Introduction
While I was working the booth at the PMA tradeshow last year, an attendee approached me with a question I’ve been asked a hundred times before. “Are these water-based inks or pigment inks?” I gave my usual response,“Well, both—actually,” then launched into an explanation about ink formulation theory, the history of inkjet printing, and the correct application for ink types. He proceeded to tell me I was wrong because someone at a printer manufacturer’s booth had just told him the opposite. He finished with a curt “thank you,” and marched off down the aisle.

It occurred to me that there has been too much misinformation doled out to the unsuspecting inkjet user. With the industry in a state of flux over the transition from traditional to digital photography, these half-truths have not helped with the acceptance of new technologies. This article is my attempt to put the record straight.

Basic ink theory
An inkjet ink formulation can be explained fairly simply (Figure 1). It consists of a carrier fluid that keeps the ink in a liquid state and acts as a “carrier” for the colorant. This fluid evaporates as the ink dries, leaving the colorant in or on the media surface. The carrier fluid used in most popular desktop and wide-format “photo” printers is water; the colorant is either dye or pigment, or a combination of both. A co-solvent, usually glycol or glycerin, is often used to control the ink’s drying time, as well as its viscosity during manufacturing. Small amounts of other additives are also present in most inks. These additives help control things like dot gain, drop formation, print head corrosion, pH level, fade resistance and color brilliance. However, in simple terms, we can think of ink as being a carrier fluid and a colorant.
The difference between a dye and a pigment is simple to explain. A dye is a colorant that is fully dissolved into the carrier fluid, and the resultant ink is a true solution (Figure 2a). Once dissolved into the carrier fluid, a well-made dye should never separate or settle out. One dye ink analogy is that it’s like diluting a fruit juice with water. Once stirred, it never separates again - no matter how long you leave it. A pigment is a very fine powder of solid colorant particles suspended or dispersed throughout the carrier fluid (Figure 2b). A crude analogy for a pigment ink is that it resembles sandy-colored water in a river or at sea. If you look closely, you can see the particles of sand dispersed throughout the water. The secret in making a good pigment ink is to keep the pigment suspended in the carrier fluid for a long period of time, particularly at the low viscosities required for inkjet printers. Pigments have a natural tendency to settle out much like the sand in the sandy water analogy.

One photographer’s test
An interesting story told to me recently is from a respected photographer who, on receipt of his new wide-format photo printer, decided to conduct a test. He installed the pigment ink cartridges that came with the machine and printed a standard color chart test print. Then, he removed the cartridges and stored them (along with the test print). He installed another set of OEM (original equipment manufacturer) pigment ink cartridges and went about printing his images. A year later, he re-installed the original set of cartridges, and printed the same color chart test print using the same media, print file and printer settings. Comparing the colors of the new and original print with a densitometer, he found the new print’s color was 50% lower than the original. He figured this was from the pigment settling out of the ink during storage. To prove his theory,he removed the cartridges from the machine and gave them a vigorous shaking. He re-installed the cartridges, and made a new test print. He discovered the color had returned to 90% of its original level. An interesting example of how pigments can drop out of ink over time, particularly when you consider this ink was from a major printer manufacturer. Fortunately, recent advances in pigment preparation technology and dispersal techniques now give water-based inks at least an 18-month shelf life without significant settling.

The ‘Golden Rule’ of inkjet printing
There’s a rule that says “when any inkjet ink dries by evaporation, the dried ink must be able to be re-dissolved by the liquid form of the same ink.” Sounds pretty simple, right? This concept is vital for an inkjet printer to function reliably. Let me explain. Most photo-quality inkjet devices use print heads with hundreds of very small jets that eject the ink onto the media. Liquid ink is always present at the jets’ opening, and if you use an ink that dries by evaporation, there’s a good chance the ink will dry up and clog these openings. The challenge is to manufacture a printer that doesn’t clog during the printing process, but nonetheless has fast-drying output.

If we take a closer look at a single jet on a print head with dried ink clogging it (Figure 3), we see that the easiest way to remove the clog is have the liquid ink redissolve the dried ink. Another way of doing this would be a separate washing system that bathes the head in an “ink dissolving” solution. However, this adds considerably to the printer’s complexity and price. It’s much simpler to stick to the concept above, and depend on the ink to “unclog” itself. One common misconception is that when a jet (or jets) clog on an inkjet printer (evidenced by lines or banding in the print), it’s caused by an impurity or “lump” in the ink. In 99.9% of the cases, the clogged jet is caused by dried ink or an air bubble in the ink. Air in the channel behind the jet on a print head is a sure way of stopping the jet from firing correctly, since the air acts as a shock absorber to the firing action.