured so carefully that the figure even goes past the decimal point.

But the best number is not the total.

Of that whole length, the wall that people actually built, of brick, of stone and of rammed earth, comes to only 6,259.6 kilometers. Another 359.7 kilometers are trenches. These are long ditches that were dug down into the ground rather than built up.

That leaves more than 2,200 kilometers.

They are mountains. They are rivers. They are sheer cliffs.

Exactly 2,232.5 kilometers of that line contain not a single brick. Where the old builders reached the foot of a towering cliff, they let the cliff itself serve as the wall. At the bank of a wide river, they let the river do exactly the same.

So the longest wall in the world is not one unbroken wall. It is a long line, and every so often a stretch of it is simply left to the mountains and the rivers.

And that is just what the builders did in the kitchen. No one set out to invent some strange new glue. People looked around, saw the pot of sticky rice they cooked to eat every day, and realized that its stickiness could be put to use. Outdoors it was the same. Where a mountain already stood, they let the mountain stand.

In the kitchen or out in the open country, there was only one rule. Use what is already at hand.

In Vietnam, sticky rice does the same job of holding things together, only in a different way. Sticky rice keeps a bánh chưng square, so that every slice stands as a neat block. It makes a handful of xôi, which is steamed sticky rice, cling together just enough to hold in the hand. In the old days, people also cooked rice flour into a paste to glue paper, fans and star lanterns.

The builders of communal houses and pagodas in Vietnam long ago had a secret of their own as well. They mixed lime with sugarcane molasses, sometimes adding handmade paper and a glue boiled down from buffalo hide. When the molasses meets the lime, it forms very durable crystals. That makes the mortar pliable when it is spread and solid when it dries. This lime and molasses mortar has held the molded decorations on the roofs of communal houses and pagodas through countless rainy seasons. Even today, restorers in Hue still cook up the very same old recipe. They use it to patch the mortar that has flaked away on the monuments of the old capital.

Two kitchens, very far apart. One cooked sticky rice porridge, the other cooked molasses. Neither copied the other. Each side simply bent over its own pot, and both arrived at the same idea. To make stone hold fast to stone, borrow a little sweetness from the rice plant and the sugarcane.

On the last night of the year, families sit up to keep watch over their pots of bánh chưng, while smoke rises and the smell of cooked sticky rice fills the kitchen. Several hundred years ago, in a place very far away, a pot of sticky rice just like that one was boiling too. The same smoke, the same smell, the same wooden paddle scraping around the bottom of the pot. There was only one difference. When that pot was done, no one ladled it into bowls. It was mixed into lime to glue brick after brick onto a wall. And that wall is still standing today.