Why is it ironic that, when the old man refers to the potion he describes to Alan, he calls it a "glove cleaner"
Here is the piece of text to which you refer. Let's look at it in context:
"Call it a glove-cleaner if you like," said the old man indifferently. "Maybe it will clean gloves. I have never tried. One might call it a life-cleaner. Lives need cleaning sometimes."
Before this moment, the old man just described the item's capability. It it imperceptible in water, colorless, odorless, and tasteless. It cannot be identified in an autopsy. After the reference to a glove cleaner, he refers to it as a life-cleanser. I think he is insinuating that this potion can clean the fingerprints off the case of a poisoned murder. Did you notice that Alan asked if it was poison and the old man did not say no, he just called it something different?
I find this ironic because Alan came for love, living life to the fullest. Here the old man introduces just the opposite, death.
It is also ironic because if you just remove the letter G, it becomes a love cleaner when said quickly. Hmmmm... I wonder under what circumstances a person might need their lives cleaned from love?