Liquid nitrogen is pumped on and freezes into a sponge of solid nitrogen.

The liquid nitrogen is in a 600 or 800 ml beaker under a shielded bell jar on top of the red vacuum cart. A cold trap is not necessary if only nitrogen is being pumped on.

It is important that the beaker of liquid nitrogen not have frozen water vapor on its side, as the view is impaired. A camera is zoomed in on the beaker, which is in a thick glass bell jar and an acrylic tube shield.

With the pump running and the bell jar vent open, pour the nitrogen and cover the beaker with bell jar. Open up the vacuum valve below two turns and close the bell jar vent. Throttle the boiling of the nitrogen by closing the valve a little. Don't let the nitrogen boil over from the beaker. The boiling gets a little slower, then a puck of frozen nitrogen forms on top of the liquid.

Open the valve more to get the boiling going faster, and more of the spongy white pucks form, pushing the previous ones up to the top of the beaker. All of the nitrogen can be frozen this way.

Venting air into the bell jar melts the solid into liquid nitrogen. No way to touch the stuff.