Here are some suggestions for archival pens to sign your inkjet prints.
The ideal pen will be labeled archival and indicate that it uses pigment based ink. Black ink is recommended because black is generally the most stable color. In our experience, your final choice will come after a number of tests. Keep in mind that archival pens are usually tipped - they have a nub that delivers ink to the paper. Unlike rollerball pens, tipped pens take more practice to use smoothly. Order a number of pens and practice with each until you find the best for your writing style.
Glossy, satin, luster, or semi-gloss inkjet papers (photo papers) require pen as pencil will not work. Most photo papers are resin coated (photobase) material. They are coated with plastic on both sides and feel like a photo lab print. In most cases, one side is coated for printing and the other is uncoated plastic. You should not sign the back of resin coated paper. Archival pens will not dry and will easily smear. Sharpie markers will work but as far as we can tell, Sharpie ink is not considered archival.
Sakura Pigma Micron
Pigma ink is derived from a single pigment to ensure color consistency, and is fadeproof against sunlight or UV light. Pigma inks will not clog or dry out like most mechanical pens. Available in a number of line width sizes and colors. Shop for Sakura Pigma Micron pens here
Steadler Pigment Liner 308
Fineliner for writing, sketching and drawing. Long metal tip, ideal for use with rulers and templates. Pigment ink is indelible (in accordance with ISO 14145-2), lightfast and waterproof. Shop for Steadler Pigment pens here
Pencils are the most popular way to sign, date, or otherwise label prints made on inkjet matte or cotton fine art media. Look for "carbon pencils" which allow for smooth writing and darker, richer lines.
One example is the General's Layout No.555 carbon pencil. You can find these at Shop for General's Layout No.555 pencils here and other art supply stores.
A number of customers have recommended Prismacolor Verithin pencils. The model 747 black and 747.5 Cool Gray are popular.