The Carrying Capacity of a Cave
The Mogao Caves outside Dunhuang hold 492 painted chambers cut over a thousand years, and the thing that damages them fastest now is visitors breathing. Since 2014 the site has run to a measured daily limit, with a film standing in for the caves you are not shown and sensors deciding which doors get unlocked.
You do not choose which caves you see at Mogao. You book weeks ahead, arrive at a reception centre ten kilometres from the cliff, sit through two films, get on a shuttle bus, and are then handed to a guide holding a ring of keys. She unlocks eight doors out of the several dozen open on rotation that week, points a torch at what she wants you to look at, and locks each one behind the group. Seventy-five minutes later you are back on the bus. No photography, no wandering, no second look.
This reads as crowd control and it is not. The binding constraint at Mogao has nothing to do with queues or wear on the floor. It is that a person standing in a sealed rock chamber raises the carbon dioxide and the humidity in it, and the cliff the caves are cut into is full of salt. The Dunhuang Academy runs the whole site to a number derived from that fact, and it can produce the sensor traces the number came from.
What is in the cliff
The caves are cut into a conglomerate face above a dry riverbed twenty-five kilometres south-east of Dunhuang, traditionally from 366, when a monk named Lezun is supposed to have seen a vision of a thousand Buddhas on the rock, and then continuously for the next thousand years. Some 735 chambers survive. Of those, 492 hold painting or sculpture — roughly 45,000 square metres of mural, plus a couple of thousand painted clay figures. The rock itself is hopeless to paint on, a cemented gravel far too coarse to take pigment, so every surface is a skin of mud plaster mixed with straw laid over it, and every layer in that sandwich is a separate thing that can come apart. UNESCO listed the site in 1987.
Sixty-two per cent
The Getty Conservation Institute has worked at Mogao alongside the Dunhuang Academy since 1989, an unusually durable arrangement by the standards of heritage conservation, and its most valuable output was not a repair but a measurement. Monitoring inside the chambers established that once relative humidity passes roughly 62 per cent, the salts held in the rock and the plaster take up water and go into solution — chlorides and sulfates together, a mixture that does this at a markedly lower humidity than either would on its own. When the air dries again they recrystallise, and they do it underneath the paint, which is levered off the wall in flakes. Blistering and loss had been visible for a century. What was new was knowing exactly which threshold triggered the cycle — and therefore that it was a threshold somebody could defend. The caves are now instrumented, and one that crosses 62 per cent humidity or 1,500 parts per million of carbon dioxide is closed and left to recover.
The conservation problem at Mogao is not vandalism, damp or time. It is that a painted cave has a maximum number of people who can stand inside it, and for most of the site's modern history nobody had worked out what that number was.
Six thousand
The published daily carrying capacity is 6,000 visitors, reached by working backwards from the volume of each chamber, how fast it ventilates, and how long it needs between groups. That is the standard ticket — around 238 yuan, sold on the Academy's own channel about a month in advance, and gone within hours for most of July and August. When more people turn up at Dunhuang than the standard ticket can absorb, the Academy releases an emergency ticket at roughly 100 yuan which admits you to four caves and no films at all. The two-tier arrangement is an honest admission of the bind: the cap is real, and turning visitors away from a site this famous is not politically available. So everyone gets in, and the people who planned get more.
The film that stands in for the cave
The Mogao Grottoes Digital Exhibition Centre opened in August 2014, ten kilometres back towards town, and its purpose is to absorb time that would otherwise be spent underground. You watch a film on the Silk Road and the site's history, then a dome film that moves through the interiors of caves photographed at very high resolution, and only then board the bus. It works. Forty minutes that used to be spent standing in the sun at a cave door now happen in an auditorium, which is why a guide can move a group briskly without anyone feeling cheated. It is also, quite openly, a substitution: you are shown a screen instead of a room and told that the screen is the reason the room is still there.
The library that left
Mogao has done this trade before, and lost it. Cave 17, a small side chamber off the corridor of Cave 16, was walled up in the early eleventh century and reopened around 1900 by Wang Yuanlu, a Daoist who had appointed himself caretaker of the site. Inside were something like fifty thousand manuscripts, paintings and printed documents in Chinese, Tibetan, Sogdian, Khotanese, Uyghur, Sanskrit and Hebrew — six centuries of the working paper of a Silk Road town. Aurel Stein reached it in 1907 and left with several thousand items for a payment of about £130. Paul Pelliot, who could actually read the languages, arrived in 1908 and took the best of what remained. The library is now in London, Paris, St Petersburg, Beijing and Tokyo, and the oldest dated printed book in the world, a Diamond Sutra of 868, is in the British Library. Since 1994 the International Dunhuang Project has been photographing all of it into one free online collection, which is the only form in which those documents will ever sit together again — and the same logic now runs the caves themselves, through Digital Dunhuang, which since 2016 has published complete chambers at full resolution to anyone who wants them. Which is the bargain the site offers you and does not disguise. You will see eight rooms out of 492, chosen for you, in the dark, at somebody else's pace. Everything else you will see on a screen, in more detail than a torch beam gives, for as long as you like. That is a worse afternoon and a much longer thousand years, and Mogao is the rare place that says so to your face.