Ink Chemistry

    Kids, did you ever wonder why newspaper ink comes off all over your fingers? Okay, maybe you haven't read too many newspapers yet, but now is a good time to start. Open up a newspaper, read the headlines, flip every page, and re-fold every section. By now you should be good and covered with black ink. Why doesn't that happen when you read a book or a magazine? Or this newsletter? Since we learned all about pencils in last month's column, now is a good time to learn something about inks.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    On the face of it, ink is simple; it is a pigment or dye that is dissolved or suspended in a liquid (solvent). This is pretty much the same thing as paint. There are two classes of inks, called printing and writing inks. The two main printing inks are for regular printing (using mechanical plates) or for digital printing (like in ink-jet printers). Writing inks are what you would expect - the type found in pens.

    First let's talk a little more about printing inks. Black printing ink is carbon black in a solvent. Color printing inks use organic (C,H,N,O-based) pigments, usually nitrogen-containing salts of dyes with fancy names like "yellow lake", "peacock blue", "phthalocyanine green" and "diarylide orange". A few inorganic (non-carbon) pigments are sometimes used, like chrome green (Cr2O3), Prussian blue (Fe4[Fe(CN)6]3), and cadmium yellow (CdS). The solvent for these dyes is usually linseed oil, soybean oil, or a heavy petroleum distillate. White pigments such as titanium dioxide (TiO2) can be used to adjust the colors also.

    In terms of writing inks, the older fountain pens used water-based solvents. In the 1950's, ballpoint pens were developed that use pastelike, oil-based dyes. The thickness allows capillary action to keep the ink flowing evenly, are non-smearing, and quick-drying. Water-based solvents are still used in markers, highlighters, and rollerball pens, though.

    So, back to our original question. Newspapers are usually printed with mineral oil ink at a very fast rate (like 3,000 feet/minute!). When other media are printed, like books or magazines, linseed oil inks are allowed to dry in the air. Or, paper printed with inks made from alcohol- or petroleum-based solvents are heated and evaporated to dryness. Because newsprint can't be allowed the time for heating and drying, the ink is absorbed by the inner fibers of the paper, and always sit there a bit damp, never evaporating completely. Also, the type of paper used is different (see 11/96 "ChemShorts").

    Are you curious about how much ink it might take to print a page like this one? A regular ballpoint pen seems to last a really long time before the ink runs out (usually we have lost the pen long before that happens). A magazine of, say, 80 pages needs 68 gallons of ink for 150,000 copies. This works out to about 20 microliters (which is 1/1000th of a milliliter) per page - a real bargain. After washing the newsprint off your hands, gather up a few different kinds of pens, highlighters, and markers, and create a piece of artwork while thinking about the chemistry coming out at your fingertips!


    Kathleen Carrado Gregar, PhD, Argonne National Labs
    May 2003


    Reference: Steve Ritter, Chemical & Engineering News, ACS, 11/16/98, 76(46), in the "What's That Stuff?" column. (