Re. Those who made paperweights & related objects circa the turn of the 20th century at Whital Tatum in Millville, NJ:
These makers include Ralph Barber, John Rhulander, Michael Kane, Marcus Kuntz, Emil Stanger to name the most famous.
These makers made weights on their own time. Whital Tatum was a glass factory that made bottles, glasses, mugs, pitchers, etc...Not paperweights.
As such, those who chose to make weights made them on their own time during lunch or before or after hours.
Back then, little was known about co efficients of expansion and glass color compatibility and the like. So there was a bit of the luck of the draw there.
Also, their annealing ovens were used by many and for Factory work, not simply for paperweights.
As such, the doors to the ovens were opened for factory product to be placed inside, this often causing undo rapid cooling & stressto the demonstrably thicker, solid cased paperweights.
So then, why did weights that came out of the oven cracked make their way out of the factory?
Probably because the makers were still proud of them, treasured their work, and did not want to destroy their labour.
Furthermore, back in those days, most of what they made were given as gifts or traded for other goods. They were Not sold for cash.
Finally, Allan is correct in that many weights with internal stressors do Not show their cracks until long after they were made.
So if many of those made at Millville back in the day often had color compatibility issues or were poorly annealed, then some would show a crack(s) immediately while others might go a long time before the stressor commenced to cracking.
Here is a nice view of a footed Millville Lily that I own that isn't cracked:http://glassgallery.yobunny.org.uk/displayimage.php?pos=-1513
I hope all of the above is helpful