There are plenty of resources defining lightfast but I wanted to know what it looked like when something wasn't lightfast.
I wanted to use brown ink as a sketch tool by filling a Niji Waterbrush with it. I was told by one of the sales people at the art supply store that pigment-based ink would clog up the brush and that I should use dye-based ink instead. But I really wanted to use the pigment-based ink because, from what I read, I knew it was more lightfast than the dye-based ink. I wanted to see for myself what the difference was.
I did an experiment based on one of the lightfast tests described in "The Painter's Handbook, Revised and Expanded" by Mark David Gottsegen, 2006, published by Watson-Guptill Publications, New York. I deviated a bit by using thick, white, acid free paper from my sketch book instead of rag paper. On the paper I scribbled two rows of graphite pencil. Then all the way across one row I painted some pigment-based ink and then going down I created a wash with the ink and water. I repeated this process on the second graphite row with the dye-based ink. Next I cut the sheet in half. I taped the loose half in a south facing window where the ink and paper would be hit with the most sunlight. The other half stayed in my sketchbook out of the light.
I checked the strip after about 24 hours and was surprised to see that the dye-based ink had faded to a value of about 60-70% from the original dark brown. The pigment-based ink had a barely noticeable loss of color or value.
Day 2 and 3 showed no noticeable loss in either strip.
Day 4 the pigment-based ink still didn't appear to have changed much. The dye-based ink appeared to have lost a lot more color and value. It was down to about 30% of its original quality.
Day 5, 6, and 7 the pigment-based ink still didn't have a visible change but the dye-based ink appeared orange. Wow, how interesting. I was not expecting that.
I left the strip in the window for over a month and there wasn't much noticeable change to the colors after day 7. The paper did get lighter and possibly a little yellow compared to the half I left in the sketchbook.
I don't take this result to mean that all my sketches done with pigment-based ink will last forever. There are many variables that affect the longevity of a picture besides the lightfastness of the pigments used. I interpret the result as: if I want my sketches to have a good chance of looking as I intended for a long time (if I put any of them on display) I should stick with my original thought of using the pigment-based ink.