Complex sentence: Watercolor painters before c.1800 had to make paints themselves using pigments purchased from an apothecary or specialized "colourman"; the earliest commercial paints were small, resinous blocks that had to be wetted and laboriously "rubbed out" in water.
Split into simpler sentences: Watercolor painters before the turn of the 18th century had to make paints themselves using pigments purchased from an apothecary or specialized "colourman". The earliest commercial paints were small, resinous blocks that had to be wetted and laboriously "rubbed out" in water.