In Star Trek IV, Scotty has some trouble procuring “transparent aluminum” in the 20th Century to make a tank to transport a breeding pair of whales to the 24th Century. He ultimately has to give the formula to a contemporary engineer, at the risk of changing history.
Only – why does the tank need to be transparent? It’s for transport, not display. Wouldn’t ordinary aluminum do?
It didn’t. But transparent aluminum was a standard material in starships, and Scotty was familiar with its characteristics.
Besides which, it confers some benefits - it’s easier to monitor the health of the whales, it keeps them in an environment more similar to that which they’re used to (the aquarium), and so on.
The whales are intelligent, they wouldn’t want to be in total darkness.
Also, the aluminum is much lighter than the plexiglas.
Sidenote: in the novelization, Scotty recognizes the engineer(by name) as the one who did invent the stuff.
I thought they traded the formula for transparent aluminum for some Plexiglas to construct the whale tank since they had no money.
Scotty just says, “How do you know he didn’t invent it, then?”
They did. But Scotty wanted to use transparent aluminum until they discovered it hadn’t been invented yet.
Exactly - The plant manager even says that it would take years to analyze the structure. If you notice the walls they are installing in The Bounty were easily six inches thick, not the one inch they could have achieved using transparent aluminum.
Exactly. This is a common mistake people make about Star Trek IV. The cage was normal plastic. They used the formula to pay for the plastic.
You’re right, I lost track of the events.
I agree; they traded the formula for the Plexiglas they needed but couldn’t pay for. I found a site with the script and it seems to suggest a trade.
“Yes, I noticed.”
I’ve missed that every time I’ve seen the movie. 
BrainGlutton: (emoting) What would. Scotty need, with transparent aluminum?
Well, God wants a view from His starship.
Because, of course, he calculates the thickness of plexiglas to hold back water all the time. Without having to know the dimensions of the tank.
Although I love the film, this line always bothers me. There are some engineers who’d rattle off a definite number like that with no observable delay for calculation (or bullshitting) and with great confidence, but Nichols doesn’t otherwise act like one.
Easily explained: he just got finished fabricating the aquarium at Bill Gates’ place.
So the real hole is that a crew of incredibly competent people from centuries in the future with advanced technology couldn’t quickly come up with a way of making a few thousand dollars without giving away advanced technology and messing with the timeline (not to mention the Prime Directive).
What bothers me is the size of that piece of Plexiglas; 60’ x 10’ x 6" is enormous. I think most manufacturers max out at whatever size can fit in the back of a semi-trailer and I think that exceeds that.
The pieces aren’t nearly that big. A UH-1 Huey is 57 feet long. Look how big the sheet is when they are lowering it into the stealthed Warbird.
Since they have phasers, I bet Scotty has a hell of a good blowtorch available, even on a Klingon junker.
Bird of Prey.
