on its outside a rotationally symmetrical shape which widens from a certain point conically bears with its outside 6 on the edge of the aperture 4 the invention is based on the recognition that the sintered product may show cracks in that during sintering in which the spinel structure is formed the moulding 5 initially expands this expansion cannot be compensated for by the holder 1 in fact the moulding 5 is jammed in the edge of the aperture 4 depending on whether the moulding 5 consists of a prefired powder or of a non prefired powder it has been found that the expansion may be from 0 5 to 4 another problem with annular cores sintered in holders of the types shown in fig1 and 2 is that they may become suspended obliquely in the aperture 4 this situation is shown in fig3 a moulding 5 which is sintered in an oblique position becomes unconcentric this means that the final product has to be ground so as to return to the desired degree of concentricity or that the deflection unit in which it is used has to be provided with correction means to magnetically correct for the u concentricity which leads to an undesired influence of the electron beams during operation of the deflection unit the invention solves both above mentioned problems in that during sintering and subsequent cooling the mould is incorporated in a holder of a new design the newly designed holder 11 is shown in.