Plate oils, sometimes referred to as copperplate oil, burnt plate oil, or litho varnish in lithography are the main ingredient in all printmaking inks besides pigments. Derived from linseed oils, processing has become less hazardous over the centuries, and it is no longer set on fire in vast vats (hence the name burnt plate oil) but rather heat polymerised to create stand oils of varying viscosities to suit the ink’s end use.
There are different measures for oil viscosities used across the world, from European ‘poise’ to US numbers but we will use weak/light for thin oil and strong/heavy for thick oil. The thicker the oil, the higher the viscosity/tackiness of the oil. Different viscosities give etching, relief and lithography ink their specific properties, with combinations used to create each ink.
To illustrate, a lithography ink will require a strong oil with high tack. This is to hold enough pigment in a very thin film of ink and to transfer to the paper as a strong colour. For etching a low tack, weaker oil is better. This is because the ink requires pushing into the etched lines and wiping from the plates surface, and the ink in the lines needs to transfer easily to the paper. Relief ink is usually made from a strong oil combined with a weaker oil to soften somewhat. A thicker film of ink is required than with lithography, but the ink still needs the right amount of tack to roll onto the block.
When it comes to adding oils to your inks to adjust them, the basic principle is that a light/weak oil will soften the ink and reduce tack, and the heavy/strong oil will increase tack and viscosity.
