Chris Salter provided an excellent response to Julie Brodeur's inquiry
on the 'effects of burning on brick'.  I don't have much to add except
to agree that the amount of heat (as associated with peak temperature
reached) to reach vitrification or melting is highly variable.  Pure
clays with less fluxing agents, as in fireclays, will vitrify at much
higher temperatures than those with more fluxing agents, which includes
common elements of regular face brick such as sand temper and iron oxide
for the red color of red bricks.  This is why one doesn't find
sandy-textured, or red, firebricks.

I should also point out that terminology here is important for brick
discussion.  The firing of green bricks (or alternatively the
heat-hardening phase for non-adobe bricks) is generally termed
'burning', as in 'burning the brick hard'.  Thus the initial query of
'properties of brick that has been burned' would normally refer only to
the manufacturing process of heat-hardening the brick.  The query here,
when combined with the effect of burning of the associated mortar,
inferred secondary burning of brick, as in the burning of a brick
structure.  This is the aspect that Chris Salter addressed.

Distinguishing a secondary burning from a primary burning (as in
manufacture) is probably most easily made by a comparison of the
interior, body or paste portion of a brick with the exterior.  If the
brick is vitrified or glassy throughout, as in 'clinker' bricks, one can
assume the vitrification occurred in manufacturing.  If it is only on
the surface, and especially on only one side with possible blackening by
a reduction-characterized environment, a secondary, structural burning
is suggested.  Heat moves very slowly through thick clay bodies like
bricks: thus primary brick burning can last several days versus the
relative rapidity of structural fires.  However, the surface treatment
of 'flashing' (I believe this is the term) a brick by exposing an
already burnt or hardened brick to an intense dose of high heat in a
reductive atmosphere, creates a dark and vitrified, glassy surface.
Such a surface will tend to be uniform across all faces of a brick, as
opposed to secondary burning (or usage in refractory contexts) where
usually only one or a few surfaces are affected.

Larry Buhr
Univ. of Nevada, Reno