“Historic”
“Historic” is an adjective that means something important or influential in history. So Denise should say, “The treaty was a historic occasion.” It was an important occasion. It would be incorrect to say, “We sell historic replicas” unless they are replicas that are important to history. You’ve probably heard TV announcers refer to “historic treaties” or perhaps you’ve visited some “historic houses” or “historic battlefields.” All of these were important or famous things in history
“Historical”
“Historical,” on the other hand, is an adjective that refers to anything from the past, important or not. Denise should say, “We sell historical replicas” because these replicas are from the past; they’re probably not so important. A “historical occasion” would be just some occasion in the past; it wasn’t necessarily an important occasion. “Historical documents” are just documents that record the past. You’ve probably read a “historical novel” or perhaps even a “historical romance,” which are books set in the past. There is nothing especially important about these books; if they were, they’d be “historic books.” The Gutenberg Bible would be a historic book, for example.
Memory Tricks
It’s common for people to mix these two words up. One style guide laments, “Examples of ‘historic’ used incorrectly for ‘historical’ could easily run for several pages” (2). William Safire said something that might help you remember the difference: “Any past event is historical, but only the most memorable ones are historic”