I’m not certain if this is the most counterintuitive thing I’ve ever seen or the most ingenious—probably a little bit of both.
In addition to their inherent fragility, Bach’s remaining manuscripts are susceptible to damage from rust owing to the iron content found in the ink he used (which he often made himself). This can do everything from changing the color of the writing to holes being eaten into the paper. To preserve these priceless manuscripts conservators apply cellulose to weakened areas of the paper, but it’s the way the do it that is incredible.
First, the manuscript is placed between two sheets of adhesive gelatin then pressed together (images are ©EuroArts):
Here is the amazing part—those two sheets are pulled apart, dividing the manuscript’s thickness in half:
Areas that are ink-damaged receive some individual attention:
Then a layer of cellulose is placed in between the two halves before they are joined back together:
I’m sure that this method was researched and rehearsed over and over again, but still, can you imagine being the first person pulling apart one of these irreplaceable documents? Talk about no do-overs…
You can watch this harrowing procedure here (I thought that the choice of music—with the dissonant suspensions while they separated the sheets—was a fitting choice). Afterward there is also some discussion about the Bach Digital project as well.
Here is an article from the Journal of the American Institute for Conservation that describes in detail the process of paper splitting. It features much of the research done by Günter Müller and Wolfgang Wächter, who provided key support to the Bach project.