4.4. Comparison of dielectric properties of various solidsA comparative data of εrand tan ı for some common solids istabulated in Table 5. These properties were measured at 2.45 GHzfrequency and at ±20 to 25◦C. Generally, solids reveals lower εrandconsiderable variation in tan ı. The εrfor wood, biomass and plasticwere found to be nearly similar. However, tan ı is much lower incase of plastic, glass, and metals compared to wood and oil palmbiomass. Thus, plastics and glass are transparent to microwaves,whereas metals are the reflector. These materials do not absorb themicrowave energy and hence cannot convert it into heat. In con-trast, wood and biomass materials (Salema and Ani, 2011) are ableto absorb the microwave energy to some extent because of mois-ture content in the materials and convert it into heat. However, thisheat is not enough to pyrolyze them and thus, are considered to bepoor absorber of microwaves (Krieger, 1994). Our previous (Salemaand Ani, 2011) research explains that oil palm shell and oil palmfibers biomass materials can barely reach the temperature of about125◦C and 95◦C respectively as discussed in subsequent Section 4.5 of this paper. The dielectric properties of oil palm biomass andbiochar are close to that of wood material (from Table 4), becausethe physical and chemical properties of biomass materials resemblecomparatively to that of wood.