Complex sentence: The techniques continued to be developed by his son, James Whatman the Younger (1741--1798), at a time when the craft was smaller paper mills to the large scale and widespread industrialisation of paper manufacturer.
Split into simpler sentences: The techniques continued to be developed by his son, James Whatman the Younger (1741--1798). At a time when the craft was based in smaller paper mills, his innovations led to the large scale and widespread industrialisation of paper manufacturer.