This feast of dragons, angels and seven-headed beasts depicting in gory style John's vision of the last days, was done about three centuries later than the better known Norman conquest era Bayeux Tapestry embroidered cloth -- but it is larger.
The work, which purports to be the longest tapestry in the world, originally stood 5.8 metres high compared with its current 4.6 metres, and was around 40 metres longer but it has lost some 20 panels and part of its border over time.
Surviving sections of the 14th century masterpiece, state property since 1905, are now showing multiple signs of wear and tear as well as the effects of gallery lighting after having been on permanent display since the mid-1950s at Angers Chateau, about 300 kilometres (190 miles) southwest of Paris.
The culture ministry has been busy collecting data for "an autopsy to decide what we do in terms of restoration and guarantee its long-term public showing," says castle administrator Herve Yannou.
As the deep clean progresses, the gallery has been plunged into virtual darkness with scaffolding erected to permit miniscule, "square centimetre by square centimetre," scrutiny of the huge work.
It involves identifying, then quantifying all kinds of deterioration, say restorers Suzanne Bouret and Montaine Bongrand from the Loire region cultural affairs department (DRAC).
- Delving into pictorial past -
The inspectors have to probe deeply and carefully. Is the tapestry dustier at the end on display at the entrance or the exit of the gallery? Is there greater deterioration towards the top or the bottom?
They must also take into account temperature and humidity levels, grime, warping or tension in the fabric linked to hanging. Everything gets measured.
Four sections out of a total of some 70 surviving scenes have been taken down. The rest remain on display while those removed from the work which, highly unusually, is reversible, undergo examination and cleaning.
Yannou points out a section depicting medical examiners on the reverse side of a scene called the Harvest of the Chosen Ones and the Sleep of the Just panel showing seven men sharing two beds.
Such scenes offer those who gaze upon them "a stunning smorgasbord of colour," says Yannou, marvelling that time has been kind to the colour rendition of the section.
"The reverse side not only tells a story of the beauty of its colour hue but also the different interventions which have taken place" over the centuries.
"Here, part has been rewoven. There, one can see retouching techniques with new wool and threads going off in all directions," says Bouret, hunched over one section as a colleague vacuums away to suck up dust before weighing a tapestry that has endured being moved several times.
- 'Like an X-ray' -
After a century in the keep of the dukes of Anjou, Rene of Anjou bequeathed the artwork in the late 15th century to Angers Cathedral.
Some 200 years later, the bishopric was faced with what to do with the tapestry when the political climate conspired to see Church art fall victim to the chaotic aftermath of the French Revolution.
That period saw various works destroyed and the tapestry was cut up and used variously as floor mats, stable insulation and anti-frost covers for fruit.
The work, which had fallen into "a thousand fragments," was rescued in 1850 by a church canon, who took a first stab at restoration, says DRAC curator Clementine Mathurin.
"On the reverse side you see a mass of things which were designed to stay hidden. It's like an X-ray... you're really inside the skeleton of the tapestry," says Bongrand, who hopes that counting up the number of threads and weaves will tease out details of the technique used to produce it.
"We hope to unearth the secrets of this artistic and historic masterpiece from the Hundred Years War (between England and France, 1337 to 1453), created in just seven years," says Yannou.
"Where was it woven and in how many workshops? How many people were involved? It's a mystery..."
For Yannou, although it's largely unknown to the general public even in France, the Apocalypse Tapestry is Angers' very own answer to the Sistine Chapel, which it predates by fully a century.