In my research I have found that a number of my German ancestors on my mother's side (Bluhm) were glass-makers from the area surrounding Usch, in modern Poland. For some reason, around 1850, they migrated to the area around Hamburg and Hannover, where they continued as glass-makers. However, they weren't alone in that migration. Several related families (Stuhm, Gundlach, Hartmann), along with their siblings and children also made that move. Can anyone with a more detailed understanding of German history suggest why several related families would have made such a move at that time?