A Guangzhou courtyard, mid-repair, in the long heat of July

The lime had been slaking in a shallow tin tub since dawn, and by the time we crossed the threshold of the courtyard it was hissing quietly to itself in a way that sounded, from a few metres off, like the breath of a sleeping animal. Mr Huang was crouched at the foot of the south wall with a wooden paddle, turning the paste, his shirt already darkened along the spine. A cicada started up somewhere above the tiled ridge. A neighbour's transistor radio carried in over the wall, Cantonese opera scratching its way through a long aria. He did not look up. He was watching the paddle, and the paddle was telling him something we could not yet read.

The house sat in a lane off Enning Road, three bays deep, a single qilou-style facade onto the street and behind it the small square sky of a Lingnan courtyard. Half the south wall had been opened up the previous week. A vertical seam of removed stones ran from waist height to the eaves, the cavity packed with damp sacking to keep the surrounding masonry from drying too fast in the July heat. The family had lived here since the 1930s, four generations under the same set of beams. Mr Huang had been called in by the granddaughter, who manages a tea shop two streets over and had grown tired of watching the lime render around the doorframe powder away with each typhoon season.

The studio that arranged the visit, an architectural conservation outfit on Tongfu Road, had been clear that the work was not theatre. There would be no demonstration. There would be a wall, a man, an afternoon, and either the mortar would set as he wanted or it would not, and we were welcome to sit on the granite kerb of the well and watch.

Mr Huang is sixty-one. He learned the trade from his father, who learned it from a Hakka mason in Heyuan in the 1950s, and the lineage matters less to him than the fact that he has been mixing lime by eye in this city for forty-three monsoons. He does not own a thermometer. He does not own a moisture meter. He owns the paddle, a flat stick of camphor wood worn smooth at the grip, a pair of zinc tubs, a sieve, and iron scrapers his father had made by a blacksmith in Foshan.

“Eleven new stones lay along the kerb beside the cavity, each one a hand-sized block of the soft local sandstone, the colour of weak tea.”

The lime came from a kiln in Yangshan, three hours north, shipped down in paper sacks. He had slaked it the night before with well water drawn from the courtyard, then left it under wet cloth to ripen. The proportion of sand to lime was something close to two and a half to one, but the ratio was the least of it. What mattered was the feel of the paste when it dropped from the paddle. Too quick and the wall would crack within a year. Too slow and the new render would slide off the old before it cured. The right paste fell in a slow ribbon, hung from the paddle for the count of two, then released cleanly. He had us watch him test it three times. We could not see the difference. He could.

Eleven new stones lay along the kerb beside the cavity, each one a hand-sized block of the soft local sandstone, the colour of weak tea. He had quarried them himself from a dismantled wall in Liwan, a house that had come down for a metro line, and he had been keeping them in his nephew's yard for two years, waiting for the right job. He did not select them in the order they had come off the cart. He set them out on the kerb and walked along the line twice, picking one up, turning it, setting it down a few places along. Some of these stones, he said, want to be together. You can see it in the grain.

He pointed to two blocks now sitting side by side. The bedding planes of the sandstone ran in faint dark lines, the way wood shows its grain, and these two had been cut from the same horizon in the original quarry, perhaps a century and a half ago. If you laid them out of sequence, the wall would still stand, but it would weep differently in a typhoon, and in twenty years the discoloration would betray the mistake. The stones remembered where they had been. The mason's job was to listen.

He worked through the long flat heat of the early afternoon, troweling the lime mortar into the bedding joints, settling each stone with two careful taps of a rubber mallet, scraping the excess paste away with a thumb. The cicadas climbed and fell. The radio went off and was replaced by a kettle whistling somewhere upstairs. Around three, the granddaughter came down with a tray of cooled herbal tea, the kind brewed with chrysanthemum and honeysuckle that Guangzhou households keep through the summer, and we drank it standing in the shade of the eaves while Mr Huang stood at the wall and watched the mortar set.

He did not leave for an hour. He stood with his hands on his hips, occasionally pressing a fingertip against a joint and reading the resistance. The lime, he said, was carbonating. The calcium hydroxide in the paste was pulling carbon dioxide from the courtyard air and turning, slowly, back into the limestone the kiln had broken down the month before. The wall was becoming, again, the rock it had once been. If the humidity dropped too quickly, the surface would skin and the inside would stay soft. If it stayed too high, the cure would crawl and the mortar would never reach its full strength. He read the air the way a roaster reads a basket of leaf.

By half past four the south wall was closed. The new stones sat flush with the old, their joints darker than the surrounding render and slightly damp, the colour difference one Mr Huang said would even out across the next two rainy seasons. He swept the courtyard, rinsed the paddle in the well, and sat down on the granite kerb to roll a cigarette. The cicadas had gone quiet, and we could hear, from somewhere along the lane, two old men beginning a game of Chinese chess, the wooden pieces striking the board in a slow, considered rhythm.

He did not tell us the wall would last a hundred years. He told us the wall would last as long as somebody was paying attention to it. The aesthetic of the Lingnan courtyard, the one written about in the books on gardens and opera, is the aesthetic of a thing being repaired in July by a man who knows which stones want to be together. We sat with him until the light shifted off the ridge tiles, and then we walked out into the lane, and the radio came back on behind us.