Opacity or Transparency – This is the inks ability for light to pass through it. Some printing inks require high opacity – such as metallic inks, scratch off or mirror inks. This quality depends largely on the type of pigment that is used. This property is also referred to as covering power. Other must be as transparent as possible, such as process colors (cyan, magenta, and yellow). This is to allow a combination of colors to perform various colors of the color
Permanence – The ability of an ink to resist fading over time. When the ink is exposed to the elements, it has the tendency to fade. Permanence is a measurement of it’s resistance to this tendency and refers to all elements that could effect it, such as air quality or sunshine. This quality is governed by the lightfastness of the ink.
Wettability – This is the ability of the ink to resist discoloring or bleeding when it is exposed to moisture. This is especially important when offset printers make packaging.
Body – This is the ink’s consistency, whether it is hard or soft. This can be manipulated by the ink manufacturer, or it can even be influenced by temperature. When offset printing ink gets hot in a running press, it tends to be quite runny which relates more to it’s viscosity.
Water pickup – The ability of the printing ink to accept water. When the oil based ink is mixed with water, the ink will accept a certain amount. This is important for running conditions in the offset press. (http://offsetpressman.blogspot.com/2011/02/water-pickup-values-and-what-it-really.html)
Viscosity – This describes the degree of the ink’s resistance to flow. Water will flow freely but honey will flow slowly. The difference between the two is their viscosity. This range goes between being very viscous (flowing freely) to very inviscid (flowing freely). This property can vary in offset printing. For example sheetfed presses that use quickset inks tend to be quite viscuous. UV curing inks tend to be quite inviscid. Each has it’s own viscosity reading measured in a unit called a poise.
Length – The ability of the ink to form long strings when stretched. The longer the strings are between the ink and the surface that is being lifted, the longer the ink is. This is an important property as it will determine how well the ink will flow through the offset printing press. Inks that are too short (similar to a butter consistency) or undesireable as are long inks. The right value will run smoothly between all the rollers in the offset press.
Tack – In simple terms, this is the stickiness of the ink. It is a measurement of the force required to split an ink film. This is important to offset printers. When printing colors in succession, each successive color being printed must be slightly sticker (or higher tack). This is so as to pull the ink off of the printing blanket and onto the successive layer of ink. This is called trapping. Inks will too little tack cannot accomplish this. Inks with tack that is too high wrap up in cylinders of the offset printing press. This happens when the tack exceeds the surface strength of the paper.
Gloss – When ink dries, there will be a certain amount of gloss to it’s finish. As a rule, the more porous the paper is the lower the gloss. Therefore coated paper tend to have high gloss inks. This is because the paper will absorb most of the ink vehicle that promotes gloss. Ink manufacturers will sometimes add varnish to their ink formula to enhance the glossy look.