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firing basics

updated tue 13 mar 01


Cindy Strnad on mon 12 mar 01

Hi, Barbara.

I wanted to send you to a book, but, on reflection, I can't think of any
books I've read that explain this question very clearly. So, here goes.

There are many different ways to fire ^6 pottery. I will share with you the
most common, and the way I fire.

First, it would be helpful to know whether your kiln has a computer
controller. If it does, you have many more options and a lot more control of
the fire, but you can manage without.

Most of us fire bisque and glaze in two separate loads. Cone 6 pottery is
very different from low-fire pottery in firing and glazing methods. Low-fire
is bisque-fired a cone higher than the glaze firing. Not so with higher temp

(Bisque or biscuit is the first firing, in which glaze is not involved, so
called because the fired ware ends up with the porous consistency of a hard

For higher-fired work, the bisque firing is much cooler than the glaze
firing. This is done in order to allow the ware to retain porosity needed to
allow the glaze to adhere easily. People may fire a bisque load at
temperatures ranging from ^010 to ^06. Some fire cooler still. I fire my
bisque to ^04.

Incidentally, when you're below the "zero" mark, a higher cone number (010)
designates a lower temperature. Cone 06 is, therefore, a hotter firing than

All the clays may be fired together for bisque--just be certain you'll be
able to tell them apart after the bisque firing.

Your bisque should be fired to a temperature which will allow it to absorb
water from the glaze in such a manner that it dries within a few minutes of
glazing. If the ware takes too long to dry, bisque at a cooler temperature
next time. If it seems to attract too much glaze, bisque fire hotter next

Now, even if you don't have a computer controller, please see my posts of
last week re: ramp firing schedules. You can find them in the Clayart
archives under "ramp firing" or under my name. To access the archives, click
the web link you'll find at the very bottom of this message.

If you do not have a computer controller, you need to know that it's
important to fire greenware very slowly. You should have the ware absolutely
dry before you place it in the kiln. If it is cool to the touch in a warm
room, it isn't dry enough. If it retains any darker areas of color, it needs
longer drying.

Use shelf cones (ask your supplier for self-supporting shelf cones) to
monitor your firing and to double-check that your kiln sitter or computer
controller is functioning accurately. Set the cones around 6 inches or more
from the peephole in a spot where you'll be able to see them. Make sure you
have a clear line of vision to the cone and behind it all the way to the
back wall of the kiln. These cones are difficult to see in a hot kiln under
the best of conditions, so you don't need a lot of ware standing behind the
cone to confuse you.

If your pieces have thick areas (handle attachments, knobs, sculptural
areas, thick-walled beginner pots, etc.), this slow firing becomes even more
important. Try to keep the kiln below the boiling temperature for your
altitude for several hours. You can use an oven thermometer for this if
necessary, but
*** remove it before continuing the firing.***

The reason for this slow beginning is explained in my ramp firing post. Keep
the lid of the kiln as far open as necessary to prevent it from getting too
hot during this time. Once you've completed this step, close the lid down to
one of the notches on the lid prop. You can keep the peep holes open at this
point. Fire at a leisurely rate up to around 500 degrees Fahrenheit if you
have a thermocouple. If you don't, you can just fire slowly until your kiln
reaches a dull red inside. This is much hotter than 500 degrees, but the
slow firing won't hurt anything.

You may close the lid now, and plug the peep holes. If you are using a kiln
sitter, you should leave the bottom peep hole open to allow for continued
escape of gasses. These gasses will escape through your kiln sitter
otherwise, and eventually cause corrosion.

At this point, you may fire as rapidly as you like. Some of us like to carry
on with a slow firing to ensure that as many gasses as possible will be
released from the clay. This practice helps to prevent glaze flaws such as

Keep a close eye on the kiln. Controllers and sitters do not always function
as intended. Anyone who has been potting for very long has stories to tell
about that one time when the controller didn't work. This is one of the
reasons for the shelf cones.

Once the cone has bent so that its tip touches the shelf, your kiln has
reached temperature and should be turned off even if the kiln sitter hasn't

For glaze firing:

You will need to test your clays to see what temperatures they can really
withstand. Manufacturer's claims aren't always accurate in regard to clay
maturity temperatures. My own clay, rated for ^6, is actually better at just
below ^6. It's possible your ^5 clay will be able to withstand ^6
temperatures, but you won't know unless you try it.

Clay melts when it is over fired. Way over fired. At first, it only bloats
and warps, and that is likely to be the reaction of your ^5 clay to ^6
temperatures. High fire clays which are intended for utilitarian ware should
be fired to their highest possible temperature. This prevents them from
being too absorbent, taking in water through the base or through glaze

You can fire any pieces whose target temperature will be reached during the
firing together. Glaze drips when it becomes too fluid (whether because it
is thicker than it should be, or simply a runny formulation) to continue to
adhere to the walls of the pot. Underglazes alone generally do not drip.

Now, for glaze firing. As before, please see my ramp firing post. This will
give you a lot of information you can use even if you do not have a computer
controlled kiln. This post should tell you most of what you need to know.
Again, do use shelf cones of the proper cone for your target temperature.

For bisque firing, it is important to go slowly at the beginning. For glaze
firing, it is important to go slowly at the end. Keeping your glaze fluid a
little longer at the high end of the firing will allow the coloring
ingredients to mingle and mature. As in many culinary dishes, this added
time improves the result in very perceptible, yet subtle ways.

Firing times will vary according to the ability of your kiln to reach the
temperature goal you've set for it, the density of your ware within the kiln
(try to allow for some air flow--don't pack too tightly), the size of the
kiln, and your own preferences.

Cooling times vary in the same fashion. However, for the best glazes, you
should never rush the cooling of the kiln. Rushed cooling is also hard on
the kiln itself, and on the heating elements. As you'll see in the ramp
firing post, there are times when you will want to cool your glaze kiln
extra slowly.

Generally, the kiln will take as long to cool as it took to fire, but this
is, I emphasize again, a generalization. Unload the kiln when the ware is
cool enough to be handled bare-handed.

Now, you may want to obtain some good general texts on pottery. For this, I
suggest you consult the archives again. Search under topics such as "books",
"Book list", "Library", and so on. Many people have posted good suggestions
for books, and I'm sure you'll find them very helpful.

Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730