Copperplate oils are the base carrier or vehicle for traditional printing inks to which pigment is added before grinding to the appropriate consistancy for the various printmaking techniques. Also known as boiled or burnt linseed oil, referring to their traditional method of modification, they are today created in a heated high pressure vessel. The process adds viscosisty and tack to the linseed oil in increasing degrees ranging from weak to strong as heating time is increased. As a rough guide the weaker (thin) to medium oil being the base for intaglio inks and the medium to strong (thick) oil the base for relief and lithographic inks. You can soften and loosen an ink by adding weak thinner oil and increase tack and viscosity by adding stronger thicker oil.