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Common Kiln Firing Faults

BUBBLED GLAZE, CRATERS AND PINHOLES

Bubbled glaze, craters or pinholes could be caused by:



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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.
Get a hold of your slip or color manufacturer should you have questions.

Glaze could be applied to heavy.

Apply only the number of coats according to the color manufacturers directions and label.

Dust on the ware or in the kiln.

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.

Firing too fast or cooling too rapidly.

With the no-lead glazes, rushing through either of these processes will bring these unsatisfactory results.

CHINA PAINT RUBBING OFF

Paint has not been fired to maturity which will then rub off.
Paint that is not mixed properly and thoroughly.
Pottery that is dirty or has a film coating the piece.
Clean the piece good with alcohol.
Improper firing procedure.

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:

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.

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.

Handling pottery and ceramics too roughly.
Greenware is very delicate and should be handled carefully.
Putting stess on a particular point could cause a crack that would not appear until after firing.
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.

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.

GLAZE OR UNDERGLAZE NOT ADHERING TO THE WARE


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.

Bisque that has not been dried completely will not accept a good application of paint. Dry the ware completely before attempting to paint.

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.

If the underglaze if applied to thick, it could pop during firing.
Read the color manufacturers instructions thoroughly prior to application.

Incompatibility could be the problem.
Be sure you know what type of color and clays with which your are working to eliminate any problems.

PROBLEMS WITH RED GLAZES

Problems with reds are not unusual and care should be exercised in firing.
Reds dislike high temperature, but love good paint, heavy paint application and good ventilation.
Some firing faults on red may be caused by:

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.

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.

Overfiring can ruin the red color.
It will turn out grayish and not the bright shiny red.

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.

RIPPLING OF GLAZE

Rippling of glazes is caused by too heavy of an application of glaze.
If you get a piece of pottery like this, it is not correctable but good to remember for future work.

WARPED BISQUE

Generally, it is impossible to salvage warped pottery and ceramic and is caused by:

Incorrectly removing ware from the mold.
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.

Overfiring can cause distortion.

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.

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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