um, good question. I think it would depend on the heat of the fire.
After the 1906 earthquake in San Fransisco, there was a fire that destroyed the rest of the city, including the Wells-Fargo Bank. As soon as the safe was cool enough to open, they did so. Inside were all the bank ledgers. The fire had gotten so hot that the papers had spontaneously burst into flame. But since the safe was airtight, the fire was immediately extinguished for lack of oxygen, and the papers were left with singe marks on the edges.
-reference: a 3-week internship in the history museum at Wells Fargo in SF. I actually got to touch (with layers of white gloves on my hands) some of the artifacts from the early days of the bank and stagecoach company. way too cool.