Geoscience Reference
In-Depth Information
351
Figures
351-353
show terracotta made by the Italian
Giovanni da Maiano for Henry VIII in the early 16th
century. Manufactured from fine clay with grog comprising
a mixture of quartz sand (specially crushed to a uniformly
fine size) and pulverized fired clay. XRD detected the
presence of the mineral mullite, suggesting a maximum
firing temperature for the terracotta in excess of 1000°C.
Figure
354
shows terracotta from the façade of a
historic London building, dating from 1905. It is
manufactured from fine clay with grog mainly
comprising pulverised fired clay. Distinctive wedge-
shaped ('arrowheads') crystals of tridymite were observed
in thin section suggesting that the brick was heated to
temperatures in excess of 870°C during firing (
355
).
Figure
356
shows an example of modern terracotta
which includes a large pellet of previously fired ceramic
material as grog.
351
Historic terracotta comprising a fired clay matrix
(red) with grog consisting of finely crushed quartz-rich
sand (white); XPT, ×35.
352
Traditionally ceramic tiles took several days to make as
the clay base was fired first and then fired again, after the
glaze and any decoration had been applied. Modern
methods use a single firing and can be completed in as
little as 20 minutes. To achieve a very low and uniform
firing shrinkage the bodies of modern tiles are made from
finely ground, evenly mixed refractory clays with
minimal fluxing agents. A typical recipe would consist of
35% ball clay, 15% china clay, 10% limestone, and up to
15% ground tile waste (Prentice, 1990). The remaining
25% would comprise silica, usually as finely ground
quartz sand but frequently including a proportion of
cristobalite. Tile glazes are essentially silica, with
nepheline syenite or feldspar added as a flux, coloured by
metallic oxides, and fused in a furnace. Optical
microscopy may be employed to investigate the
composition and condition of ceramic tiles and can be
complemented by SEM and microanalysis (Kopar &
Ducman, 2005). Figures
357
and
358
show a section
through the outer surface of a modern ceramic tile. This
consists of a fired clay base with an underglaze
containing a coloured crystalline aggregate, over which
a transparent silica wet glaze has been applied to seal the
tile surface.
Ceramic tiles are fixed to walls and floors using tile
adhesive (
359
). The failure of tiled surfaces may involve
loss of adhesion or cracking often caused by a lack of
provision for movement, use of inappropriate bedding
materials, or poor workmanship (Jornet & Romer, 1999).
Petrographic examination can provide information
helpful to the identification of failure causes, by checking
352
Close view of historic terracotta showing fired
clay matrix (red/brown) and grog particles consisting
of angular quartz particles (grey/white) and previously
fired clay (red, right); XPT, ×150.
353
353
Very close view of historic terracotta showing
fired clay matrix (red) and grog particles consisting of
angular quartz particles (grey/white) and muscovite
mica (pink, centre); XPT, ×300.
