What effect is created by this combination of concrete and mystical language at the beginning of The Scarlet Letter?
In describing the scarlet letter, Hawthorne combines concrete descriptive details with a concluding sentence focusing on the strange feeling the letter evokes in him.
What a wonderful observation you have made in the confines of Nathaniel Hawthorne's "The Custom-House" section of The Scarlet Letter! Let's look at the instances of both concrete and mystical language you mention. We'll take the concrete first:
It was the capital letter A. By an accurate measurement, each limb proved to be precisely three inches and a quarter in length. It had been intended, there could be no doubt, as an ornamental article of dress.
Concrete details, indeed! An exact measurement and description of its absolute use! It is significant to note here that Nathaniel Hawthorne actually worked in the Boston Custom House in the mid-1800s. It is also important to realize that Hawthorne had Puritan ancestors and many critics believe that he felt guilt associated with their cruel tactics of penance and punishment.
Here is the mystical language (where the word "mystic" is actually used) which immediately follows the concrete above:
Certainly there was some deep meaning in it most worthy of interpretation, and which, as it were, streamed forth from the mystic symbol, subtly communicating itself to my sensibilities, but evading the analysis of my mind. ... I happened to place it on my breast. ... I experienced a sensation not altogether physical, yet almost so, as of burning heat, and as if the letter were not of red cloth, but red-hot iron. I shuddered, and involuntarily let it fall upon the floor.
So, the speaker (Hawthorne himself?) in "The Custom-House" stands there contemplating the letter and, when involuntarily placing it in the exact play where Hester wore hers, the speaker feels a burning sensation! We have entered into the realm of feeling and sensibility of the author.
Now to approach the point of your question. It seems to me if you combine the two sets of language (the concrete and the mystical) the basis serves as a sort of historical context to the novel as well as a precursor to the symbolism of the letter. In short, "The Custom-House" itself, and particularly these excerpts I've mentioned, give the story verisimilitude. It makes us believe the story of Hester. It gives the speaker, dare I say Hawthorne himself, a connection to the story and he, being already moved by the burning sensation of the letter, sets up a nice bit of suspense for the reader who must surely ask, "Why, oh why, does this happen?"
It's the truth of the concrete details coupled with the mystery of the burn that provide suspense. Even further than the wondering why, the reader should enter into the advanced thought that the letter must surely "mean" something. It must be a symbol in this novel, as it also graces the novel's title. And it does, dear reader, at first it symbolizes "Adultery" and later it serves to symbolize Hester being "Able." God bless her!