Dampening and Fountain Solution
In convention offset printing, there is a constant balance made between ink and water. In the printing process, the image is passed by the special chemical properties of the printing plate itself. In some areas, it is hydrophobic and in other areas it is hydrophilic. These two terms essentially mean that the plate will attract or repel water. In the areas that are hydrophobic, the plate will pick up ink from the ink-train. In areas where the plate is hydrophilic, the plate will pick up water and repel the ink.
In single color printing, there is a single dampening system. As additional printing units are added for more colors, each printing unit will contain its own dedicated dampening system. Often these individual dampening systems are plumbed together to share a dampening solution. This is done for a couple of reasons: the first of which is ease of use for the operator who can keep a single reservoir full. The second reason is consistency - with a single reservoir of fountain solution, the operator only needs to maintain a proper mix of water and fountain solution, and each of the printing units will have the same formula.
The dampening system consists of several major components. The first and most noticeable of which is the fountain. This is typically the most exterior part of the system, and it's where the fountain solution enters the system. In smaller presses, the fountain is a small tank with a bottle. In larger presses, the fountain solution is pumped into a trough. Within the trough would be a fountain roller that will sit partially submerged in the fountain solution in order to pick up the fountain solution and roll it into the water train. From there, a complex system of rollers will meter out the fountain solution and ultimately reach a set of form rollers, which physically touch the printing plate.
In the early years of printing, fountain solutions were comprised of mostly alcohol. Alcohol exhibits the desirable property of being extra wet. How can something be more wet than water? To answer that question would get into some rather complex and confusing principles of physics, but, to sum it up quite nicely, we can talk about surface tension and dispersion. Alcohol has a much lower surface tension that straight water; that property allows it to flow out into a larger pool and evaporate much more quickly. Surface tension is what causes water to bubble up in certain situations. Think of a cup of water filled right to the top; water would be able to actually bubble over the top just a slight amount before the surface tension broke and the water spilled over the edge. Alcohol, with its lower surface tension, would flow right over the edge as soon as the liquid level rose above the edge of the cup. This property allows the fountain solution to meter out into the water train much more easily and predictably when compared to water, which can sometimes behave unusually.
Fountain solutions in modern printing have become a sophisticated set of formulas for a multitude of reasons. The first of which is environmental; alcohol, as it evaporates, releases fumes into the air, which can be somewhat toxic to nearby operators and the environment. Another reason why it has been phased out of the printing process is that there are other chemicals that mimic the properties without the negative aspects. Also, other chemicals are introduced into the mix in order to fine tune the chemical properties and behavior of the fountain solution in action on the press.