I've been looking at some valve swag and noticed the company motto (?) on top of the Aperture Science logo. Why is "Salt Asbestos Curtain" the company motto?
First off, it's not the company motto, it's an ad campaign.
Aperture Science was a shower curtain company producing deadly shower Curtains before they became better known for developing the portal gun.
Basically, you're reading the phrase the wrong way. It's not "Salt Asbestos Curtain", it's "Salt", "Asbestos", and "Curtains". I'm not sure quite where the Salt comes in.
Its not one of their motto's, it was just a marketing line of the products they were selling in 1940, their motto's are:
As listed here.