Understanding the characteristics of fireclays and how they are mined and processed can offer valuable information regarding their use in any ceramic project.
As the term implies, fireclays
associated with fire or heat. While the refractory nature of the clay becomes
evident when used, fireclays can also contribute particle size variation to a
clay body to enhance its forming characteristics.
As a type of clay, fireclays offer potters the most cause
for concern when used as part of a clay body formula, and they require vigilant
inspection and care when introduced. While other components of the clay body
have their own potential for producing imperfections in the forming, drying, glazing
or firing stages, fireclays are statistically more likely to be the origin of
faults as opposed to ball clays, stoneware clays, bentonites, earthenware
clays, feldspars, flint, talc or any of the other raw materials that can
compose clay body formulas. Understanding the characteristics of each fireclay
and how they are all mined and processed can offer valuable information
regarding which to use in any ceramic project.
The geologic formation of fireclays depended on rock from
the earth's crust being weathered by wind, rain, heat, cold and abrasion. The
raw materials were then transported to acidic waters containing roots and
vegetation, resulting in finely divided minerals containing silicates of
alumina, among other materials. After the Glacial Period (800-600 million years
ago), the waters receded, leaving sediment and silt ground up by the receding
Streams moved the remaining fine particle material to
other locations, resulting in different types of fireclay depending on the
local geology. Almost all fireclays are formed by their movement along streams,
eventually forming into beds. Clays of this type contain impurities gathered on
their travels, which can contribute to their plastic qualities, fired color,
particle size, organic content and refractory nature.
Consequently, the American Ceramic Society defines
fireclay as a "sedimentary clay of low flux content," which enables
it to withstand high temperatures before deforming. Generally, fireclays can
have a pyrometric cone equivalent (PCE) from cone 31 (3022°F) to cone 36 (3268°F).
The pyrometric cone equivalent is a classification of the softening point of a
material, in this case fireclay.
To determine PCE, the fireclay is shaped into a
"cone" and placed in a cone holder with a series of known Orton (www.ortonceramic.com
) cones close to the temperature
where the fireclay is projected to soften. The cone holder is placed in a
specially designed furnace and the temperature is raised on a schedule until
the fireclay cone bends over and the tip touches the cone holder. At this
point, the sample is removed, and the cone in question is matched with the
"standard" cone and given a PCE value.
Christy Minerals offers an example of one type of clay
mining operation. The mine is located approximately 50 miles west of St. Louis, Mo.,
and uses selective open pit excavation to extract Hawthorn Bond fireclay. The
top layer of overburden is taken away and the clay seam exposed. After
carefully removing the clay, it is stockpiled and weathered for 12-18 months to
cause it to break down.
The natural freeze/thaw conditions, plus rain and sun,
hydrate and dehydrate the clay platelets, which ultimately improves consistency
and plasticity. The clay is then transported to the plant where it is dried and
placed in a covered shed until it is ground to the appropriate mesh size.
Several grinding methods can be used depending on the desired grain size and
distribution of the product. The material is then tested for particle size
distribution and placed in packaging that ranges from 50- and 100-lb paper bags
to larger 3500-lb supersacks.
Raw Clay is rarely taken from the mine site without some
form of processing. Over 99% of clays are destined for use by industry and the
commercial sector of the economy. Potters represent less than 1% of the total
market, and their limited role affects the quality of clays they can purchase.
Clays for commercial applications require a degree of uniformity and quality
control, but not necessarily the same kind of processing that potters require.
Two machines used in the preparation of raw clay are the roller mill and the
In a typical roller mill operation, some of the free
moisture is removed from the clay and it is fed between a ring and a series of
rolls that spin around the inner diameter of the ring where it is ground and
dried. It is then thrown between the ring and rolls by plows. A stream of air
lifts the finer clay particles up to a whizzer (several fan blades); the faster
the blades spin, the finer the clay particles need to be to pass through the
whizzer. Once through the whizzer, the clay/air stream goes to a cyclone
collector where the clay is separated from air during the air floating process.
