Dear Folks,
Adding to what Larry Buhr wrote:
"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."
There are exceptions: many of the pre-1880s buildings I have examined are
composed of bricks which display vitrification on one or a few surfaces.
These bricks, particularly from country areas, were burned in brick clamps.
The clamp process often results in uneven burning (as will 'Scotch' kilns).
Parts of the bricks closest to the heat of the fire vitrify, most often the
headers (due to the stacking in the kiln).
Other signs of secondary firing may be the mortar turning a reddish colour
(but including certain mortars in fireplaces). Mortars from 'recent'
fossiliferous calcarenite stones tend to redden under heat. If the fire is
extreme, surrounding material will tend to fuse with the vitrification on
the surface of bricks.
Regards, Robert
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