Monday, February 23, 2009

Cloud Tank effect

People have been asking me to provide some of the details of the Cloud Tank so that's been on a long list of subjects. Recently I had to write up some of the notes so thought i'd post it here as well.

When I start in visual effects I was hired by Doug Trumbull as his assistant. The first day I was asked to create clouds in a liquid for a film that he had just started working on called Watch the Skies. You may remember it by the final film title, Close Encounters. They needed to create clouds and they liked the look of pouring cream in tea or coffee.

I was given a 20 gallon aquarium and $20 of petty cash. I worked it out in a few days with additional ideas from Doug, Wayne Smith and others.

This same process was later used in a number of films and commercials. (Raiders of the Lost Ark, James and Giant Peach, ID4, etc) Actually most of these used the same exact tank as well.

I've attached a couple of primitive drawings to make it clearer. My photos are packed away deep in boxes otherwise I'd post some here. This was also covered to some extent in the Close Encounters book.

[Update: 12-22-11 To make things a little clearer I'm adding some notes here. The basics are a large all glass aquarium was filled halfway with salt water and then fresh water was carefully added to the top. Salt water is heavier so tends to stay on the bottom but you want to avoid mixing them if possible. That's what some of the details below are about. Also best to avoid getting the water too warm. It's very difficult to see any difference in the water once filled since it all looks the same (assuming you've also cleaned the tank and filtered the water).  under the right light and angle you may be able to just make out a slight difference.

Next white liquid tempra paint is injected in the fresh water portion (top), usually just a few inches from the dividing line of the fresh and salt water. Think of a large syringe with an aquarium tube going into the water. When the tempra paint is injected it billows outward like cumulus clouds and will tend to sink a bit. But the salt water prevents it from going lower so the 'cloud' tends to flatten it's base on the salt water line and and billow outward, similar to real clouds based on air pressure levels. Avoid going below into the saltwater since the clouds will just drop to the bottom of tank.

Once you're setup, light the water and create the clouds. You can film while injecting the clouds to get the large billow action or you can film once they're in place. Filming can be under-cranked (less than 24 fps) or over-cranked (over 24 fps) depending on the look desired. Over-cranked will tend to increase the sense of scale and by placing smaller clouds in background you can create force perspective. You'll need to completely drain the tank, clean it and repeat after each shot.]

Process in a nutshell as used on Close Encounters:
2000 gallon glass tank approx 7 x 7 x 4 feet deep

Water was purified and filtered in 2 large hot tubs/wine tubs(?) 1000-2000 gallons in size.
One had rock salt added (probably close to seawater specific gravity but I can't remember the details)
The salt water was pumped in first usually to halfway mark, dependent on the shot.
Sheet of thin plastic (visqueen) was laid on top. Think of the plastic used for heavy duty trash bags or painter's drop cloths, flexible but not easy to tear. This was longer than the tank front to back and would drape out the back of the tank.

A PVC pipe running the width of the tank was placed on top of this with a rig. Later to remove the plastic it would be pulled out and this PVC pipe was used to keep it down.
A PVC piping system that looked like a fork from above was placed just above the plastic and the fresh water was pumped in.
When done this was removed and the thin plastic sheet was removed.

Now it appeared as just a tank of water but there were two layers if you looked very carefully.
A large syringe was used with aquarium tubing to add small forced perspective clouds in the distance.
The mixed white tempera paint was injected right above the slat water layer. if it went below the tank was contaminated and would require a full redo.

Next an atomic arm was used to inject the main clouds during shooting. This allowed the operator to stand in front of the tank near the camera and as they move it around a brass tube was moving in the tank.
An electric trigger on the handle would cause the tempera paint to be injected as the arm was moved. The tube came from a pressure cooker filled with the paint.

Pressure was supplied from a compressor and an electronic valve was connected to the atomic arm trigger.
We also had a light source (with rotating colored lights) that went down this same tube as a fiber optic.

Frame rates could be under cranked or over cranked depending on the look.

Close Encounters Book

Close Encounters - BluRay

Close Encounters - DVD

Close Encounters VFX Video 1 - YouTube

Close Encounters VFX Video 2  - YouTube

Updated: Polaroid from setup 6-14-76

Cloud tank articles from others:


  1. Wow! Cool! Thank you for posting this!

  2. Wow! Sounds stunning. Thanks.

    image coloring

  3. Hi there, how can one do this low-tech? What I mean is I won't have access to an atomizer arm or specialty plastics. I will, however, be able to use a fish tank, make saltwater, use water, get tempera paint, and I can make tons of stuff with PVC for very little money. I can also use a DSLR T2i camera to overcrank a bit. I know it's asking a lot, but I'm a huge fan of conventional special effects (despite digital). I was thinking I could use PVC and beverage tubing with a small, handheld pump to push paint into the tank.