The clay then goes to a packer where it is bagged and ready for shipment.
Greenstripe is an example of a roller-milled, air-floated fireclay.
In a hammer mill, crushed clay that is 2 in. and smaller
enters from above. High rpm hammers rotate and pulverize the clay, which is
pushed through screens at the bottom. The screen size determines the mesh size
of the clay and the size of possible contaminates processed with the clay.
Hawthorn Bond is an example of a hammer-milled fireclay.
As opposed to the process of screening clay to remove
comparatively outsized coarse clumps of clay and large-size contaminates from a
given amount of clay, the platelet size of clay is best observed on a
microscopic level. Platelet size distribution simply means how many small,
medium or large platelets are present in a given amount of clay. The
distribution of platelet sizes also affects the texture (or "tooth")
of the moist clay during forming operations.
The shape of the platelets, their direction in relation
to each other, and the colloidal water adhesion forces that bind them together
also play a part in determining the handling characteristics of the moist clay.
Platelet size distribution can influence plastic qualities, shrinkage rate,
forming potential and drying characteristics. A greater percentage of larger
platelets will result in a fireclay that is less plastic with decreased ability
to bend when moist. It will shrink less and dry faster than a fireclay with an
increased amount of small platelets.
Fireclays having a finer platelet size will shrink more
and have greater plasticity when moist. They will take longer to dry and
require more extensive water films around the increased amount of platelets in
the fireclay to achieve plasticity. However, most clay body formulas contain
other types of clays, feldspars, flint and grog, which can be mitigating
factors to the above-mentioned fireclay platelet size characteristics.
Figure 1. Raw fireclay nodules caught on a 30 mesh
screen. Particles can range in size from 2 to 6 mm.
The mesh size of the screen is a factor in determining the
amount of contaminants that are allowed to pass through with the fireclay (see
Figure 1). A large 20-mesh screen can allow greater quantities of
"tramp" material or contaminants (e.g., twigs, stones, coal, or
processing debris such as bolts, wire, and metal parts) to enter the final bag
of clay compared to fireclay passing through a smaller 50-mesh screen.
Air floated fireclays, such as Greenstripe 200 mesh,
which is naturally finer than clays like Hawthorn Bond, offer lower defect
rates from contaminants but also change the "clay body feel" when
moist. The higher percentage of fine mesh fireclay used in a clay body formula
results in less "tooth" and a gummier feel to the moist clay. Such
clay bodies do not stand up on the wheel and feel overly soft. They can take on
more water in forming operations and deform more easily due to each small clay
platelet being surrounded by water. While these are subjective factors for each
potter, there is a recognizable difference when a coarse fireclay like Hawthorn
Bond 35 mesh is used in place of the finer Greenstripe 200 mesh as a component
in a clay body formula.
Fireclays with comparatively large mesh sizes (28, 35 or
40) can have periodic runs of irregular contaminants. This happens when rips in
the screen occur or operator error allows large particles to enter the clay
batch. Simply stated, the larger the contaminant size, the greater the chance
of it causing a problem in the forming, drying and firing stages. Using a finer
mesh fireclay is one option that can diminish the problem, but, again, the
moist clay can lack "tooth" or stand-up ability in forming
operations. If a coarse mesh fireclay is used, splitting the total fireclay
requirement 50/50 with another fireclay of similar or finer mesh can reduce the
risk of a bad batch of fireclay disrupting the clay body.
As with any screening procedure, whether it's a 35 mesh
or 200 mesh fireclay, some residual material will get through the screen. Clay
is not a round or symmetrical material, but has an aspect ratio making one
dimension larger than the other. In some cases, depending on how the clay
particle hits the screen, it may pass through or be retained and removed. If
screened again, the off-size material could be removed.
