In any given matrix control over the final particle size distribution requires a constitutive understanding of the mechanisms and kinetics of the particle evolution. In this contribution we report on the formation mechanism of silver nanoparticles embedded in a soda lime silicate glass matrix. For the silver ion exchanged glass it is shown that at temperatures below 410 degrees C only molecular clusters (diameter <1 nm) are forming which are most likely silver dimers. These clusters grow to nanoparticles (diameter >1 nm) by annealing above this threshold temperature of 410 degrees C. It is evidenced that the growth and thus the final silver nanoparticle size are determined by matrix assisted reduction mechanisms. As a consequence, particle growth proceeds after the initial formation of stable clusters by addition Of silver monomers which diffuse from the glass matrix. This is in contrast to the widely accepted concept of particle growth in metal-glass systems in which it is assumed that the nanoparticle formation is predominantly governed by Ostwald ripening processes.