Generic term that indicates various species of natural resins deriving from the excretion of certain plants, both alive and in fossil state. Copal resins are distinguished from other natural resins due to their insolubility in the majority of solvents. It is soluble in oils when left for a long period of time and heated at a high temperature: in this phase the resins undergo a chemical process. The varnishes that have come from this process are particularly hard, resistant and brilliant, yet present the inconvenience of darkening considerably in time, that may decisively compromise the conservation of the painting due to the great difficulty of removing the resins, due to its insolubility. Another inconvenience is caused by the strength of the resins, as it easily provokes cracking in the pictorial film.