I'm not certain if there are any absolutes for dating brick but I usually
assume that the tempering agent used on the brick can tell something about
its relative age. For example earlier bricks are usually prairie grass
tempered, then later straw tempered and after machine manufacturing of
brick you usually have a grit or sand temper. In addition many of the early
pioneer sites were making their brick on site and unlike Ian's experience
of uniformity in brick size in Australia, I have found extreme variability
in brick size and irregularity in shape. I again assume that irregular
shape and size is a product of on site manufacture. There is  a very good
article on Nineteenth Century Midwestern Brick by William Walters, in
Pioneer America 1982, Vol 14:125-136. In addition I have found a lot of
variability from site to site. On two 1840s farmstead sites one in
northeast-east central Iowa and one in southeastern Iowa, the brick from
the southeastern site was twice as large as the brick from the northeastern
site. I tentatively suggested that these differences were due to the
different regions of the eastern U.S. where the pioneer settlers were
coming from. I have also been trying to figure out if calculations of brick
density can tell anymore than who highly fired they were and what type of
kiln/firing situation was used. I think that in general bricks made on site
tend to be less well fired in addition to their irregularity. Therefore I
think it is possible to distinguish between on site homemade brick and
brick made by a professional brick maker prior to machine manufacturing of
brick.
 
Sorry for rambling, hope this is of some help.
 
Susan R. Snow
Project Archaeologist
Office of the State Archaeologist
University of Iowa
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(319) 335-2396
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