Apparatus for controlling the filter channel opening size of a ceramic filter element, said ceramic filter element having flared pores, the apparatus comprising: a lapping machine having a lapping plate; and a filter holder to hold a ceramic filter element for controlled lapping of said flared pores; wherein said filter holder comprises: a filter element mount to mount the ceramic filter element to be lapped such that a surface of said ceramic filter element lies substantially flush with a lower face of said filter element mount, wherein said filter element mount comprises a piston having a flat face, against which the ceramic filter element bears; an adjustable actuator to controllably move said surface of said ceramic filter element relative to the lower face of said filter element mount so that it remains substantially flush or projects beyond said lower face of said filter element mount during lapping, wherein the adjustable actuator controls movement of the piston; means for urging said filter holder towards said lapping plate; and a system controller to: sense or measure one or more parameters selected from the group consisting of: a pressure of said ceramic filter element on said lapping plate, a rotational speed of said lapping plate, and a duration of lapping of said ceramic filter element; and provide one or more outputs to the adjustable actuator to control one or more of said parameters and control movement of the piston to urge the ceramic filter element downwards to remove a portion of the thickness of said surface of a ceramic filter element to thereby control the filter channel opening size. a lapping machine having a lapping plate; and a filter holder to hold a ceramic filter element for controlled lapping of said flared pores; wherein said filter holder comprises: a filter element mount to mount the ceramic filter element to be lapped such that a surface of said ceramic filter element lies substantially flush with a lower face of said filter element mount, wherein said filter element mount comprises a piston having a flat face, against which the ceramic filter element bears; an adjustable actuator to controllably move said surface of said ceramic filter element relative to the lower face of said filter element mount so that it remains substantially flush or projects beyond said lower face of said filter element mount during lapping, wherein the adjustable actuator controls movement of the piston; means for urging said filter holder towards said lapping plate; and a system controller to: sense or measure one or more parameters selected from the group consisting of: a pressure of said ceramic filter element on said lapping plate, a rotational speed of said lapping plate, and a duration of lapping of said ceramic filter element; and provide one or more outputs to the adjustable actuator to control one or more of said parameters and control movement of the piston to urge the ceramic filter element downwards to remove a portion of the thickness of said surface of a ceramic filter element to thereby control the filter channel opening size. a filter element mount to mount the ceramic filter element to be lapped such that a surface of said ceramic filter element lies substantially flush with a lower face of said filter element mount, wherein said filter element mount comprises a piston having a flat face, against which the ceramic filter element bears; an adjustable actuator to controllably move said surface of said ceramic filter element relative to the lower face of said filter element mount so that it remains substantially flush or projects beyond said lower face of said filter element mount during lapping, wherein the adjustable actuator controls movement of the piston; means for urging said filter holder towards said lapping plate; and a system controller to: sense or measure one or more parameters selected from the group consisting of: a pressure of said ceramic filter element on said lapping plate, a rotational speed of said lapping plate, and a duration of lapping of said ceramic filter element; and provide one or more outputs to the adjustable actuator to control one or more of said parameters and control movement of the piston to urge the ceramic filter element downwards to remove a portion of the thickness of said surface of a ceramic filter element to thereby control the filter channel opening size. sense or measure one or more parameters selected from the group consisting of: a pressure of said ceramic filter element on said lapping plate, a rotational speed of said lapping plate, and a duration of lapping of said ceramic filter element; and provide one or more outputs to the adjustable actuator to control one or more of said parameters and control movement of the piston to urge the ceramic filter element downwards to remove a portion of the thickness of said surface of a ceramic filter element to thereby control the filter channel opening size.