I have a clay humidifier in my Don Alfredo humidor. Bygone technology long ago surpassed by polyacrylics ('crystals') to do the same thing, but better. Similarly, foam has been surpassed by silica gel ('beads') that can be 'trained' to hold a fixed relative humidity (whereas PG solution can only hold 70% RH). Cost is the main reason one still sees foam.

Pop quiz: What was Don Alfredo's last name?