This information was sent to me by Bill Gill:
Chuck, Saw your post on the Westlake forum about concerns over Silverwood fading. I don't know what its made from, but if it includes iron filings like the homemade vinegar and steel wood formulas it probably will fade/change when exposed to light like medieval writing inks made with iron gall.
see:http://en.wikipedia.org/wiki/Ink (scroll down to "writing and preservation" to see comparison of carbon inks and iron inks)http://en.wikipedia.org/wiki/Iron_gall_ink
this link has a less pessimistic view of iron gall inks.
This is a topic that constantly floats in the back of my mind whenever I'm making a model. (Must have rubbed off from working a while in a museum, even though I wasn't directly connected with preservation of stuff),
I'm always dismayed by model RR articles recommending using a fine tip Sharpie to color the gaskets around windows on a locomotive or to touch up small paint chips. Those markers fade pretty quickly in bright lights and fade overtime in less light.
"Permanent" markers or inks are misleading because most only mean that they are water resistant, and have nothing whatsoever to do with their light fastness or not (mostly they are "or not").
I have experimented with some Pigma Micron markers (from art stores, A.C. Moore and Michaels among other places, though suddenly harder to find). They say right on the barrel that they are archival, waterproof and non fading. They come in several size tips and in a red, blue, green and burnt sienna brown as well as black. So far, so good.
I have some really nice Pelikan Drawing inks in a couple shades of brown that can create some great looks on wood, but alas, they too are pretty fugitive. Even some "India" inks have iron in them and can fade. I have (had?) an older article from some old museum publication that studied the fade resistance of a number of brands of India ink because that was/is what many curators use/used to write I.D. numbers (on top of a small white shellac coated area) on the artifacts. The researchers found some black "India" inks faded and others were corrosive as well.
I think (but am not sure) that Pelikan (Koh-i-noor) India ink and Koh-i-noor Fount India #9150-D (a non waterproof India ink for fountain pens) are strictly carbon black inks and will not fade or corrode materials. There are also one or two Higgins India inks that are just carbon black, but I don't know which specifically.
I also experienced first hand a surprisingly different kind of "fading" on the tiny HO layout my son and I are building. A stone bridge abutment has driftwood piled against the upstream side of one end. The driftwood was carefully selected small twigs that had a beautiful silvery sheen that nothing else I've tried closely duplicates. It looked great for several years, but I suddenly noticed that the wood seemed "new", all the gray had vanished and the wood looked raw like it had just tumbled downstream that day (still acceptable but not what was intended). I knew it couldn't have faded even though that side of the layout did see strong sunlight in the winter. A close examination with a magnifying glass turned up some tiny little critters on the wood. They had eaten the silvery oxidized surface right off the wood! I have no idea what they were. I'd heated the wood in an oven before gluing it to the layout, so they must be in the house. A bit of rubbing alcohol did them in and so far all has remained Ok, thought the beautiful natural silver color has not returned.