Argentia

"Silver brings ruin to all beasts, but even a brave knight bearing a silver blade may not have the strength to defeat the most dreadful monsters. There is a way, though, to awaken the spirit of a silver sword — by using the moon oil known as Argentia."

Witchers coat their silver blades with Argentia, which increases the effectiveness of silver, but weakens steel blades. Damage inflicted is increased by 60% for silver swords, but reduced if the oil is applied to a steel sword.