DRYING AND FIRING
Water
Water of plasticity
20% by weight of wet clay- gone by leatherhard stage
-lubricant that allows clay particles to slide past one another
Pore water- also called atmospheric water- 10-12% by weight
-still present in air-dried clay, fills pores between particles of clay, held there by surface tension of the
water layer around each particle(imprisoned)
-not removed until clay has reached temp higher than water’s boiling point and stayed there for awhile
(water smoking period)
-finished by 250 degrees, why we bisque slowly because steam can rupture piece
Bound water- 10% by weight
-chemically bound in the molecular structure of the clay-Al 2SiO 2H2O
-stays in the water until red heat (1112 F really842-1292)
-not so dangerous because it gradually escapes
-once complete clay cannot be slaked down and is not called clay but ceramic or fired clay
Stages in a bisque
1. The first 150 °C (to 250°F) are the most critical. The water of plasticity and pore water is driven off.
Your work will explode if the water turns to steam too fast. Most bisquing (the explosion issue)
problems happen in this temperature range. At this time you should assume your piece is still a little
wet even if it passes the evaporation test (testing whether clay feels cold to the touch).
2. The time from 350-500 C (840° -1110°F) is when chemically combined water is driven off. After this
point the clay cannot be slaked down in water and it looses all of its potential for plasticity.
3. 1060°F (573°C) Quartz Inversion. At this specific temperature, just at red heat, microscopic quartz
crystals (free silica) in the clay body change from alpha to beta form. This change results in a small
(+2%) volume increase. The quartz inversion is reversed during cooling and the volume decreases
again (-2%) as the quartz returns to its alpha form. Because of the sudden volume changes at this
temperature, both the firing and the cooling must proceed slowly in order to avoid cracking the ware.
4. Oxidation, or burning of the organic compounds found in the clays begins from 1290° - 1650°F (686896° C). This burning process produces sulphur dioxide, carbon monoxide and other combustion byproducts. The mineral compounds in the clay are also converted into an oxide form during this time.
Oxygen must be present and available in the kiln atmosphere during this part of the firing to complete
these processes.
5. At red heat the process of vitrification (1470°F or 802°C) begins. It continues throughout the
remainder of the firing, as the particles that make up the clay body begin to melt and to fuse together.
Vitrification gives clay its characteristic hardness, strength and density. Vitrification proceeds gradually,
requiring both time and increasing temperature. As various compounds in the clay body begin to fuse
together, their products serve as fluxes and draw other materials into the melt. Vitrification is always
accompanied by shrinkage and the amount of shrinkage will increase as a greater degree of vitrification
is achieved.
6. At 1830°F or 999°C the formation of mullite crystals begins. These needle-like crystals form in the
glaze/clay body interface. They provide an intimate connection between the glaze and clay and give
structure to the glaze, increasing the toughness and durability of the glaze surface.
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WHAT DO ALL THESE FACTS MEAN??????
In general a bisque firing should take about 12 hours to fire from room temperature to
1015°C, 1859o F.
Go slowly in a bisque firing until you see the first sign of color in the kiln pore water then
quartz inversion both need slow temp rise!!!
Dull color appears at 600-1022 degrees C, at this point its ok to speed the firing a small bit.
Temperature measures of the intensity of heat (in degrees) not the quantity.
Firing can only be done successfully with the proper amount of heat-work or time allowed to
develop any given temperature. For example one could raise the temperature of a kiln to
cone 12 in two hours. However, upon cooling one would find that the ware was still porous
and soft because the ware had an insufficient amount of time for the proper reactions to take
place.
Pyrometric cones-measure heat work not temperature. Relationship between time and
temperature. Only work in one temperature- a climbing temperature, not a falling
temperature. Small, large shelf cones are all different and are not interchangeable.
How do I tell what temperature the kiln is at? Until you see some color, this is a little
difficult. This is one reason for initially raising the kiln temperature very slowly. In the later
stages of a gas firing, when we get closer to the temperature we want, we will follow the
sight cones. One cone should fall several cones before our desired temperature (this tells us
we are close and should check the kiln more frequently). Then, we want to have three
cones in a row. The middle cone is our desired temperature. The other two are just above
and just below in temperature. This is so that we can balance our desired temperature with
any possible unevenness in the actual kiln firing. If we are firing to C/08, our cones would be
C/011, C/09, C/08, and C/07.
