Agriculture Reference
In-Depth Information
Conclusions
It is likely that the historic rubbing bricks were fired at slightly lower tempera-
tures than their modern counterparts. This would explain the difference in
physical properties observed, as a lower firing temperature would result in a
greater porosity.
Apart from the usual cements generated through firing (e.g. haematite
and goethite), a number of additional mineral cements were microscopically
observed, interspersed within the brick matrix. These arose from reactive min-
erals contained within the temper of chert and volcanic rock fragments. This
reactive temper was found to form reaction haloes and cements in the sur-
rounding matrix.
The petrography of these reaction haloes and cements is very similar to that
of hydraulic reactions involving certain types of pozzolans. These have been
often observed during petrographic analysis of hydraulic lime mortars (Pavia,
1995-2001).
The petrography of the reaction haloes and newly-formed cements in the
rubbing bricks suggests that the temper was probably activated when the
bricks were soaked prior to using. The temper within the rubbing brick - rich
in microcrystalline silica - is acting in a similar manner to a pozzolan in a
hydraulic lime mortar. The temper has reacted with lime in the presence of
water, forming reaction haloes and cements in the surrounding matrix.
The great ability of the historic rubbing bricks to absorb water by capillary
action, together with their high porosity, implies that their fine pores are bet-
ter inter-connected and more effective at transporting water, than the coarse,
open pores of the modern rubbers. This high efficiency in transporting fluids
allows free movement of water throughout the brick, and, therefore, does not
restrict the local crystallisation of cements from solution.
Although modern rubbing bricks may be considered to be low fired, in com-
parison with other building bricks, they have probably been fired at a higher
temperature than their historic counterparts, resulting in slightly different
physical properties.
In both cases, however, the firing temperature has been sufficient to induce
sintering. It is this sintering process, and the presence of reactive temper,
inducing localised cementation, that are the main contributors to the observed
durability of the rubbing bricks.
Further research is needed to investigate the differences in physical proper-
ties between historic and modern rubbing bricks and their effect on mineral
cementation and related durability.
