Classic movies, practical effects - how did they do that?

[Roger]

So I was watching the tail end of Raiders tonight and I know they used all old-school effects for everything, some stuff I knew already or had a rough idea how they did it, other stuff I had to get my Google-fu going and see what I could find.

 

Techniques I knew they used were matte paintings, miniatures, and time lapse photography.

 

I had always wondered how they did the clouds at the end where the fire shoots up into the sky.

It turns out they used a cloud tank to get the clouds but I can't for the life of me figure out how they got the clouds to iris like that. Anyone have any ideas? My first thought was maybe it was something disturbing the cloud material (usually milk or paint injected into freshwater layer of water on top of a denser saltwater layer) like maybe a fan, but that seems unlikely. Maybe instead they injected something into the tank to push the cloud layer away in a perfect circle like that?
I've done several additional searches so far and found some interesting info but not anything on that specific aspect.

[largento]

I remember from an old issue of Starlog about the sfx in Raiders and found it on the Internet Archive (it's issue 54.)

 

The article had a couple of photos.

 

 

 

As you can see in the second image, they used a robotic arm to inject pigments into the inversion layer. As stated in the article, they could create the inversion layer in several ways, the main way being a layer of salt water above a layer of fresh water, but they mentioned that temperature differences could also be used, which might allow for different effects.

 

 

The second shot indicates something like a situation where they could stir the water in a circle to make a whirlpool. Presumably they used high speed cameras to slow any movement down to make it appear to be at a larger scale.

 

 

[Roger]

Yeah I figured they used something to stir/push the paint out of the way and then just spliced 2 copies of the footage together, only the one running in reverse, so that it would appear the clouds open and then close perfectly. As you said, running the film at high speed and then playing back normally would give the appearance of massive clouds.