We consider a model of diffuse cloud dust in which carbon is deposited slowly on the surfaces of silicate cores. The solid carbon is also processed by the interstellar radiation so that its optical properties change with time. The computed interstellar extinction curves (ISECs) for this model evolve in time, lie close to the observed mean ISEC for substantial periods but generate at later times significant dispersions in the far-ultraviolet (for ISECs normalized in the visual). The requirement on the carbon budget is well within that available, while essentially all the available silicon must be in the silicate cores. The variety of observed ISECs may therefore be interpreted (at least, in part) as due to evolutionary changes in a time-dependent model of interstellar dust.