Zinc oxide eugenol (ZOE) is a material created by the combination of zinc oxide and eugenol contained in oil of cloves. An acid-base reaction takes place with the formation of zinc eugenolate chelate. The reaction is catalysed by water and is accelerated by the presence of metal salts. ZOE can be used as a filling or cement material in dentistry. It is often used in dentistry when the decay is very deep or very close to the nerve or pulp chamber. Because the tissue inside the tooth, i.e. the pulp, reacts badly to the drilling stimulus (heat and vibration), it frequently becomes severely inflamed and precipitates a condition called acute or chronic pulpitis. This condition usually leads to severe chronic tooth sensitivity or actual toothache and can then only be treated with the removal of the nerve (pulp) called root canal therapy. The placement of a ZOE "temporary" for a few to several days prior to the placement of the final filling can help to sedate the pulp. It is classified as an intermediate restorative material and has anaesthetic and antibacterial properties. It is sometimes used in the management of dental caries as a "temporary filling". ZOE cements were introduced in the 1890s.