In most instances, however, the material is not screened
again and is simply packaged for shipping. Unfortunately, potters require a
greater degree of quality control for their fireclays than processors typically
offer. As in any economic profit motive activity, some fireclay processors are
more in tune with serving the larger industrial market than the smaller pottery
Several ceramic supply companies have taken it upon
themselves to screen fireclays. While the additional cost might be a few cents
more per pound, the extra expense can be worth the potential failure of the
clay body. Sheffield Pottery, Inc. (www.sheffield-pottery.com
) is one ceramic supplier that
can screen fireclays or any other material used by potters. Typically, a
fireclay is passed through a 30-mesh screen to remove contaminants, though
other mesh screening sizes are available.
In addition, the clay can also be moved through a tube
containing a powerful rare earth magnet that removes ferrous iron contaminants.
The clay will still produce an iron spot pattern when fired, but large
particles of iron will not be present. Both systems bring an added degree of
processing necessary for clays used by potters.
Today's potters find themselves in a difficult economic
situation. In a perfect world, they could dictate the exact specifications of
the materials required in their craft. However, the extra processing of clays
and other raw materials would generate a higher, possibly prohibitive, cost.
Individual potters are often forced to accept material as is, which in many
instances means clays with particle size variations, chemical composition
deviations and tramp material contamination. Inconsistent raw materials and the
potter's inability to change this economic fact are recurrent areas contributing
to the loss of product.
While ceramic suppliers, as a general policy, have
limited liability when it comes to the clay they sell, it is the potters who
suffer a loss as their profit margins are tight. It is not unusual for a potter
to lose an entire kiln load of pottery due to a raw material causing a defect.
A ceramic supplier will replace "bad" clay, but the potter will not
be compensated for time and labor spent in the production of defective pots made
with substandard clay.
As a general rule, the potter will not be remunerated for
the damage caused to the kiln, kiln shelves, adjacent pottery in the kiln or
kiln furniture caused by a defective clay or raw material. In many instances,
the potter must prove that the defect originated in the raw material and not
their own forming or firing procedures.
Figure 2. Particles of iron in fireclay can
cause large blemishes in pottery. The defect is more pronounced in
reduction-fired ware since the iron fluxes more than in an oxidation atmosphere.
Fireclays are the weakest link in any of the clays that
compose clay body formulas. There are several geologic and economic reasons for
the presence and continued existence of contaminants found in this necessary
but troublesome clay. Fireclays can contain carbonaceous materials like
lignite, peat, coal and other associated tramp materials. They can also have
variable particle sizes of silica, iron and manganese. At the mine site, even
with careful excavation, some of these impurities can find their way into the
seams of fireclay.
On a technical level, all of these "bad"
qualities can be refined out of the clay. However, as mentioned previously,
intense processing is not required on an economic level since the majority of
fireclay users (e.g., steel mills, casting foundries and brick manufacturers)
use the clay with minimal processing. Since potters represent less than 1/10%
of the raw material market in the U.S., it does not make economic
sense for mines or raw material processors to refine a product for such a small
market share. Unfortunately, this is a common downside for potters who use raw
materials specified for industrial requirements.
The major (but certainly not only) contaminants found in
some types of fireclays are lime and iron. With iron, the size of the particle
is critical as potters want small random brown specks (1/2 to 1 mm in diameter)
in the fired clay body. However, large nodules of iron (4 to 6 mm in diameter)
in the fired body can cause outsized brown blemishes on pottery surfaces (see
Figure 3. Large (2 to 3 mm) particles of manganese form
concave defects (white boxes) in a reduction-fired c/9 (2300°F) clay body.
Fireclays may also contain aggregate lumps of manganese,
which can cause brown/black concave or convex defects in fired clay surfaces
(see Figure 3). In functional pottery, these disruptions can trap food or
liquid, causing unsanitary conditions. The same, but more pronounced, defect
occurs when manganese decomposes between 1112 and 1976°F in a reduction kiln
atmosphere. The manganese nodules are unaffected by reduction, but melt in
irregular "spit out" patterns on the clay body's surface.
Figure 4. Lime pop caused by the expansion of lime in the
fireclay component of the clay body.
