Review the original item:
Fred blew and blew on the globs of liquid whiteout, but they refused to
dry when the professor called time, Fred closed
the cover of his exam book and hoped the pages would not cement together.
You wanted to fix it this way:
Fred blew and blew on the globs of liquid whiteout, but they refused to
dry,
when the professor called time, Fred closed the cover of his exam
book and hoped the pages would not cement together.
To add a comma between dry and when would cause an equally bad problem, a comma splice. A comma splice occurs when you have two complete sentences joined with just a comma. They refused to dry is the first sentence. When the professor called time, Fred closed the cover ... starts the second sentence.
The spot between dry and when needs a stronger break than a mere comma.
You might want to consult the rules for fixing comma splices and fused sentences.