Mini World wishes
to thank the
Edward Orton Jr. Foundation and
Evenheat Kiln Inc. for the information found on these pages.
For comprehensive
firing information on your particular type of ware and color, we
suggest contacting the manufacturer. It is important that all manufacturers
directions are followed to obtain good results. Read and follow
their directions carefully.
The Edward
Orton Jr. Foundation publishes a circular called "Firing Line"
and "Firing Tips". You can find excellent information on firing
at http://www.ortonceramic.com/. Direct links to some of
the tips and information found on the Edward Orton Jr. Foundation
website are listed below. To read these files requires Adobe Acrobat
(download free http://www.adobe.com)
Common
Firing Faults |
| BUBBLED
GLAZE, CRATERS AND PINHOLES |
| Bubbled
glaze, craters or pinholes could be caused by: |
|
1
|
Underfired
or immature bisque. Most often the bisque is fired to a cone 03 or
04. The glaze firing is generally fired 1 or 2 cones cooler, 05 or
06. If the bisque has not been fired completely through, gases will
escape from the immature bisque causing small pinholes, craters or
bubbles in the glazed surface. Read all directions on the color manufacturers
products for correct firing temperatures. Contact your slip or color
manufacturer should you have questions on this issue |
|
2
|
Glaze
could be applied to heavy. Apply only the number of coats according
to the color manufacturers directions and label. |
|
3
|
Dust
on the ware or in the kiln could cause some glaze defects. The ware
should be free from dust by brushing the dust off with a soft brush
or a damp sponge. Vacuum your kiln, including inside the grooves,
to keep it free from unnecessary dust. |
|
4
|
Firing
too fast or cooling too rapidly could cause these firing faults. With
the no-lead glazes, rushing through either of these processes will
bring unsatisfactory results. |
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| CRAZING
AND CRACKING |
| Crazing
is recognized by many hairline cracks on the glazed surface after
the ware has been fired. This will not necessarily occur immediately
after the final firing but over a period of time the hairline cracks
will become visible. Crazing and cracking could be caused by: |
|
1
|
Bisque
or greenware that has not been thoroughly fired. Greenware must be
completely dry before doing a bisque firing. The piece should be fired
to a cone 03 or 04 making sure the pieces are not crowded and adequate
heat circulation surrounds all the pieces. If the pieces are not thoroughly
fired, moisture will be trapped inside the ware. This will cause the
crazing and cracking to occur during the glaze firing. The moisture
and gases are escaping and disrupt the smooth glaze finish. |
|
2
|
Either
cooling the kiln too rapidly or removing the ware before it is completely
cool. Do not hurry the cooling process or raise the kiln lid before
it is cool. An extreme temperature change could put stress on your
ware thus causing crazing. |
|
3
|
Handling
ware too roughly. Greenware is very delicate and should be handled
carefully. Putting stress on a particular point could cause a crack
that would not appear until after firing. |
|
4
|
Cracking
of flat pieces could be caused from not enough ventilation surrounding
the piece. Flat pieces should not be placed directly on the
kiln bottom or shelf. They should be raised about 1" from the firing
surface by using posts or stilts. This gives good ventilation around
the entire piece and eliminates cracking during the glaze firing.
|
|
5
|
Inability
for your clay and glazes to work together. Make sure you know what
type of clay you are using and follow the paint manufacturers directions.
Contact your slip or color manufacturer should you have questions
on this issue. |
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|
GLAZE
OR UNDERGLAZE NOT ADHERING TO THE WARE
|
|
1
|
A
dirty surface on the bisque ware caused from dust (including greenware
dust), hand lotion and skin oils are the most common causes. Make
sure the bisque is free from dust and sponged to removed any foreign
material or oils. |
|
2
|
Bisque
that has not been dried completely will not accept a good application
of paint. Dry the ware completely before attempting to paint. |
|
3
|
Underfired
or overfired bisque could be a problem. If the bisque is underfired,
refire to the correct cone. If the piece was to have a stain application,
the bisque firing will not be as hot as the glaze firing. Know your
design plans prior to your bisque firing. |
|
4
|
If
the underglaze if applied to thick, it could pop during firing. Read
the color manufacturers instructions thoroughly prior to application.
|
|
5
|
Incompatibility
could be the problem. Be sure you know what type of color and clays
with which your are working to eliminate any problems. |
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Problems
with reds are not unusual and care should be exercised in firing.
Reds dislike high temperature, love good paint heavy paint application
and good ventilation. Some firing faults on red may be caused by:
|
|
1
|
The
glaze may be applied to sparingly. Unless heavily applied, reds will
bleach out into tattletale grays and weary looking whites. Follow
the color manufacturers direction for proper application and firing
temperatures. |
|
2
|
Firing
reds with other glazes that are not chemically compatible with the
color red. The color will be distorted. Check with you color supplier
on which colors are compatible. |
|
3
|
Overfiring
can ruin the red color. It will turn out grayish and not the bright
shiny red. |
|
4
|
Inadequate
air circulation causing insufficient oxygen may result in black
spots in the red glaze when matured. Good ventilation is a must
while firing reds. Sometimes reds are best fired near the bottom
of the kiln where it has a tendency to be a little cooler.
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Rippling of glazes is caused by too heavy of an application of
glaze. This piece of ware is not correctable but good to remember
for future work. Make sure you read the color manufacturers directions
prior to application and follow directions. Contact the color manufacturer
should you continue to have problems.
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Generally, it is impossible to salvage warped bisque.
|
|
1
|
Incorrectly
removing ware from the mold could cause the piece to warp. Ware should
not be removed from the mold until it is leather hard and releases
easily. This is extremely important in porcelain casting. Porcelain
has a memory and cannot be reshaped. |
|
2
|
Overfiring
can cause distortion. |
|
3
|
Placing
the ware too close to an element could cause one section of the ware
to mature prematurely and distort. Keep the ware approximately 1"
away from the kiln side walls and tube assembly (if one is installed),
and thermocouple (if one is installed) to insure proper ventilation.
|
|
4
|
Improper
support on stress areas could cause warping. Large flat pieces should
be supported with either stilts or posts. Porcelain should be supported
with prop or silica sand. It is suggested that firing instructions
be taken for firing porcelain. |
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