In "My Grandmother's Love Letters," Hart Crane's tone is one of uncertainty.
Tone is not the mood of the poem but the author's attitude towards his subject:
...similar to mood, [it] describes the author's attitude toward his material, the audience, or both.
In this poem, Crane speaks of what is seen and what is not. For instance, the stars are not seen but the memory of them is present. (He "sees" his memory of his grandmother but not who she is.) Here, the idea of memory is introduced, and there is, according to the speaker, a great deal of room for memory; it can encompass a something the size of "loose" and "soft" sheets of rain.
There are no stars tonight
But those of memory.
Yet how much room for memory there is
In the loose girdle of soft rain.
The presence of the rain may help set the mood but, in terms of tone, Crane conveys the importance of what confronts him (the speaker) and the fragile quality of the letters themselves and the information they contain--perhaps even unknown knowledge of himself in this information or his grandmother or of the two of them with each other.
There is even room enough
For the letters of my mother’s mother…
That have been pressed so long
Into a corner of the roof
That they are brown and soft,
And liable to melt as snow.
He must proceed carefully. The letters are fragile and, in his uncertainty, might not the information or what he learns of her also be fragile as well? Could what he learns affect how he sees himself or how she might see him if she knew him not as a grandson but as a man?
Steps must be gentle.
It is all hung by an invisible white hair.
It trembles as birch limbs webbing the air.
It is suggested that in preparing himself to discover an unknown side of a woman who was not yet his grandmother at the time (nor even yet his mother's mother), but rather "Elizabeth," he suspects that a stranger will emerge. Is he prepared to know her, assumed to be so different from his grandmother, a young woman probably not yet married?
Uncertainty is present by the line that stands alone—he stops to ask himself questions about what he is able to do: will he be strong enough to face this challenge? Will he be able to reach far enough into the past to make the connection between the child he was and the man he is, and his grandmother?
And I ask myself...
However, as the author speaks of leading his grandmother by the hand and helping her to come to terms with things she may not understand--of the world--or of him--he "stumbles," almost a sure sign of uncertainty, and then hears pitying laughter in the sound of the rain.
Yet I would lead my grandmother by the hand
Through much of what she would not understand;
And so I stumble. And the rain continues on the roof
With such a sound of gently pitying laughter.
Does the "pitying" laughter symbolize his grandmother's laughter, as if knowing what he is going through—a part of life—of meeting those you love face to face as people, not as labels (grandmother, friend, boss, etc.)? Or does this laughter symbolize his uncertainty of understanding her, or moreover, her understanding him, as an impossible task?
The questions far outweigh the answers and, even at the end, we don't know what will happen. This, too, leaves the reader with a sense of uncertainty, the same emotion the author seems to feel as he approaches the unknown in that attic corner.