Table 1.
Combination of double impregnation and heat treatments

Impregnant I	Concentration (%)	Impregnant II	Heating
No Preservative	-	No MMA	90 °C, 4 hrs
Boric acid	5	No MMA	90 °C, 4 hrs
Borax	5	No MMA	90 °C, 4 hrs
No Preservative	-	MMA	90 °C, 4 hrs
Boric acid	5	MMA	90 °C, 4 hrs
Borax	5	MMA	90 °C, 4 hrs
No Preservative	-	No MMA	180 °C, 4 hrs
Boric acid	5	No MMA	180 °C, 4 hrs
Borax	5	No MMA	180 °C, 4 hrs
No Preservative	-	MMA	180 °C, 4 hrs
Boric acid	5	MMA	180 °C, 4 hrs
Borax	5	MMA	180 °C, 4 hrs