Lime can cause clay body defects depending on the
particle size (see Figure 4). Nodules of lime can expand in the fired clay body
as it takes on atmospheric moisture, and the resulting half-moon-shaped crack
caused in the clay body disrupts the surface. The same material in powder form
does not exert the considerable internal forces of expansion when in contact
Figure 5. Glaze blisters are sharp-edged craters caused
by carbonaceous material in the fireclay exiting the clay body as a gas at high
Fireclays mined near the coal fields of Pennsylvania
are laid down next to seams of coal and lignite (an intermediate carbon-based
material between peat and bituminous coal). If not removed by a clean-firing
oxidation kiln atmosphere, such contamination can cause black coring/bloating
in clay bodies or blistering in glazes due to incomplete combustion of
carbonaceous material in the fireclay exiting as a gas through the glaze layer
(see Figure 5).
Figure 6. Sharp edge cooling crack in ware caused by
cristobalite inversion in the kiln or oven. The larger part of the crack
(right) is where the crack started to form.
Aside from tramp material, large silica particles can be
associated with coarse (35, 40 and 50 mesh) fireclays that can also cause
defects in a clay body. Since potters often prefer coarse material with tooth
or grit, fireclays will include coarse particles of silica by design. Both the
fine and coarse particles have the potential to form cristobalite as the clay
body is heated past cone 8 (2280°F). Excessive cristobalite buildup in the clay
body is often due to the kiln stalling or taking an excessively long time to
reach temperature past cone 8.
By physical nature, the larger particles of silica in
fireclay that convert to cristobalite will produce a larger
expansion/contraction, thus exerting a greater pressure on the body during
firing and cooling than small particles. However, all alumino-silicate
materials (e.g., talc, feldspars and other clays) will contain some level of
free silica that can also be subject to the same transformation to cristobalite
and the eventual cooling inversion.
If considerable amounts are present in the clay body, an
inversion of high-to-low cristobalite takes place around 392°F that can cause a
pinging noise in the pots as they cool in the kiln.1
intact functional pottery with a buildup of cristobalite can also crack later
when heated in a hot kitchen oven due to cristobalite inversion during a
similar cooling range (see Figure 6). Cracks can occur anywhere on the pot and
have a sharp edge, almost like the pottery has been hit by a hammer.
Fireclays are refractory, relatively coarse particle
size, low shrinkage clays that are mainly employed as a component in medium-
(2232°F cone 6) to high-temperature (2300°F cone 9) clay body formulas.
However, they can be used in low-fire (1828°F cone 06) clay body formulas to contribute
a coarse particle size and tooth to the forming qualities of the moist clay.
Fireclays can constitute 5 to 30% of a clay body,
depending on the firing temperature, forming method, fired color and intended
Potters use fireclays in diverse ceramic products like
floor and wall tiles, functional pottery, and sculptures. The amount and type
of fireclay in a clay body formula also depends in part on the cost of bringing
the fireclay to the point of manufacture, whether by a ceramic supplier or
individual potter. The actual cost of shipping fireclay or any clay is often as
expensive as the cost of the material. This economic consideration is apparent
when ceramic suppliers stock raw clay and use clays in their moist clay body
formulas based, in part, on the lowest shipping cost.
Frequently, the decision to stock one clay over another
is decided by transportation costs. It is unusual for West Coast ceramic
suppliers to use Midwest fireclays in their
stock clay body formulas when West Coast fireclays are readily available and
offer lower transportation costs. However, some large West Coast suppliers can
make the capital investment to also stock Midwest
fireclays as part of their inventory. East Coast ceramic suppliers are also
reluctant to carry West Coast mined clays for the same reason.
Economic reasons, not geologic depletion, can dictate a
clay or raw materials' disappearance from the pottery market. Some discontinued
materials do live on in ceramic supply houses' inventories or individual
potters' storage bins, but they are not currently being mined. Before
developing any clay body or glaze formula, always inquire if the material is
still in production. In some instances, potters have used a clay, feldspar or
other ceramic raw material found in their studio only to discover when it is
time to reorder that the material is no longer in production.
