Regim Ignaul Neratum, more commonly known as Bone-Tar, is a dangerous transporting agent used in artificing.
Bone-Tar is an extremely volatile and corrosive substance. If it were to be spilled onto the skin it would be able to eat through the bone in about ten seconds. The agent boils at room temperature, making it important that it be transported in a chilled container. It is also important it be transported with a pressure cap as it is easily evaporative. As a gas it exhibits surface tension causing it to have a thickness similar to mercury, that does not dissipate. Once it warms sufficiently it will burn on contact with air, cascading into an exothermic reaction. It is heavier than oxygen and also coheres to itself.
In The Chronicle
Kvothe first encounters Bone-Tar in The Artificery where Kilvin introduces it to the students with a small performance in which he burns a small vial in the firewell. Despite the danger the substance poses it is extremely valuable as it can be used to make high priced sympathy lamps that emit a blue light instead of the traditional red. Shortly after completing his apprenticeship with Manet, Kvothe decides to make a batch of the blue emitters so that he might make back the money he spent on building his "thieves lamp," it is at this time he notes the container for the substance as too frosty. When his concern is assuaged by a more experienced artificer, Jaxim, he moves on with his project. However, a few hours later the container is compromised due to the previously noted coolness and cracks, leaking the Bone-Tar onto the table and thus destroying it. When the table gives way the entire canister falls and breaks, exposing the bone-tar which quickly catches fire. Due to the grating system featured in the workshop the liquid travels in two different directions trapping Fela behind a wall of flame. Kvothe notices this and quickly leaps to action, he makes a sympathetic link with the twice-tough glass of the drench and destroys it, soaking himself before traversing the flames to rescue her. However, during the rescue he inhales too much smoke and upon completion of his mission is rendered unconscious, waking up sometime later in the Medica. It is this incident that causes Kvothe to miss his date with Denna at the Eolian, where she ends up meeting her Patron, Master Ash. However, this freelance also leaves Fela feeling indebted to him, a fact he later exploits when he finds a secret entrance to the Archives and also when he is poisoned with the plum bob by Ambrose. It also further adds to his already egregious reputation.