Why are blue ball mason jars numbered on the bottoms?

Answer 1

Collectors frequently refer to these numbers as "mold numbers." However, in some cases, this term oversimplifies what the numbers represented.

Originally when jars were blown by hand, the number represented a specific glass blower and his team. At the end of the day the blower and his team would get paid for the amount of jars they produced as determined by the number of jars made with a given number on them. I.E. glassblower #3 made 200 jars that day and he and his team therefore gets paid X number of dollars at X cents per jar produced. Later, when glass making went to machine the numbers represented the mold or machine the jar was made from (usually 4-8 molds per machine or one to several machines per factory.) That way the plant manager could check quality control, production, etc.

Today you can frequently find numbers on new jars that indicate date of manufacture, plant location, job number, etc.

There is a rumor that jars with the number 13 were more valuable because superstitious people were afraid to can in them, broke them or threw them away. However, I have never found any concrete evidence to back up this claim. Lately, these jars have sold for more on on-line auctions such as eBay.

Another View: Having spoken with many distinguished Southern Gentlemen that grew up during the days of moonshine I was informed by more than one distinguished Southern Gentleman that these bottles were usually broken by the Moonshine Runners because yes they were superstitious and felt like this increased their risk for getting stopped by the law. This tale has been passed on for many years by Southern Moonshiners and their runners.

Answer 2

There is usually a makers mark on the bottom of the jar and the numbers are generally the mold numbers that the jar was made from. They can also contain date codes as to when the jar was made, but normally are only mold codes. They can also give reference (via the mold codes) as to where the jar was manufactured.