Once a fireclay is removed from production, it is highly
unusual for it to be brought back on the market. A little research will save
much time and effort. Some fireclays that are no longer in production are
P.B.X. fireclay, North American fireclay, and Pine Lake
fireclay. Green Ribbon fireclay and G.S. Blend fireclays were blended by Laguna
Clay Co. from Lincoln
60 and Irish Hill Cream fireclays when Greenstripe fireclay was temporarily
For medium- to high-temperature clay body formulas (cone
6 and above), the total lack of a fireclay component can affect the drying
qualities, handling characteristics, fired color and fired maturity of the
clay. Keep in mind that porcelain clay body formulas traditionally do not
contain fireclays when high temperature, whiteness and translucency are
desired, but in a great percentage of high-temperature stoneware clay body
formulas, fireclays are a viable addition to the formula.
Author's AcknowledgementsShane Bower of Christy Minerals LLC supplied
information on the mining and processing of fireclays. Jon Pacini, clay
manager, and Jon Brooks, president, of Laguna Clay Co. were most informative
and helpful in contributing information on West Coast fireclays. Jim Kassebaum,
general manager of Laguna, supplied samples of fireclays for evaluation. Eric
Struck, technical service director at Industrial Minerals Co., supplied detailed
information on West Coast fireclays and processing operations. Dave Jenkins,
president of Ione Minerals, Inc., supplied technical information on the mining
of Greenstripe fireclay. Mort Gensberg, Seaway Shipping & Trading, directed
my research into West Coast fireclay mines. John Cowen, president of Sheffield
Pottery, supplied images and descriptions of the clay screening process. John
Benedict, clay manager at Sheffield Pottery, screened samples of fireclay for
the article. Jim Keating, technical manager of Gladding McBean, supplied
information on Lincoln
SIDEBAR: Fireclays Commonly Used by PottersLincoln Fireclay
The Gladding McBean (www.gladdingmcbean.com
) plant in Lincoln, Calif.,
has been in operation since 1875, and the Lincoln Clay Products plant, a
division of Gladding, has been in operation since 1898. The mine produces Lincoln 60, 8, and 9
fireclays, which are hammer milled. The sedimentary clays are formed by the
erosion of the Sierra Mountains in California
and are selectively open pit mined. Scrapers peel off overburden soil, exposing
seams of clay 2 to 5 ft thick. It is then processed and shipped in 50 and 100
lbs bags and bulk packaging ranging from 20 to 34 mesh. All of the Lincoln fireclays have
excellent drying properties with minimum cracking and warping. Technical data
sheets are available.
Lincoln Fireclay 60
fires to a
buff/cream color and is frequently used in pottery production. Its name is
derived from the one to six layers of clay that form the deposit. Over 30,000
tons per year are shipped to pottery manufacturers and ceramic suppliers. It is
hammer milled and has coarse iron specks (PCE 28-29).
Lincoln Fireclay 8
has a higher
percentage of iron than Lincoln Fireclay 60 and fires to an orange color. It
has good plasticity and excellent strength (PCE 28-29).
fires to a buff/cream color and has granules of siderite
(iron carbonate and iron sulfate). It has a low iron content and exhibits good
plasticity and excellent strength (PCE 30-31).
is Lincoln 60 fireclay
mined by Gladding McBean that is then roller milled and air floated by
Industrial Minerals Co. (www.clayimco.com
). Industrial Minerals sells 400
to 500 tons per year each of IMCO 400, IMCO 800, Howard Clay and Irish Hill
Cream Clay. IMCO 400 has greater strength and fires to a buff/cream lighter
color than IMCO 800, both of which are mined in layers. Technical data sheets
is a smoother fireclay than
IMCO 400 and has a yellow-fired color. It is a finer ground fireclay than Lincoln 8, cutting down on
the size of contaminants and iron specking.
is a roller-milled,
air-floated fireclay mined by Industrial Minerals. It has a lighter fired color
than IMCO 400 or IMCO 800.
