Geoscience Reference
In-Depth Information
Mineralogical analysis by XRD can be used to identify
crystalline constituents that cannot be resolved by
microscopy. It should be noted that the semiquantitative
estimates of composition derived from XRD will be
biased due to the presence of amorphous (noncrystalline)
phases in ceramics, which cannot be detected by the
method. Microstructures and composition of ceramics
can also be successfully investigated using SEM and EDS
microanalysis (Hernández-Crespo
et al
., 2007).
Colour may not always be uniform and bricks of mixed
colour may result from variations in the raw materials, or
from thermal gradients developing in the brick during
firing. Black patches and black core may result from
inclusions of carbonaceous and combustible materials
that create reducing conditions within the brick. The
superficial colour of the brick may be controlled by
treating the outer surface with sand or pigments before
firing.
Brick porosity depends both on the clay composition
and firing temperature/duration and may range from 1-
50%. The vitrified matrix usually contains few
micropores (<1 μm across) but there is usually a
continuous network of larger voids.
Under favourable circumstances, petrographic
examination techniques can be used to identify the
source of brick raw materials and the method of
manufacturing. The most effective approach is to use a
combination of optical microscopy, X-ray diffractometry
and SEM with an EDS microanalysis (Pavía, 2006). The
origin of clay raw material for bricks is difficult to
establish because the raw material is often processed
prior to use and then the clay is more or less wholly
converted to other minerals by the firing process. If
residual clay minerals are present and can be detected
and characterized by XRD, it is sometimes possible to
trace fired clay products back to a particular clay deposit.
When minerals formed at high temperatures are
identified the firing temperature in the kiln can be
estimated (Dunham, 1992).
Figures
338-340
show historic clay brick from the
Great Wall of China (Ming Dynasty, circa 1368-1644
AD
).
An abundance of quartz, feldspar, mica, and granite sand
particles are enclosed in the brick. Many of these
particles are angular suggesting that crushed granite was
deliberately added as 'grog'.
Figures
341
and
342
show historic clay brick from a
Jacobean manor house in London (built in 1623). It is
relatively compact and includes no large air voids. Quartz
and flint sand particles are abundant and these may have
originated from the clay raw materials or been added as
grog. One flint particle exhibits red discolouration and
another appears calcined, suggesting that the brick was
heated to between 600°C and 900°C during firing. At
higher magnification (
343
) the fired clay matrix has a
distinctive red appearance but the detailed structure is
still too fine to resolve with the optical microscope. A red
clay-rich particle may represent crushed brick grog from
a previous firing.
Clay bricks are made by shaping suitable clays or shales
to units of standard size, which are then fired at a
temperature of 900-1150°C for a period of 8-15 hours.
The fired product is a ceramic composed predominantly
of silica (55-65%) and alumina (10-25%) combined with
as much as 25% of other constituents. Quartz sand is a
common and desirable ingredient of brick clay as its
presence reduces shrinkage, aids demoulding, and
encourages the drying process by creating an open
texture. Additional sand is sometimes added to the clay
as a 'grog' (or inert filler) and other commonly used grog
materials include crushed rock, incinerated domestic
refuse, and recycled burnt brick. It is common practice to
add some fuel to the brick to produce even firing, for
example, coke or powdered coal. Bricks from historic
structures can contain all manner of unusual inclusions
(Papayianni & Stefanidou, 2003).
Colour is an important characteristic of clay bricks
that is dependent on the chemical composition of the raw
materials and the nature of the firing process. As a
general rule brick colour is determined by the iron
content and the calcium carbonate content of the raw
materials, combined with the amount of oxygen in the
kiln as follows (after Ashurst & Ashurst, 1988):
Brick colour
Causative constituents and kiln
atmosphere
Buff
Iron oxide (up to 2%) at 900°C in
a reducing atmosphere
Bright salmon pink
Iron oxide (up to 2%) at 900°C in
an oxidizing atmosphere
Red
Iron oxide (up to 2%) at 1100°C in
a reducing atmosphere
Blue
Iron oxide (up to 7-10%)
Black
Iron oxide (up to 7-10%) plus
manganese oxides
White
Lime (high %) with iron traces
Grey
Lime (low %) with iron traces
Cream
Chalk (low %) with iron traces
