Hi. Richard;
The brick pictures you posted to me are only showing impurities that were in the clay used to manufacture this brick. Some of those impurities will absorb more heat than the clay and produce this affect when firing.
Below is a little more information that might help some understand the process.
PROPERTIES
"All properties of brick are affected by composition of the raw
materials and the manufacturing processes. It is for this reason that
most manufacturers blend clays to reduce the possibility of impurities
from one clay source affecting the overall quality of the finished
product. Similarly, the standardization of the manufacturing processes
permits the manufacturer to limit variations due to processing and to
produce a more uniform product.
Product Use
The properties that most concern the users of brick are 1) durability,
2) color, 3) texture, 4) size variation, 5) compressive strength and
6) absorption."
http://www.bia.org/BIA/technotes/t9.htm
“Common clay is clay or a claylike material that is easily molded or
shaped and changed into a glasslike material at temperatures of 1,100
°C (about 2000 °F) or less. The minerals found in common clay are
biotite, illite, muscovite, and sericite; other minerals, such as
kaolinite, smectite-group clays (montmorillonite, saponite,
nontronite, and so forth), mixed-layer clays, and chlorite, also are
present in many deposits. Common clay is usually higher in alkalies,
alkaline earths, and iron-rich minerals and lower in aluminum than
ball clay, fire clay, and kaolin. Common clay is mined in hundreds of
places across the United States. When common clay is processed, it is
used mainly in the manufacture of building bricks.”
Brick Clay: suitable clay for the manufacturing of bricks or coarse
pottery. Often an impure clay containing iron, calcium, magnesium and
other ingredients is referred to as brick clay. The chief chemical
composition of brick clays is silica, alumina, iron oxide, magnesia,
lime and alkalis. Either more or less than the required amount of any
of these constituents may cause serious damage to the products. The
presence of some harmful constituents like iron concretions, stone
particles, soluble salts and limestone has a detrimental effect on
brick."
http://banglapedia.search.com.bd/HT/B_0626.htm
"All properties of brick are affected by composition of the raw
materials and the manufacturing processes. It is for this reason that
most manufacturers blend clays to reduce the possibility of impurities
from one clay source affecting the overall quality of the finished
product. Similarly, the standardization of the manufacturing processes
permits the manufacturer to limit variations due to processing and to
produce a more uniform product.
http://www.mii.org/Minerals/photoclay.html
http://www.state.ar.us/agc/clay.htm
Marcel 