Irish Hill Cream
is a roller-milled,
air-floated fireclay mined by Industrial Minerals. It has good green strength
and is not as refractory as Howard Clay, but is darker in its fired color.
The Ione Minerals (www.ioneminerals.com
) plant has been in continuous
operation since its construction by Gladding McBean in the early 1950s.
Fireclays have been mined in the Ione basin of Northern
California on and off since the mid 1800s. The
Ione basin is about 20 miles long and one to four miles wide, along the western
Sierra Nevada foothills.3
The company has eight mine
sites over 21,000 acres, where it solar-dries and processes its fireclays and
The mine produces Greenstripe
fireclay, a very plastic, 200 mesh, air-floated fireclay that features
comparatively high shrinkage and excellent green strength. It fires to a light
buff color (PCE 31-32). Technical data sheets are available.
Christy Minerals LLC (www.christyco.com
) mines Hawthorn Bond
fireclay and sells 80% of its production of fireclays to the pottery market and
the art clay industry, which manufactures architectural ceramics. Missouri
fireclays, such as Hawthorn Bond and A.P. Green (mainly processed for the brick
and refractory industries), take longer than West Coast fireclays to absorb
water in the clay mixing process due to their harder granular structure. This
characteristic can result in clay bodies with a Missouri fireclay component becoming stiffer
after the initial mixing process as water is eventually absorbed into each clay
fireclays may also be associated with seams of lime and gypsum that can have
nodules from 1/4 to 4 in. in diameter, causing lime pop in the bisque and fired
ware if not screened properly in the processing stage. The fireclay can also
have nodules of silica disrupting a clay body surface (PCE 31-32). Technical
data sheets are available.
SIDEBAR: Clay Body Formulas
Many low-, medium- and high-fire clay body formulas
utilize a fireclay component, and it is useful to understand the reasons. It is
also important to note that fireclays act in conjunction with other clays and
raw materials in a clay body formula.
Listed here are examples of clay body formulas that can
be used in oxidation or reduction kiln atmospheres. They can be used on the
potter's wheel or in hand building. The primary functions of each fireclay in
the formula are described. As with any clay body or glaze, it is always best to
test the formula before committing to a large quantity.
Low-Fire Cone 06 #1 (white fired color)
Hawthorn Bond fireclay 50 mesh: 20
Thomas ball clay: 40
Hawthorn Bond fireclay contributes tooth and a
coarse particle size to the clay body. Without the fireclay component, the
moist clay may feel gummy and soft when used on the potter's wheel or in hand
building operations. As a general rule, particle size differentiation from the
use of dissimilar particle size clays is preferred, as it contributes to a
mechanical interlocking of clay platelets in the forming stages.
Medium-Fire Cone 6 #2 (light brown in
reduction atmosphere/off white color in oxidation atmosphere)
Greenstripe fireclay: 16
EPK kaolin: 16
Goldart stoneware clay: 23
Nepheline syenite 270x: 20
Kentucky ball clay OM #4: 15
Silica sand 85x: 6%
Greenstripe fireclay contributes a refractory
clay component that enables the clay body to withstand the higher temperatures
reached in the stoneware firing range. The fireclay also acts to reduce dry and
fired shrinkage, as well as lessen or eliminate warping. The variation in
particle size of the fireclay aids in forming operations.
High-Fire Cone 9 #3 (medium brown in
reduction atmosphere/cream color in oxidation atmosphere)
IMCO 400 fireclay: 20
Goldart stoneware clay: 40
Newman Red stoneware clay: 10
#9 ball clay: 10
Custer feldspar: 12
Grog 48/f: 5%
The addition of IMCO 400 fireclay contributes
refractory strength and reduces dry and fired shrinkage. The fireclay also lessens
any warping potential in the drying and firing stages. The variation in
particle size aids in forming operations.