Geoscience Reference
In-Depth Information
Figure
344
shows a modern red clay brick consisting
mainly of fired clay matrix and quartz-rich sand
particles. The brick is compact with no visible air voids
and the clay appears to have been well mixed except for
one unmixed lump. Figures
345
and
346
show a modern
orange clay brick comprising lumps of fired brick clay
that have been pressed together. There is a relatively high
content of macropores present as a continuous network
of interconnected voids. Figure
347
shows a modern
yellow clay brick with uniform texture comprising a
matrix of fine, fired clay with almost no sand or other
coarse particles. There are some interconnected
macropores visible. Figure
348
shows a modern black
brick manufactured from Carboniferous shale in England.
The brick contains black carbonaceous inclusions
originating either from fuel or domestic waste additives.
The main causes of brick masonry decay are eminently
suited to diagnosis aided by optical microscopy. The most
important deterioration mechanisms are salt
crystallization, attack by acid gases in the air, and frost
action. Salt crystallization occurs when a solution of
salt/s in water is deposited under drying conditions on
the surface of the masonry and/or within its pores. The
growth of the salt crystals is an expansive process that
causes powdering, scaling, and delamination of the outer
masonry. Figures
349
and
350
show scaling of the outer
surface of brick masonry caused by salt crystallization.
Terracotta and faience are moulded clay products made
from fine, pure clays mixed with 'grog' (or inert filler) to
reduce shrinkage considerably. The grog may comprise
sand and/or pulverized fired clay. Manufacture involves
pressing the carefully prepared clay on to the inside of a
plaster mould to a thickness of 1¼ inches (just over
30 mm). The mould is then set aside and the plaster
absorbs moisture from the clay. When sufficiently dry
the clay is separated from the mould and hand finished
as required. After further air drying the clay piece is kiln
fired at high temperatures (up to 1250°C). In the case of
faience, a high temperature glaze is applied to the unfired
clay prior to firing in one operation.
Petrographic techniques used to investigate clay brick
can be applied to terracotta to determine the ingredients
used, the method of manufacture, and to screen the
material for defects and evidence of deterioration.
Terracotta units suffer from a similar range of weathering
and deterioration mechanisms to those affecting clay
bricks. Inadequately fired units are particularly
susceptible to frost attack and corrosive atmospheres. In
addition, a range of cracking, spalling, and staining
issues can arise from corrosion of the iron and steel
anchoring systems of terracotta façades.
344
344
Red clay brick comprising particles of quartz and
feldspar (white/grey), with a matrix of fired clay (red).
A rounded unmixed lump of fired clay is seen lower
right; XPT, ×35.
