The present invention relates to a ceramic surface in a casting tool, in particular a surface which is arranged outside the contouring region and to which a liquid or free-flowing aluminium material can be applied, wherein there is applied to at least one ceramic surface a coating consisting of an infiltration layer diffused into the ceramic surface, structural parts of the form Al2O3 and/or SiO2 and/or ZrO2 and/or TiO2, and a polymer, enclosing at least a region of the primary parts, the structural parts and the sliding parts, of one or more polymerized phosphates. In addition, the present invention relates to a separating means and to a method of producing such a coating.