Montresor seems to encounter Fortunato by accident at night during the carnival. He lures him back to his palazzo and down into the catacombs. There he shows Fortunato the trowel he has concealed under his cape. Eventually he uses the tool to build the wall entombing his victim. Why was he carrying a trowel on that particular night? Why didn't he have it hidden somewhere near the niche where he planned to chain Fortunato to the wall?
Re. Post #12. It is true that Fortunato might have stopped before they reached the niche where Montresor intended to wall him up, but Montresor couldn't use the trowel at some other place en route because there was no other place for him to construct a wall and no stones or mortar at any place but the niche. He might have had to kill Fortunato with his rapier if Fortunato balked and refused to go any further. Then Montresor would have had a problem--but the trowel wouldn't have helped him solve it. Probably he would have had to drag the corpse all the way to the niche and wall him up as planned. That would have been awkward but not impossible. And he had plenty of time to complete his task, even if it took more than one day and he had to mix a new batch of mortar.
Perhaps, Montresor carries the trowel because he does not know at which point in the catacombs Fortunato may stop. After all, the passages grow narrower and narrower, and Fortunato may become wary and/or fearful at some point.
Montresor could have had no idea that Fortunato would bring up the subject of the Masons. Therefore Montresor could not have been carrying the trowel for the purpose of proving he was a "Mason" in a joking fashion. And he would not have shown the trowel if Fortunato had not brought up the subject of the Masons. He had the spot for Fortunato's execution already picked out. He had the stones and mortar waiting at that spot. Why carry the trowel around with him instead of leaving it where he intended to use it?
The passage quoted in the preceding post suggests that Fortunato can't believe that Montresor is a mason -- that is, a member of a secret brotherhood. He expresses his unbelief in a way that Montresor must find frustrating and condescending. Fortunato thus seems to insult Montresor in yet one more way. Montresor's emphatic insistence that he is a mason implies his frustration. Why he happens to be carrying a trowel as convenient proof of his Mason affiliations is anyone's guess, although the earlier arguments about sadism and foreshadowing seem to make the most sense.
"You are not of the masons."
"Yes, yes," I said "yes! yes."
"You? Impossible! A mason?"
"A mason," I replied.
"A sign," he said.
"It is this," I answered, producing a trowel from beneath the folds of my roquelaire.
"You jest," he exclaimed, recoiling a few paces. "But let us proceed to the Amontillado."
"Be it so," I said, replacing the tool beneath the cloak, and again offering him my arm. He leaned upon it heavily. We continued our route in search of the Amontillado. We passed through a range of low arches, descended, passed on, and descending again, arrived at a deep crypt, in which the foulness of the air caused our flambeaux rather to glow than flame.
The quoted section shows Montresor revealing his trowel. Poe contrives this interesting scene that plays upon a secret Masonic symbol of ritual and fraternity to neatly foreshadow what Montresor will soon do to Fortunato. So (1) in the sense of literary device, Montresor carries and produces to trowel as a clue to what is to come. (2) In the sense of the action of the plot, he carries and reveals the trowel because he has a cunningly thought out and executed plan for--well--executing Fortunato.
Montresor, of course, has a trowel to work on the wall he constructs around Fortunato. (There is a vague joke between the two about the "masons," whereas Fortunato refers to the secret order of the Freemasons, while Montresor takes the reference quite literally and waves the trowel which he is about to use.) As to why he carries it, perhaps he plans to use it as a weapon in case Fortunato sobers up or decides to go no further. Placing the tip at one's back through clothing might feel like a knife to Fortunato, when it really is nothing but a trowel—though in a pinch, it could serve the same purpose. It would also draw less notice than a knife or pistol, and should anyone notice a trowel, he could make a joke, and like Fortunato, some might infer that it was a hint about the Freemasons.
The idea that it serves as foreshadowing, as mentioned previously, is a good one.
As the other posters have suggested, one can justify the carrying of the trowel for a few different reasons. First, it does offer an example of foreshadowing. Another reason may be that Montresor simply does not care. His revenge is the most important and that is all he is concerned with. Another suggestion could be that it is carnival time and what people are doing is not as scrutinized during this time.
Montresor assures the reader that he has planned for every eventuality, and by keeping the trowel on his person he shows that he is confident. He has no reason to think his plan will be discovered, or that Fortunato will figure it out, and so he smugly shows it off. The Masonic joke is significant only in that Montresor uses a mason's (bricklayer's) skills to imprison Fortunato, who is actually a fraternal Mason.
The trowel is an excellent example of foreshadowing, and demonstrates how Fortunato had no idea what was coming- even he laughed at the pun when he showed him the trowel to prove he was a "mason." I also agree that it probably gave Montresor a great deal of pleasure to show Fortunato the murder weapon. Perhaps this was intended as a play on the convention, common until the eighteenth century, of having people condemned to die by beheading inspect the executioner's blade.
The trowel, a mason's tool, is carried by Montresor because he knows he will need it to mix the mortar. Poe incorporates it as a foreshadowing device as well as for making a great pun when Fortunato asks Montresor if he knows the sign of the Masons. As for why it wasn't planted with the mortar, then Poe wouldn't have been able to use the joke or foreshadowing. Or, perhaps, it was Montresor's one mistake--forgetting to leave the trowel ahead of time along with the mortar.
I think that he is trying to bait Fortunato. I think that he wants in some way for Fortunato to know what is going to happen to him. It's kind of oblique as a hint, of course, but I suppose he wouldn't really want to give the game away since that would rob him of his revenge. But I think that it gives him some kind of sadistic pleasure to display what will, essentially, be the murder weapon.