Aside from being larger than chicken eggs, duck eggs have different material qualities and react differently to the vinegar than the chicken eggs.
Whilst in vinegar the chicken egg shell dissolves layer by layer relatively ‘as one’, the duck shell, like the layers of a sedimentary rock or a particularly thick toe nail, comes away in flakes.
However, the duck egg shell responds well in the heated vinegar. A far better solution of a clean removal of the hard shell.
Duck egg shells have an altogether more rocky or marble-like quality than their chicken counterparts which I think of more like a ceramic material.
Secondly, duck egg membrane is thick, rubbery and less malleable than membrane from a chicken egg. Thus, I have found it to be an unsuitable material for taking impressions.