Firing Cycle
From Clay to Mullite
Igneous rock
Igneous rocks are formed from the solidification of molten rock
material. There are two basic types: 1) intrusive igneous rocks
such as diorite, gabbro, granite and pegmatite that solidify
below Earth's surface; and 2) extrusive igneous rocks such as
andesite, basalt, obsidian, pumice, rhyolite and scoria that
solidify on or above Earth's surface. Pictures and brief
descriptions of some common igneous rock types are shown
below.
• http://geology.com/rocks/igneous-rocks.shtml#granite#granite
Granite
• People commonly think of lava and magma as a liquid,
like molten metal, but geologists find that magma is
usually a mush — a liquid carrying a load of mineral
crystals.
• Magma crystallizes into a collection of minerals, and
some crystallize sooner than others. Not just that, but
when they crystallize, they leave the remaining liquid
with a changed chemical composition.
• Thus a body of magma, as it cools, evolves, and as it
moves through the crust, interacting with other rocks,
it evolves further.
Feldspar
• Feldspars are the most important part of rocks
like granite, syenite, gabbro, basalt, gneiss, and
others that make up about 75% of the earth’s
surface. These feldspars contain potassium,
sodium, calcium, and barium, sometimes all
together, sometimes one or two at a time. There
are many, many types of feldspar. Each is mostly
Al2O3 • 2SiO2 but each has a certain amount of
other ingredients. The most common are potash
(potassium) and soda (sodium).
Granite to Clay
• Granite is high in quartz (silica) and the mineral
feldspar. This rock has spent years eroding,
weathering into clay.
• After the rock is broken down, it picks up the
chemical water (H2O) while just lying around in
the bottom of swamps and lakes.
• The Al2O3 • 2SiO2 (feldspar) adds 2H2O
• Therefore, the basic chemical formula for clay is
Al2O3 • 2SiO2 • 2H2O
Weathering Process
• During the weathering process the rock
(granite) is mixed with other minerals (iron,
magnesium and titanium are the most
important).
• Each mineral makes the clay different, but
each has the same basic formula.
Al2O3 • 2SiO2 • 2H2O
Firing Cycle
• Once the clay has been shaped and is bone
dry, (feels completely dry-room temperature)the water of plasticity has evaporated. It is
never completely dry because our atmosphere
is not at 0% humidity. Therefore, the last 2%
of the water of plasticity (mechanical water) is
evaporated during the firing cycle.
Water Smoking
• When the temperature in the kiln reaches 212°F
all remaining water of plasticity (mechanical
water) is burned off.
• This is the first in a series of critical points. If the
steam leaves the clay too fast, the pot explodes.
• Fire the kiln slow and easy with the door open a
crack or the vent on to allow the steam a chance
to escape.
• In large kilns, enough steam is often generated to
be seen. For this reason the early part of the
firing cycle is called water smoking.
Dehydration
• As the temperature increases to 660°̊F, the
chemical water starts to burn off. This is the
2H2O part of the clay formula.
• By 950°F the clay is completely dehydrated-all the
chemical water has been burned off.
• The clay is now a chemically different material
than it was when put into the kiln.
• It is now an aluminum silicate known as mullite.
(Al2O3 • 2SiO2) This change is not reversible.
(notice the 2H2O is now gone)
Quartz Inversion
• The next critical change begins at about
1000°F. The quartz crystals in the clay
undergo a change called the quartz inversion.
The crystals grow and change in shape. This
change must be done slowly as the crystals
expand or the ware will crack.
Oxidation
• Oxidation (the burning off of organic
materials) occurs at 1600°F to 1700°F. This
has no chemical effect on the clay, but it does
leave the clay more porous. This is an
advantage when glazing bisque ware.
Vitrification
• Partial vitrification begins as the temperature
rises. This will be determined by the type of clay
body.
• Clay with a high alumina content (stoneware and
porcelain) will vitrify more slowly and at a higher
temperature than clay high in fluxes such as iron
or talc (earthenware).
• Vitrification is the melting of the clay platelets.
Complete vitrification results in a glasslike
material: therefore, clay ware is never completely
vitrified. (or it would be a puddle!)
Cooling
• The clay should cool slowly-especially as it
drops to around 1063°F. The quartz inversion
is reversed and the crystals shrink. Again, this
is done slowly or the ware will crack.
• The ware is now and forever more mullite and
will never again be able to be recycled and
reused.