I tried googling the answer, but articles explaining it are more geared towards oil paintings. I apologize for the more science-related nature of this question, but it's specifically regarding historical documents. I recall visiting the National Archives in Washington D.C. and the copy of the Declaration of Independence there in a dark area, dimly illuminated with very faded ink. Signage was posted all around in the Archives advising against any flash photography, but I always wondered but would happen if a tourist were careless enough to try. If it were a movie, I'd imagine it immediately turn to dust.
What makes the flash photography damaging to the Declaration, or any historical document? Is it the age, quality/kind of paper, ink, or all of it?
Secondary question: Is it the same regarding temperature? TIA
I’m not a paper expert but I have some familiarity with pigments and textiles and their care, which is adjacent.
I’ll answer your first question from direct experience many times over: if a tourist takes a forbidden flash photo of a document/object 99% of the time nothing happens. Often a docent or other staff member will give them a stern reminder that flash isn’t permitted. Sometimes they’ll just yell “NO FLASH” across the gallery in a bored or annoyed voice. Sometimes they don’t care.
When it comes to flash photography around historic documents and objects, the threat comes from the cumulative exposure to light from the camera flashes of many tourists, constantly, for days and years on end. This is in addition to the document’s exposure to any display lighting, natural light, and nearby ambient light.
Light exposure deteriorates many organic pigments. So ink, paint, and dye require minimal light exposure to minimize fading. Light contributes to the breakdown of mediums and binders like egg white (in the case of tempera) or organic oils (hence why some varnishes yellow or become otherwise discolored). Light and the changes it causes to the atmosphere, such as increasing/decreasing temperature, causes structural changes to substrates like wood, paper, vellum, and organic fabrics.
Next time you go to a museum you may notice that paper and textiles are lit more dimly. Manuscript pages are turned periodically if the book is on display to distribute the effect of light evenly across its folios over time. Objects and documents themselves are regularly rotated on and off display to give them a “break” from the elements and help to preserve them longer. These are just several strategies among many others (i.e. air conditioning, humidity control, air pressure) that museums employ to monitor and control the environment around historic objects. They’re developed through the collaboration of curators, conservators, and display technicians.
In short - one flash won’t do anything and won’t get you much other than a scolding from a museum employee. Many flashes for a long time will break down the object’s constituent materials and lead to more severe and rapid damage. If you love this stuff, turn off your flash (but if you goof and forget not to and accidentally take a flash photo no one is going to send you to history or art jail for it).
Source: am an art historian trained in technical analysis of materials and I work with conservators and conservation scholarship in my everyday work.