A paper document, as it ages, has 4 effects of note that are standard:
- it has acid degradation
- it has in ink migration or ink/toner de-fixation
- it suffers in oxidation of ink
- it becomes brittle
Parchment, being a subset of leather, has similar issues, but in different order:
- Ink migration
- Ink oxidation
Vellum can be either a type of paper (modern vellum), plastic, or leather.
These are in fairly standard order; high quality low-acid rag bond swaps acid degradation and ink migration in sequence. And these are just the acts of age alone, not abuse.
Acid degradation is a yellowing, browning, or reddening of the paper. Quite literally, the paper changes color due to the acids in the paper and the environment altering the chemical nature of the fibers. This can be mimicked with dying in watercolors really low quality papers, by baking in a particular mix of chemical vapors, or by soaking in acids, especially tannic acid. Note that low acid papers next to high acid ones suffer acid migration - as the acidic paper suffers acid migration, it also releases acids into the neighboring paper, which begins to suffer acid degradation as well. A single sheet of low quality paper in a box of archival bond can cause acid damage up to several millimeters per decade away.
Ink migration is when a saturation ink (most writing inks, many printing inks) expands through the paper over time. This happens faster with water soluble inks in humid climates, including most hand-written documents. At low levels, it is almost imperceptible; I've handled 110 year old hand written documents on quality bond that had no perceptible ink migration, and 10 year old ones exposed to humidity that the ink formed a shadow line half a millimeter from the original written line.
Ink or Toner Defixation: certain forms of printing involve depositing inks upon the surface, bonded to the surface of the paper. Intaglio printing, lithographic printing, wax-spray ink-jet, plastic or wax ribbon typewriters, thermal transfer printers, carbon paper, and laser printing all work this way, each using different means to affix their ink. if the paper is in the right temperature and humidity conditions, the ink (or laser printer toner) can lose its bond to the paper, literally falling off. This is mostly going to apply to items of the 19th and 20th centuries, but intaglio printing has been in use since the 1430s, and lithography is older still. some forms use wax-based inks, which can readily detach. Individual letters, parts of letters, and sometimes whole words may simply be missing from documents suffering defixation. Note that wax-carbon copies usually drive wax deep enough not to have this happen, but on high-clay papers (which tend to be glossy), it can and does.
Oxidation of inks can change their colors. The vibrant reds and greens often become browns; the blacks can become brown, red, or even blue or green, depending upon the chemicals used to make the color. This is a particularly slow process, but accelerates in high acid papers. Iron based black inks usually turn red-brown.
When coupled with acid degradation, old paper and old iron ink results in red-brown fuzzy letters on pinkish brown paper. Certain documents, in under 100 years, can become completely indecipherable to the human eye.
Finally, as paper ages, it often dries out and suffers chemical changes. As it does so, it becomes less flexible. Eventually, it becomes inflexible enough to be described as brittle.
All of these effects can be replicated visually with acidic solutions in an oven. Tea is popular, because it generates a relatively even acid damage in short order, migrates water-soluble inks, and brittles paper in a matter of a few hours.
Distress is the term for things that happen other than the chemical changes of age.
Most paper or parchment documents of great age have edge wear. Mimicking this with a single sheet isn't all that easy, but stacks aren't terribly hard. Clamp a bundle between two boards, right near an edge, and sand lightly. This softens the edge. A much read document will have this happen far faster than one referenced only rarely. If you're doing oil damage, alternate sandings and oil. Ideally, you want a slightly frayed edge.
Cracking. Once the substance begins to become brittle, routine page turning can result in surface cracks. Emulating this requires careful cuts with razors. This can be triggered by baking and flexing, as well.
Moisture stains. Until relatively recently (17th C), most documents were read aloud. This results in spittle spray, and small directional dots which are more acidic. This is easily replicated. Take a sip of coffee, dark tea (herbal works fine), or soda, then speak loudly with the paper about 6 inches from your chin, at about a 30° angle from flat. If at all possible, emulate the techniques of firebrand preachers, charismatic dictators, or angry politicians, to get that spray. A more cautious approach uses the same substances, and a stiff paintbrush. For best results, use multiple concentrations, then while wet, bake at 300° for a bit, in order to get the proper pucker. For a multi-page document, at least some pages should be pressed together to transfer to the backs.
Smoke and fire damage. Generally, most medieval documents will have some smoke damage, due to fire-based light sources. A great many survived fires. This is easily done using a barbecue grill. Set the document in a pan, which itself is on a second pan, and set it to smoking. For good directions on how to set up a grill to smoke, or an oven, for that matter, any cooking show by Steven Raichlen will have good directions. To actually show burn damage, carefully, in an area away from flammables, light the edges and put them out. In either case, keep a CO2 fire extinguisher to hand and a steel bucket; it won't damage the paper much, but will put out the fire; drop the document in the bucket and fill with CO2.
Finger smearing. Fingers have oils, and premodern readers often used a finger to follow the texts. Further, edges often accumulated finger oils. Further still, many books were read by people shortly after eating greasy foods. Get oil on one's fingers, and rub the edges. Oil them, and run them along the text like an elementary student. Thump at important points. Extra oil at paragraph starts is a good trick, too, as not everyone followed the text; some just kept track of which paragraph they were on. Be careful not to use too much. Oil oxidizes to dark smudges; baking can increase the apparent age.
Spills and rings. Modern and near-modern documents tend to have spills and rings as a form of distress. Take a beverage container, and dip it in a fluid of choice, then set on the paper. Nothing makes a better coffee-ring than a genuine coffee ring. A fresh wet one promptly baked at about 110°C will nicely pucker.
Tears and Rips. These are easily overdone. Most historic tears and rips are in books, not loose documents, and tend to be towards the spine. Some loose documents were cut, ripped, or cracked loose from a book; these will show spine side damage, often including binding holes ripped through. Staples through cardstock, then pulling loose, can emulate this beautifully.
Crumpling. This is actually rather rare for historic documents other than identity papers. Documents were, until the 19th century, fairly few, and usually kept in good order. It can, however, mimic the natural cracking patterns of old parchment.
Folding and corner wear. Carried documents were often folded. So, fold it, then sand the edges lightly with fine grit sandpaper; hit the corners harder. Oil the edges as well. When unfolded, the characteristic fold-damage will show through. While folded, flex it as well. Steam it a bit, and flex it.