Screen printing isn't exactly a new process - it dates back at least a thousand years to China when silk screens were used for the mesh. You will be pleased to know that silk is no longer used outside purist craft circles, although the process is still sometimes referred to as silk screen printing, and the screens themselves are often called silks.
Essentially, the process is similar to the stencilling you might have done as a child. You create a gap in a solid card, you can paint over the whole card, and when you lift it off, only the part that was a gap will have been painted.
You have probably also seen the military stencils that are used to quickly spray numbers and letters onto tanks. If you look closely, however, you'll see that the characters aren't whole - there are gaps in any letter that has a "counter", which is the name for an area of the character that's completely surrounded by ink, such as in the characters A, B, O, g, q, 4, 8, and 9.
Obviously, a stencil can't have a piece of card with the counter floating in mid-air, so in the stencils, they need to be joined to the rest of the stencil with a "bridge". With screen printing, however, that isn't necessary, as the whole stencil is held on a fine mesh, a bit like your kitchen sieve that you didn't wash quickly enough, straining sauce through it. The mesh matrix means you can have any shape you want, and there's no need for bridges. Now, you can effectively "paint" over the whole mesh, and the parts that are covered with stencil won't be inked on the fabric underneath, but the gaps will.
Historically, the required shapes were painted or glued onto the silk mesh. They form a "negative" of the required image, so if you wanted a circle, you'd block off the area surrounding the circle on the mesh.
Nowadays, the process is done in a similar way to photography. The mesh is coated with a special emulsion that reacts to light, and a transparent acetate with the image printed on it is placed on the emulsion-coated mesh and exposed to bright light.
Where the light reaches the emulsion, there's a chemical reaction, and it instantly hardens and forms an impervious negative of the image. The parts hidden from the light by the acetate's printed areas will remain liquid and can be simply washed away. What you're left with is the negative of the image - gaps where the ink will go and dried emulsion where it won't.
For multiple coloured images, there are two main ways of screen printing. If the colours are in simple blocks, you can make separate screens for each colour and print them separately. Imagine the French flag, for example. You'd need three screens, each containing a rectangle positioned left, central and right, and you'd print the blue, white and red separately.
If the image is more complicated, you can use a process similar to how colour newspapers are printed or how pixel-based computer screens work - that is, using RGB (red, green, blue) or CMYK (cyan, magenta, yellow, black) inks.
The colour image is scanned into a computer, and the image editing software separates it into the colours that combine to make the final image, so for RGB, you'll have one screen that's just red, one for green and one for blue. Only when all three images have been printed will the final full-colour picture emerge. The three screens must be positioned with pinpoint accuracy for this to work.
There are other effects available, such as halftone (basically different-sized dots to give texture, like Roy Leichtenstein's comic book art), greyscale and duotone, but the ones above are the most common.
The actual printing is pretty simple. When you're hand printing, you'll pour a line of thick ink at one end of the image, then spread it out like butter over the whole screen with a squeegee. Next, you'll pull the squeegee back, pressing down onto the fabric, and the ink will seep through the mesh where there's no solid emulsion and leave that colour on the fabric. The ink is heat cured to make it permanent and washable.
The process will need to be repeated for each other, whether it's in blocks or CMYK/RGB. With a machine, the process is much quicker, and tens of t-shirts can be printed in a minute.
The same screen can be used multiple times, which is why it's cheaper to get larger runs per unit. Even the emulsion can be chemically removed after you've finished with it, so the mesh can be reused, making screen printing an efficient and environmentally friendly way of getting colour images onto all sorts of garments.