  4. No need for high tech. Pour in in saltwater. Let it settle. As gently as possible pour in fresh water. Use large spoon or something to prevent the water from breaking into the salt water.

    Once it settles you can use a large syringe like those for glue or lab. attach 1/8 inch aquarium tubing taped to a bendable wire. Slowly push plunger and avoid going below the water split layer.

    1. I used to do exactly that as a kid to replicate this effect! Big syringe, aquarium tubing and white gouache produced very beautiful and mesmerising effects. And once the cloud dissipated in the water I would shine a light through a matte on the side of the tank and emulate Carpenter's The Thing opening title ;)

  5. Awesome, thanks for writing back. I'll send you a video if it turns out okay. :) -JZ

  6. In case that wasn't clear just place something above the saltwater to diver the water to the sides instead of straight into the salt water. Slowly and gently in any case.

  7. Hey Scott, thanks for Posting this, its very helpful.

    One question, what does the pressure cooker do in this effect? Is it somehow beneficial to heat the paint?

  8. Pressure cooker not actually used with any heat. The point is to contain the paint and some amount of compressed air.

    The pressure cooker was modified with 2 holes and nozzles. One would be connected to a standard air compressor (like used for tools) the other nozzle had a brass tube that would go almost to the bottom of the pressure cooker and it also had an electronic value t allow liquid to flow through when it was energized.

    The pressure cooker was filled up to a certain level with paint and then it was pressurized with a compressor. The pressure cooker gauge read out the amount of pressure. When it was at the right pressure the compressor was turned off and the pressure cooker would keep the pressure. Now when the button was pressed on the atomic arm the valve would open on the pressure cooker. The paint was pushed by the pressurized air up the brass tube, through the valve and then down through the plastic tubing into the tank.

    The amount of pressure adjusted would control whether it rushed out or whether it dribbled out.

  9. Hey Scott,
    Was wondering, was there a drain at the bottom of that thing, or did you guys place a pump in there to get all that water out? Also, do you think a big acrylic tank would work as well as a glass one?

  10. As I recall there was a drain at the bottom but it wasn't a thing I focused on.

    Acrylic - The problem with plastic is the scratches and the potential chromatic aberrations.
    I'm no aquarium expert so I'd suggest checking elsewhere for the various tradeoffs as well.

  11. Hi Scott! Was wondering if you could elaborate a bit more on what/how you lit the clouds? Even better, if you were to do a "lo-tech" version of this today, what would you use? Waterproof LED's?

  12. I had the opportunity to see Douglas Trumbull speak here in Tucson not long ago. He mentioned that the salt water needed to be heavily saturated with salt and really cold. I'm still working on this here, but haven't had time due to being busy. Holidays are coming up tho and that means less work, but more time for my little projects. Oh, a (clean) 5-gallon paint bucket (these are everywhere on construction sites and they seldom mind you taking one - just ask - or talk to professional painters) with holes for air and liquid and tubes running in/out works good. I use this method for squirting blood with a hose. ;P

  13. Lighting the clouds -
    Depends on the shot. Many had standard movie lights outside the tank. If there as a light within the clouds then it was a fiber optic bundle (1/8 inch) that went down the same same tube as the paint.

    Today you could use LEDs as long as you waterproof connections (heat shrink tubing should work)
    One advantage of the fiber optics is we had it go up to a special light source with a rotating glass wheel. By putting colored gels, ND filters on the glass or paint on the glass you could have the light change color, change intensity or flicker.

    Temperature of the water - We did some tests but it wasn't a huge deal. (i.e. room temp will work) We didn't refrigerate the water but cooler water tended to maintain the specific gravities longer without mixing and would tend to slow the paint dispersal.

    If salt water was warm then thermal action would try to move it up into the freshwater.
    If the temperature of the fresh water or paint was warm then it would tend to blend out quicker.

  14. Exactly the info that need for a still photo project shot with pentax 67.
    much thanks!!!

  15. You don't need a pressure cooker with a compressor if you want to keep it lo budget, you can get a pressurised water sprayer from a garden centre. Just use the pump to pressurise and connect the aquarium tube to it.

  16. Excellent topic discussed

    Hope to see more visual effect related to this

    anyone knows about water tank effect.


  17. What is the Atomic arm?

  18. An atomic arm is something like a pantograph. It's a mechanical device that has a long tube 6-8 feet long, 3 inch diameter (measurements are guess at this point) and then a 3-4 foot pole that comes down each end. When you move one 'arm' up/down/left/right then the other end does the same thing. That way if Doug wanted to place clouds more to the left he simply moved his hand (holding on to one end of the atomic arm) to the left and the part that was in the water (the other end) would move to the left the same amount, as if he was standing in the tank.

    The term atomic arm comes from it's usage in atomic work where you had to move something that was radio active in a special container or room but didn't want to be exposed to radiation. They use the same devices for some biological experiments.


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