Spark keeps history of all messages in the ~/.Sparkfirstname.lastname@example.org/transcripts/ folder.
Although there is no way to view the entire history in the application itself.
I developed this functionality. This window allows to view the messages grouped by the date tag as well as search the text.
Please see some screenshot below.
Also I attached the patch.
To me it could be handy for users.
Please let me know what do you think about this functionality.