There is a Schuylkill County Estate File for “Mertz Minors” that contains the 1 Jan 1822 petition of Joseph Mertz, son of Henry Mertz and grandson of John Blatner. The petition said that an unnamed daughter of John had married Henry, now all deceased. The petitioner also referred to his, also unnamed, brothers and sisters. Joseph was above the age of 14.

A family tree I found seems to cite the baptism of Joseph at Zion’s Red Church 5 Dec 1805 son of Henrich and Maria.

I believe his last mention was the 1840 Census and that he died not too long after that. He is designated N3a3.