In the context of hand blade sharpening, honing oil is used on a sharpening stone to protect the stone, carry away the debris (swarf), and to more efficiently produce a keen edge on a metal blade such as a knife.
[citation needed] In a machine shop it also carries away excess heat and depending on composition, may prevent unintentional tearing and welding of the metal.
[citation needed] To achieve maximum cutting rates and abrasive life with petroleum (mineral) based machining oils when honing difficult materials like stainless steel, a higher level of surface active lubricity agents are combined with sulfur extreme pressure additives.
Oils were once exclusively used in part because the high carbon steels of the time, such as 1095, could rust using simple water-based solutions, and the term honing "oil" is used today even for water based honing solutions.
Importantly, it is also used to "float" off generated swarf and thereby prevent clogging of the sharpening stone, which would diminish its future cutting ability.