Should all clays be bisqued to the same temperature??? No. Often porcelain is more
refractory and should be bisqued to C/04. Most other clays are fine at C/08. It is quite
common to bisque lowfire clays higher than their final glaze temperature. This is especially
true if one wants to vitrify the clay but the glaze temperature is lower than C/04.
FIRING IN A GAS KILN
BISQUE
FIRING A BISQUE IN AN ELECTRIC KILN IS MORE COSTLY. IT SHOULD BE AVOIDED
WHENEVER POSSIBLE.
There is more to a gas kiln than temperature and time. In a gas kiln we have to control the
atmosphere of the kiln. There are three options, oxidation, reduction, and a neutral kiln. For a
bisque firing we want a neutral or oxidation atmosphere.
It is a good idea to candle the kilns overnight using just the pilots to ensure that ware is dry. Please
make certain the shut off mechanism is turned up to around 300 degrees (ADJUST THIS TO YOUR
FINAL FIRING TEMPERATURE THE MORNING OF THE FIRING).
REDUCTION FIRING INSTRUCTIONS
Please arrange for there to be someone watching the kiln at all times on the day of the firing. During
much of the firing it is only necessary to check the kiln once every 30 minutes or every hour. In the
morning please follow the turn ups recommended in the instructions for your kiln. Because it is
important not to miss body reduction (cone 07 or cone 08) you should check the kiln every 15 minutes
once you have color and think the kiln is close. Typically you should control the reduction by pushing
in or pulling out the damper, the more you push it in, the more reduction you get. The body reduction
should last from cone 07 or 08 to cone 04 or no longer than 45 minutes. During this time there should
be a strong bushy and orange flame coming out of the peeps when you pull them. There should not
however be a strong smell of unburned fuel or carbon and you should not see black smoke (unburned
fuel) coming out of cracks in the kiln or at the peepholes. There should not be any black buildup on the
peeps themselves. These are all signs that the reduction is too heavy and you need to pull the damper
out a bit. After the 45 minutes of body reduction, pull the damper out slightly to set a slight reduction
that is characterized by a gentle 6-10 inch flame coming out of the peeps. Hopefully this will be the
very last adjustment on the kiln until the very end. Every time you adjust the kiln, it is important to stay
for 5 minutes and check the atmosphere again because it takes a few minutes for your adjustment to
take full affect and the perfect reduction you have after two minutes may be far too strong after five
minutes. During body reduction I think you should check the kiln every 15 minutes. At the end of the
firing people frequently reoxidize the atmosphere for 15 minutes.
ATMOSPHERE
There is more to a gas kiln than temperature and time. In a gas kiln we have to control the atmosphere
of the kiln. There are three options, oxidation, reduction, and a neutral kiln.
OXIDATION This is an air rich flame, tends to be blue in color and has clean efficient combustion. This
firing has adequate oxygen present and available in the kiln atmosphere throughout the firing process.
Electric kilns always fire an oxidation cycle. Fuel burning kilns (i.e.; kilns which burn natural gas,
propane, oil, or wood) can also be managed in order to fire an oxidation cycle.
NEUTRAL This is an even mixture of air and gas, it is the most efficient flame to gain heat in the kiln.
You should get occasional wisps of flame out the peeps and damper in a fuel burning kiln.
REDUCTION This is a gas rich flame. You should get flames out of the peeps continuously and should
also smell unburned gas in the air. To fire a reduction kiln is to fire a fuel burning kiln with incomplete
combustion. This produces an atmosphere with an excess of hot carbon. The excess carbon in the
kiln atmosphere draws oxygen from metallic oxides in the clay body and the glaze ingredients to
combine with in forming carbon monoxide and carbon dioxide. When the oxygen is removed from
these oxides they are "reduced", and the colors that they produce are changed, from those which are
produced in an oxidation atmosphere, to those which are produced in a reduction atmosphere.
HOW DO I CHANGE THE ATMOSPHERE IF IT IS NOT WHAT I WANT?
1. If you have an oxidation firing and want neutral, either decrease the air intake or increase the gas.
Always adjust the air intake first.
2. If you have a reduction atmosphere and want neutral, increase the air intake or decrease the gas.
Again adjust the air first.