Offset ink is a specific type of ink used in conjunction with offset printing presses such as those used to produce letterpress or lithography prints. Offset ink needs to be very rich in pigment so that its full color vibrancy is perceptible, even in minute quantity. It is a paste rather than a fluid. Most offset inks dry rapidly without the need for additional equipment, but there are also inks optimized for infrared, ultraviolet or electron beam curing. All offset inks contain water-repellent materials, needed because offset lithography is based on the fact that oil and water do not mix.