Agriculture Reference
In-Depth Information
Table 9
Reference values for porosity, water absorption, and water suction of historic, hand-made,
and machine-made clay bricks (Pavia and Lynch, 2003).
Reference values
Brick type
Porosity (%)
Water absorption (%)
Water suction
(gcm
2
/min)
Hand-made
Irish, seventeenth century
36.15
18.47
0.08
range
(mean of 10 samples)
Spanish, seventeenth century
37.13
21.29
0.22
(mean of 19 samples)
Machine-
Gault facing
38.5
-
-
made range
Keuper marl
24.6
-
-
Flettons
34.8
-
-
London stock
48.9
-
-
action (suction). The mean porosity value for both the calcium-bearing bricks
and the haematite-rich bricks (39.87
2.00, respectively) fall
within the typical range of historic hand-made and machine-made bricks (see
Table 9).
The average value of water absorption for the calcium-bearing bricks (19.00
2.51 and 33.27
2.10) is similar to that of the historic hand-made range. The haematite-rich
group, on the other hand, has a significantly lower water absorption value
(13.42
1.50) than the reference values given. The mean water suction
values for the two groups (0.60
0.10), however, are both
significantly higher than the reference values given. The difference in phys-
ical properties between the two groups could be due to differences in firing
temperature.
The results have shown that the historic and contemporary rubbing bricks
can be distinguished from each other through both their mineralogical content
and physical properties. In order to substantiate the mineralogical difference,
a further 10 historic and 18 contemporary rubbing brick samples (Table 10)
were sampled and subjected to mineralogical analysis (XRD).
The mineralogical compositions of the samples (Table 11) have not revealed
any distinctions between the historic and contemporary rubbing bricks, and
no calcium-bearing minerals were identified in the historic samples. Five out
of the 10 historic rubbing bricks showed the presence of goethite, an iron min-
eral formed during the firing process. As this is often found in contemporary
rubbing bricks (see Table 7) it is not, therefore, considered to be a discriminat-
ing factor.
0.11 and 0.36
