Take another look at the original sentence:
You wanted to fix it this way:
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.