Hi Duped --
Writers do not write how speech actually sounds.
Dialog which appears on the page as
"Do you have a jar of honey?'
may actually sound like
"'Dchu h'vva jarra hunny?"
or
"Dooya haf a joirra hoony?"
or
"D'yal huv a jura hoona?"
The writer doesn't write phonetically.
The writer uses standard spelling,
and the reader creates, within his own mind, character voices.
Spelling is not an approximation of how speech sounds. It's a convention for representing speech.
OK. On to contractions.
Before C20, the words 'I am' -- two words, no contraction -- were most generally used to represent that vast constellation of sounds --
"Ahm' and Ah ahm' and 'Ah yam' and 'Ay Yam' and 'Ahrm' and 'Ermm'
that means first person nominative and the verb 'to be'.
It isn't that folks generally pronounced this in two syllables -- or in one syllable.
It isn't that this pronunciation changed at any particular time.
It is that the convention, especially in C19, was to write, 'I am', regardless of what was actually said.
In C20, convention changed.
It became the custom to differentiate between a slurred pronunciation and a pronunciation that breaks into two syllables.
Since a pronunciation that breaks into two syllable is most often emphasizing one word or the other
or is part of a careful and precise speech pattern,
using 'I am" has acquired specific connotations that it did not have in, for instance, C19.
Should you use contractions?
By using 'I am' exclusively you are not more accurately representing historical speech,
you're just using a historical convention for representing speech.
You do not 'sound' more like a speaker in 1870 or 1710.
(Some of whom used one syllable and some of whom used two and we'll never really know who was doing which.)
You just sound like a Victorian or Georgian writer.
(This of course may be what you want to do, in which case I say 'cool' and wave enthusiastically from the sidelines.).
If you use contractions ...
ISTM you are just as likely to be 'accurate' in your actual sounds.
And using contractions in a C20/21 manner will communicatemore clearly and effectively with your audience
who are expecting to read a C21 author, using C21 conventions, when they look at the copyright date.