A replenischment is probably necessary after 6 or so plates, maybe you can find a replenisher formula for the process.
I found this detailed description of this process on the Albumen site
http://albumen.conservation-us.org/l...9/albnegp.html
If you can speak french here's the link to a book written by the inventor of the process Abel Niepce de St. Victor with a description of the process. Incidently he also experimented with gelatine.http://books.google.at/books?id=af-z...page&q&f=false