Abstract: Sand cores are bonded to form an assembly, using paste usually containing clay or silicates. Bonding is carried out by (a) coating at least one interfacing surface of intermating core parts with paste in a thickness of about 0.005-0.20 inches; (b) mating the coated interfacing surface of one core part to the other core part to form an assembly; and (c) subjecting the paste in the assembly to microwave energy at an energy level of about 5-7 kilowatts to effectively dry the paste in a manner proceeding first from the interior of the paste body to its outer peripheral extremities. The subjection to microwave energy draws the core parts closer together by shrinking the paste coating and by causing the paste to boil and drive the paste into pores and/or voids of the intermating core surfaces; such subjection is preferably carried out for a time period of 10-30 seconds.