The method is characterized by moulding at least one slab of a thermoplastic, normally solid material under heat treatment against a matrix having a form substantially matching that of the final mirror to form a shell (6) introducing into the shell a material selected from the group consisting of; (1) glass bubbles of a thermoplastic, normally solid material; (2) a composition of a powder of a thermoplastic, normally solid material together with an expanding agent, and; (3) mixtures thereof, and bringing the glass bubbles and the powder particles of the composition resp., the latter after expansion, to adhere, on one hand to each other, and on the other to the shell to form a porous supporting structure. The mirror blank according to the invention comprises a shell (6) of a thermoplastic, normally solid material and a supporting structure of the material in said group filling the shell, the glass bubbles and the powder particles resp. being attached to each other and to the shell. If sheet glass is used for the shell and as a powder in the composition, the composition will further include 0-2% by weight of viscosity reducing Li.sub.2 CO.sub.3 and 4% by weight of CaCO.sub.3 as an expansion agent (all counted on the glass powder).