Dan experimented and discovered new looks and uses for the versatile material. "I noticed that liquid nitrogen could look and behave much like fire. If you think of fire as a substance, it moves very quickly and dissipates with sharp blades of flame.So if you have liquid nitrogen running down on a slanted black velvet surface, gravity will pull it one way and then if you blow it back with air guns the other way, it breaks up and looks like flame, especially if you overcrank the camera, shooting at 120 frames per second." This operation required several people to man the nitrogen, air jets, and fans. Dan relied on his then effects coordinator, Ronald B. Moore, and veteran ‘nitrogen wrangler’ and effects rigger, Dennis Hoerter, for patient assistance.

Planet on fire

Dan used his discoveries in the TNG episode, ‘Lesson’ where Captain Picard has a relationship with Neela Daren, a member of the crew. Picard must assign her to a dangerous planet covered with conflagrations, much like giant forest fires. "So we created huge banks of flame coming across the horizon with liquid nitrogen," says Dan. "That same year there was a huge brush fire behind my house and the mile wide walls of flame coming down the hill, threatening to burn my house, looked surprisingly like the liquid nitrogen gag we had done." (Fortunately, Dan's house was saved when the winds changed direction.)

Dan's use of liquid nitrogen helped him win an EMMY Award for his visual effects in the TNG episode ‘A Matter of Time.’

His challenge was how to show the Enterprise NCC-1701-D cleaning off the atmosphere of a planet in about fifteen seconds. "We made a vat of liquid nitrogen about four inches deep. We were toying around with it and swatted it with some cardboard and saw that the liquid nitrogen billowed out and then came back in an interesting tidal wave.

"By wrapping it around a digital sphere we were able to get the illusion that gasses were being sucked into space.

We actually used a vacuum cleaner and drew the nitrogen vapor out of the tank, then because it was heavier than air, it would fall back in.

By placing a garbage can lid on this vat and quickly pulling it up, the natural suction of the lifting lid created these wonderful tendrils of nitrogen vapor." This same footage was later reused by visual effects supervisor, Ronald B. Moore, for the VOYAGER episode ‘Thirty Days’ where a water planet was evaporating of into space.

Liquid nitrogen elements are archived onto tape reels and have been used on all three series by Dan and other visual effects supervisors for nebulas, heat ripples, blowing dust, volcanic smoke, water falls, and flowing rivers. But the filmed liquid nitrogen footage requires additional treatment; by its nature, liquid nitrogen is white and is usually filmed over black for use as visual effects elements. During digital compositing, Dan manipulates the white-on-black footage to achieve greater realism in his effects.

Airbrush technique

"With filmed liquid nitrogen footage,” he says, “whether using it for fire or time-space anomalies, if you just tint it with the color of fire it always looks like you colorized a white element. I airbrush tonalities, whether it is in the orange or the red or the purples, and prebuild dissolves between these subtle forms that are airbrushed to suit the shapes of the phenomena we are trying to create. Keying (matting) that prebuilt dissolve through the liquid nitrogen footage gives a much more natural luminescence about it than if you just made it a solid color."

When asked why he has not turned to computer generated imagery for organic effects, Dan responds, "Computer animation is wonderful and has given us a lot of freedom, but some things it doesn't do all that well. Certain kinds of organic things are not completely convincing. There are computer procedurals that create wonderful moving textures and have nice kinetic properties, but computer fire, for example, still does not look real. So when we have something that calls for that kind of organic effect we still like to rely on liquid nitrogen. Even the Badlands' in the VOYAGER pilot, which a lot of people think was pure CGI, really was not. We shot a lot of liquid nitrogen elements that we then gave to Amblin Imaging; they assembled them into layers and that became one of the building blocks of the Badlands sequence."