I buy my blotter paper at the local art supply store. It comes in 18"x22" sheets, so you can get a lot of 4"x6" blotters out of a sheet. There is a bit more texture and the sheets are heavier than the old style ink blotters but work well. I've been printing on blotters I send out to pen pals and FredRydr will be receiving one in the next letter if I get around to it.
Printmakers use the blotter paper to remove excess moisture from printmaking paper before printing.
Bookmarks