ceramic

The Firing Process
The green body is heated to produce the desired microstructure. The changes occurring during this stage may be fairly complex, depending on the complexity of the starting materials. In the ceramics two terms have been used to refer to the heating stage: firing and sintering. Generally, the term firing has been used when the processes occurring during the heating stage are fairly complex, as in many traditional ceramics produced from clay-based materials. In less complex cases, the term sintering has been used.
Firing Shrinkage
All clays shrink during drying. Firing shrinkage increases as does plasticity, and with that increase comes more drying cracks. This happens because plastic clays have finer particle sizes and thus greater particle surface area and more inter-particle water holding things together. As that water is removed during firing , the resultant particle packing shrinks.
Firing shrinkage can easily be measured this enables you to compare one clay with another. The shrinkage measurement results are more meaningful when methods of sample preparation and water content or stiffness are controlled and when they are done as a matter of routine over time.
Fired shrinkage (shrinkage from dry to fired) is a comparative indicator of the degree of vitrification. As a clay is fired higher it shrinks more and more to a point of maximum shrinkage (after which swelling occurs as a precursor to melting).

Fired Strength
The fired strength of clays can be measured. The test is sometimes called modulus of rupture, recognizing the fact that brittle ceramics fail suddenly (as opposed to others that fail after some plastic deformation). This is also known simply as tensile strength (because the point of failure is always where the sample is under most tension). Ceramics perform much better under compressive strength testing than they do when stressed flexurally, compressive testing is more common in the structural ceramics.
Assume that higher temperatures produce stronger ware. The growth of mullite crystals in porcelain at high temperatures can contribute to a lot of strength. However other factors also contribute to fired strength (particle packing, vitrified vs. sintered, shape and surface properties) and products fired at lower temperature can rival the strength of high fire.
Ceramic is brittle, so any surface discontinuities (e.g. micro-tears made during forming from poor plasticity), large cavities or pores (e.g. from material burned away during firing) or aggregate particles (coarse particles are often surrounded by micro-cracks as a product of drying and firing) provide places for failures. A body matrix can have coarser particles, but these must be complemented by a range of sizes that produce an overall matrix that has densified well during drying and firing.

Defects Generated During Brick Firing Operations
The purpose of brick firing is:
(1) to completely vitrify or densify.
(2) to partially vitrify a body in the case of porcelain.

Porcelain, the body is taken to a temperature to which enough strength is developed to be automatically handled in glazing operations (in modern factories). Porcelain ware have to be supported during brick firing and defects generated are usually chips or breakage from handling.
Kiln dirt can cause defects here, but rarely. If they are formed, they often can be removed by grinding methods.
Case of clays the body is heated on a temperature / time cycle where full strength is developed in non-glazed industrial ceramics.
For fine china and bone china, the body develops maximum translucency through vitrification.
Black Coring Oxidize carbon compounds for removal before the final stages of firing. Can be exasperated by iron compounds causing early melting.
Scumming Caused by impurities such as vanadium.
Distortion Plastic formed ware may have to be supported during low or high brick firings. Refractory setters serve this purpose.