|Bloating and Black Coring
|[A brief analysis of a complicated problem]
Bloating occurs when the firing cycle does not match the clay being
fired. The symptom is a void created in the walls of the pot, but
the remedy does not lie with the clay: it is the firing that needs
to be adjusted. We can generate some guidelines for adjusting the
firing based on an understanding of the chemical processes that lead
Why do pots bloat? In the simplest terms, it is because gases are
formed in the body that cannot escape. With no place to go, they expand
while still inside the pot and generate the voids called bloats. So
where do these gases come from, and why can't they escape?
The gases are generated from carbon and sulfur in the clay components
of the body. As a natural part of the weathering and deposit of clay
particles, organic materials containing carbon and sulfur settle throughout
the clays we mine and use for modern ceramics. When these organic
materials reach temperatures ranging from 1290° to 1650° Fahrenheit,
they combine with oxygen, form gases, escape the clay body through
the pores, and exhaust from the kiln. Under optimal conditions, all
of the organics will be expelled from the clay through this process
during the bisque firing. However, it is possible to leave carbon
and sulfur in the clay under the following two circumstances.
First, the bisque firing can be too fast. Not only must the firing
go to or beyond 1650°, but it must allow enough time for the maximum
quantity of carbon to burn out. The process is not instantaneous;
it takes time for all the carbons and sulfurs to combine with oxygen,
and it takes time for the subsequent gases to work their way out of
Second, the atmosphere in the bisque kiln can be oxygen poor. If this
is the case, some of the organics will remain in the body because
they can find no oxygen with which to combine. We see this phenomenon
in both gas and electric kilns. In electric kilns with poor ventilation
and tightly packed ware, the organics in the clay quickly combine
with what little oxygen is available, and there is no means for new
oxygen to get to the pots. Gas kilns have good ventilation, but that
does not mean that oxygen is available. If too much reduction is used
in the 1290° to 1650° range, the atmosphere will be oxygen
poor and carbon and sulfur will be left behind.
Once the organics have been left in the body, they are likely to bloat
in the glaze firing. As the glaze begins to melt, and the clay-glaze
interface becomes vitreous, it forms a barrier. The left-over carbon
and sulfur combine with the oxygen in the kiln, but the now non-porous
surface prevents the escape of these gases. With nowhere to go, the
gases form "bubbles" or bloating within the walls of the
These problems are compounded in iron-bearing clay bodies. In low-iron
bodies, gases contend only with the clay-glaze interface. In iron-bearing
bodies, early reduction causes the formation of "black glass"
(reduced iron and silicate) within the body. This black glass is a
powerful flux which becomes active at 1650° F. As temperatures
rise, the black glass throughout the body gets softer. The net effect
is that the body becomes better at trapping the gases at the same
time that its softness makes it less capable to resist the expansion
of the gases! This sequence of events results in bloating as the body
reaches its normal maturing temperature.
Unfortunately, we may not know that the carbon or sulfur remain in
the body after glaze or bisque firing. Sometimes, improperly, fast-fired
bisque may exhibit a pink cast on the surface, which, if broken open
will often reveal black coring. Black coring is the evidence of trapped
organics in the form of a layer of black visible just beneath the
surface of the piece. The layer of black may be in a "bloat"
in a glaze-fired piece, or it may just be a black layer beneath the
surface of the glaze. It is even possible for the black coring to
exist throughout the fired clay body. Black coring will be present
in glaze fired pieces where organics have been trapped. Black coring
can explain existing bloats, or it can warn us that the firing process
is prone to create bloats even if none have yet appeared.
With this understanding of the bloating phenomenon, how can it be
prevented? It is as simple as eliminating the causes:
The above information offers a brief sketch of how to prevent bloating.
Different clay bodies require different treatment as clays vary in
their organic content. The following are a few of the many sources
for additional information on bloating and proper firing. Time spent
improving firing techniques is money in the bank!
- Fire slowly enough 1290° to 1650° F to give the carbon
and sulfur in the body time to combine with oxygen and work its way
out of the clay body. The burnout process must be done slowly and
completely before the glaze begins to melt.
- Provide an oxygen rich atmosphere. In electric kilns, use plenty
of ventilation in the bisque fire. If your kilns are densely loaded,
use a downdraft kiln vent. In gas kilns, keep the kiln in oxidation
from 1290° to 1650° F.
- When using iron-bearing clay bodies, fire up to 1950° F in
complete oxidation to allow for proper and complete burn-out of the
organics; then, if desired, begin the reduction process.
Laguna Clay Co.