A corrosive history
In the archives of the former Bokhara Khanate, historical documents are slowly being consumed by the very ink used to write them. The culprit is iron gall ink, the standard writing medium for over 1,400 years across Asia and Europe. This ink's permanence, which made it ideal for official records, comes from a chemical reaction that etches the writing surface. But this same chemistry is responsible for the ink's destructive nature.
Iron gall ink is made from iron(II) sulfate and tannic acids extracted from oak galls. The reaction between these components forms an iron-tannin complex that gives the ink its dark, permanent color. However, this process also releases sulfuric acid, making the ink highly acidic, with a pH that can fall between 1 and 3. This acidity directly attacks the cellulose fibers that constitute paper, breaking down the long polymer chains through a process called acid hydrolysis. The paper becomes brown and brittle. The documents from the Manghit dynasty (1756–1920), which chronicle the political and diplomatic life of the Emirate, are particularly susceptible.
The chemical clock
The damage does not stop with acid. A second destructive process, oxidation, is often catalyzed by excess iron(II) ions in the ink formula. These ions can participate in Fenton-type reactions, generating highly reactive radicals that further sever cellulose chains, causing the ink to "burn" through the paper, sometimes leaving lacelike holes where text once was. This degradation is visible as a browning that spreads from the inked lines into the surrounding paper.
Conservators can model these degradation pathways. The rates of acid hydrolysis and oxidation are predictable under controlled environmental conditions. By analyzing the chemical composition of the ink and the current state of paper decay—often using non-destructive techniques like X-ray fluorescence—scientists can calculate the remaining lifespan of a manuscript. This allows archivists to prioritize which of the thousands of documents in collections like the Qushbegi Chancellery of the Bukhara Emirate require immediate intervention. Cool, dry storage conditions, with a relative humidity between 30% and 40%, are recommended to slow the decay.