Re: What is the 'shockwave' made of?
A star is a fairly dense ball of gas (Sun's average is about 1.6 x density or water). When the star's core finally collapses not all of the star gets to form the remnant neutron star/black hole. There is a rebound shock wave generated in the remaining gas (now going outwards) which is the explosion blowing it all away. When this fast moving gas hits the shell(s) of gas and dust previously expelled from the star it forms the shock wave. It's this that generates the photons seen by us and other wavelength instruments; the photons from within the star are adsorbed by it's gas and don't get seen.
All elements heavier than iron are generated during the rebound compression of the non core part of the star. Normal fusion can't 'burn' heavier than iron even in massive stars.
During the actual core collapse massive numbers of neutrinos are generated; these are not adsorbed and as they move damned near c we see them first and then the visible flash later when the shock wave eventually hits the shells and generate broad spectrum photons.
By the by, current thoughts are that some forms of dust and mixed element molecules are formed by quantum-chemistry in gas in the colder parts of space. Low temperature = slow moving and gives time for electron shells to interact when they are close, not like our normal terrestrial chemistry where they have to based into each other with heat (fast brownian motion).
Sorry but I don't rate the clarity of the article much.