It’s that time of year when houses and trees glow with holiday lights and, here in the Midwest at least, cars and windows boast thin layers of glimmering frost each morning. A few of the books in the Rare Book & Manuscript Library are getting a sparkly coating as well, thanks to a phenomenon known as efflorescence.
Efflorescence can affect a number of materials, including stone, concrete, and leather. It occurs when the amount of moisture in an object exceeds the amount of moisture in the air around that object. This causes the moisture to migrate towards the object’s surface. Once the moisture reaches the surface, it evaporates, leaving behind any salts that may have been dissolved in it.
Leather bindings often contain salts left over from the tanning process. If these salts are dissolved in any moisture that a book contains, then the drier air of fall and winter will bring them, in dissolved form, to the book’s surface. Once the salts reach the surface, the moisture evaporates, and they appear as a sparkly white powder. Fear not, the environment in the rare book vault is carefully controlled and kept at a constant temperature of 60 degrees with a humidity of 44%. Our conservators are never concerned about a little efflorescence in the winter.
When books are sitting next to each other on a shelf, the efflorescence tends to appear only on their spines and top edges, as these are the only places where the leather is exposed to the air. The powder has no harmful effects, and a single swipe of the finger will remove it. As the weather warms, the holiday lights will come down, windows will no longer be etched in frost, and the efflorescence will fade. But we’ll know it’s there, waiting, a little bit of sparkle just under the surface. -BS