This site is about printing t-shirts, but there's an underlying agenda: twenty years ago the fabric printing industry was compelled, for environmental reasons, to stop using solvent-based colors. What resulted from this shift was a class of water-based printing inks that are safe and easy to use.
Two kinds of inks are generally available for fabric printing: solvent- based, and the (much newer) water-based products. Most T-shirts are printed with with plastisol, a solvent-base cousin of vinyl that facilitates large-scale production because it only dries when it's baked: because of this feature shops can let ink sit the screen, but are able to print a shirt and ship it in three minutes. These kinds of inks are generally toxic in the long run--mostly because the solvents evaporated out of the ink as well as those used for cleanup are often carcinogenic, usually bad for your liver and your central nervous system, requiring worker safety precautions, air scrubbers and special disposal schemes.
On the other hand, most of what are called piece goods--the stuff in fabric stores--are printed with water- based systems. These don't require special handling of any sort. Waterbase cures by oxidation (air dry), doesn't release toxins into the air, and screens can be washed out in the sink.
(you can put water-base washout on plants, which is a good alternative to putting it into the sewer).
Most waterbase colors need to be heat-cured if you want to wash them right away, which can be accomplished by tumbling in a hot clothes dryer. This facilitates crosslinking or polymerization, bonding the colors to the kinky cotton fibers (man-made fibers such as polyester, which look like monofilament fishline, aren't a good surface for these polymers to bond with, which is another reason why plastisol has become so universal: it bonds like hot-melt glue. An alternative to heat setting is to add a catalyst, which also helps with polyesters.
It's important to remember that this is the same chemical process that makes compounds like dried mayonnaise so hard to remove from fabric. Mayonnaise is an emulsion, a whipped-up mixture of oil and proteins. As this mess dries and combines with atoms in the air it forms big molecules that tangle up with the textured silk fibers in your tie. Waterbase colors--also known as pigment emulsions--utilise this process to bind particles of color to fibers. Properly cured, which in some cases can be accomplished as easily as letting the material sit in the air for a week, the colors will stay bright while the shirt falls to pieces from age.
Over time I've begun to take seriously the caveat printed on every container of fabric color, "wash fastness is not the manufacturers responsibility". Since a run of shirts usually starts with a couple of practice prints (I try to keep track of my family's obsolete shirts for this purpose) it's easy to be continually testing your color chemistry by throwing practice prints in the wash.
Another appeal of waterbase color is that the print is nearly as soft as bare fabric. This is because the colors are more intimately bonded to individual fibers than plastisol, which is actually the same material used for some kinds of heat-transfer prints. Plastisol transfers have been popular with stores because of retailers' reluctance to commit to preprints that they might get stuck with. The aestetic appeal of a continuous, badge-like layer of color is that it can be very opaque, which in particular suits the large black-only segment of the shirt market such as rock concert shirts and biker gear. There's a price to pay for this combination of look and convenience, the loss of what tha garment industry calls "hand", the softness and breathability of fabric.
Waterbase handles differently than oil-base inks, mainly because it's runnier, which means that it soaks in rather than sitting on the surface. For this reason it's usually more difficult to print on dark materials, although some products, most notably from Matsui, are excellent in this regard (and, amazingly, in air-cure washfastness).
Waterbase generally requires a thicker application for good coverage, which can be effected by using a more open screen mesh , 80-160 threads per inch. (It's actually the thickness of the individual fiber that
determines coverage, for two reasons: percentage of open area plus the fact
that to some extent the thickness of the printed coat is equivalent to the thickness of
the screen.) Screen mesh is available in monofilament (like fishline) and multifilament
varieties. Monofilament is popular because it's easier to push ink through, but it costs
about three times as much as multifilament, which works fine with waterbase--a little
resistance actually helps control the aforementioned runniness.
Most screenprint suppliers carry some kind of waterbase textile colors. They're all
different, probably because the market ranges from at-home to mega-industrial. The
best working properties I've encountered are from product lines originating in
countries where textile design is taken seriously: MATSUI (Japan), and
MANOUKIAN (Italy). The latter is supplied as a clear base to which pigments need to be added which makes it less convenient to work with, however it has a mysterious compensatory virtue: it never dries in the screen
(This is a big nightmare of screenprinters early in the learning curve. The best way to eliminate the problem with waterbase is to get some control over the room's relative humidity, for instance with a coldwater mist-generator of the type used to relieve coughs, or simply a hand mister. An even more basic way to combat the problem is the flood coat, where the squeegee is used to fill the screen's open areas while it's raised off the printing surface. It's also used to insure a heavy deposit, which is usually needed with waterbase).
MATSUI, for it's part, offers a line of opaque colors (4T series) that competes with plastisol on its own terms: On cotton or 50/50 it doesn't have to be heat cured, it's easy to work with and resists drying in the screen. Using 85-100 mesh, a single layer of white will dry completely opaque on black fabric; alternatively it can be flash cured with hot air, which allows use of higher mesh counts and subsequently finer detail. Every other fabric ink I've tried to print on a dark background, including oil-base, invariably shows the background, because the color sinks in as it dries. Matsui 4T has the same opacity as acrylic artists colors--it dries to a soft, flexible print--much like plastisol--that will outlast the fabric it's printed on.