The mean pore number of conventional open-celled PUR foams having at least 8 ppi can be significantly reduced if the foam is stored in a liquid compound having an aromatic skeleton and at least one hydroxy group or in another compound dissolved in a solvent. During the treatment, the foam experiences an increase in volume of at least 10% without the pores collapsing. Ceramic foams produced from open-celled PUR foam having a mean pore count of less than 8 ppi are particularly suitable as flame zone structure for pore burners.