Are tennis cans vacuum sealed, or are they pressure packed?
Today, cans are pressurized. Vintage metal cans were also pressurized.
The difference, is that early metal cans were often lined with a coating of lacquer. The balls were placed inside, all air was withdrawn creating a vacuum. Pressure was then shot into the can to exactly balance the pressure inside the ball. There are some cans that are labeled “vacuum packed”. This was either a marketing error, or they were referring to the above mentioned process.