Hi Blah. Good question. I would answer it as follows:
The basic "formal logic" sense of "and" (conjunction)is neutral, but (linguistically) it can be given a supplementary positive or negative charge:
"and" with a +ve charge means "and on top of that, as if that was not enough..."
"and" with a -ve charge (but) means "and, contrary to what you might expect,..."
The linguistic embellishments need not be treated by formal logic until they are made into explicit theses. They are not always so translated, because often they merely betray a prejudice or an unsupported supposition, etc.
For the formalities relating to conjunction see Future Logic, ch. 23: http://www.thelogician.net/2_future_logic/2_chapter_23.htm
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