Epoxy adhesives are thermosetting resins which solidify by polymerisation and, once set, will soften but not melt on heating. Two-part resin/hardener systems will solidify on mixing (sometimes accelerated by heat), while one-part materials require heat to initiate the reaction of a latent catalyst. Epoxies offer very high shear strengths, and can be modified to meet a wide variety of bonding needs. Generally, epoxy bonds are rigid; they fill gaps well with little shrinkage. The properties of epoxy adhesives are also varied through the use of fillers. For example, quartz fillers can impart improved impact resistance, ceramic fillers can offer improved abrasion resistance and silver can be used to produce epoxies which are electrically conductive.