Complex sentence: Progress in developing a reliable pen was slow until the mid- 19th century, due to an imperfect understanding of the role that air pressure plays in the operation of pens and because most inks were highly corrosive and full of sedimentary inclusions.
Split into simpler sentences: Progress in developing a reliable pen was slow, however, until the mid- 19th century. That slow pace of progress was due to a very imperfect understanding of the role that air pressure played in the operation of the pens and because most inks were highly corrosive and full of sedimentary inclusions.