The Wikipedia entry for John Berkey (1932-2008) is way too brief so far as I'm concerned. That's because he painted imaginary spaceship scenes that were astonishing when they first appeared on covers of science fiction paperbacks and continued to astonish in the years thereafter.
You need to understand the context. Pre-Berkey, sci-fi spaceships were usually depicted as (1) Buck Rogers style open-cockpit jobs, (2) Flash Gordon spindle-like craft, (3) variations on the German V-2 rocket of World War 2, (4) extrapolations of Apollo-era spacecraft or (5) combinations of these with some other details added.
Berkey introduced to the genre huge spacecraft that often combined delicate equipment detail with large, smooth, reflective surfaces. His style was basically loose, yet when reduced to book cover size, gave the impression of monster machines.
As for Berkey himself, he was a Minnesotan who from age 15 was determined to become an illustrator. His training was on-the-job at commercial art studios. In 1955 he hit the big time, being hired by Brown and Bigelow, the large St. Paul calendar company. From then to 1963 he produced around 500 illustrations for the company. After that he went freelance, continuing calendar work while moving into magazine and book cover illustration -- his long-term goal. By the early 1990s he had done about 200 book covers, most in the science fiction field.
Looking at reproductions of his work, an observer might guess that Berkey worked in oil. Not so. Most of his paintings were done in casein or a combination of casein and acrylic; occasionally he used tempera.
I used to paint in casein when doing commercial art projects in college and never came remotely close to what Berkey was able to do with it; creating smooth (non-water blotched) areas was something I found difficult and illustration board curling when the paint dried was another annoyance. Clearly Berkey's work in commercial art studios allowed him to get lots of useful tips from professional artists regarding casein-handling and other needed skills.
As for the combined casein-acrylic work, my guess is that he used acrylics to block in large areas, reserving casein for the details. If I'm wrong, please let me know.
Below are examples of Berkey's science fiction art.
If your browser allows it, click on the images for larger (and sometimes clearer) views.