In a method of ultrasonically bonding paperboard, first and second pieces of paperboard are supplied, with at least one of the first and second pieces of paperboard being provided with a clay coating. The first and second pieces of paperboard are moistened and positioned in a gap between a sonotrode and an anvil of an ultrasonic welding unit. The ultrasonic welding unit is activated and the first and second pieces of paperboard are compressed between 30-70%, more preferably between 40-60%, to cause the clay coating to mobilize and force the clay coating material into fiber matrixes of the first and second pieces of paperboard, followed by cooling of the first and second pieces of paperboard, to bond the first and second pieces of paperboard.