Slack logs all messages up to 10,k message history (because this is a 'Free' account team). You can always scroll to the top and then click to see more than the current loaded messages, or I have also installed an app to export a channel's history (not a bad idea to do after a scene completes, so you can be sure that you don't lose any of the old messages when they start getting trimmed. the command to do that is "/export"