22
Discolored glazes are often caused by one of the following reasons;
• Contamination of glaze by chemicals in the ware, especially if one-fired.
• Dirty brushes.
• Loading pieces too closely, causing cross contamination of glazes by fumes or bubbles.
• Placing pieces very close to the kiln elements, creating hot bands across the piece due to direct radiation from the ele-
ments. This can leave a band of slightly faded glaze across the piece.
• Overfiring. Too high a heat will frequently cause colors to fade.
• Glazes applied directly to the greenware.
• Incompatibility of colors.
Pinholes, bubbled glaze and craters may be attributed to one of several causes;
• Immature bisque. If your bisque is not fired hot enough to complete the vitrification and remove all vapors from chemical
reactions that come from the ware, they may erupt into the glaze.
• Dust on the ware or in the kiln.
• Firing too rapidly.
• Cooling too rapidly. Craters are sometimes formed by cooling the ware too rapidly, which freezes the crater formed when
the glaze bubbles. You can try merely refiring to minimize the effect of these defects, or you may apply a thin coat of
glaze and refire to the proper cone which may correct the problem.
Faded decals are usually due to under or over firing. Check the manufacturers’ recommendations and refire underfired ware
to the proper cone.
Trouble with reds is common; they are quite sensitive and often they appear faded or contain washed out areas or dark spots.
Probable causes and corrective measures (where possible) are listed below:
• Too thin a coating of the red glaze. Try reapplying a thicker coat and refiring.
• Overfiring. Reds are generally fired to a temperature ranging from cone 07 to cone 06. They do not do well at greater
heats.
• Incompatibility with other colors. Some colors (such as green and yellow) due to chemical composition tend to make
achievement of bright reds difficult. If you suspect this problem, ask your ceramic supply dealer which glazes may be
used with his reds.
• Insufficient air circulation during firing. This usually results in black spots. Reds need sufficient oxygen to mature. Try
leaving peephole plugs out until true red (no pun intended) heat has been reached.
• Soaking too long at maximum temperature.
Cracking and crazing (crazing is characterized by many very fine cracks running throughout the glaze surface) may be
caused by one or more of the following reasons;
• Entrapped moisture. Make sure greenware or bisque is bone dry before firing.
• Internal stress due to rough handling.
• Too rapid or uneven heating or cooling (especially in heavy, thick pieces.) If this occurs, then all of the piece does not
expand or contract at the same rate, resulting in stress which is released by crazing or cracking. You can usually tell if a
glaze cracked during heating or during cooling by carefully examining the crack. Smooth rounded, and/or sealed cracks
indicate crazing during the heating phase & sharp angular edged or separated cracks indicate too rapid cooling (possibly
a draft from opening the lid or peephole too fast). If you suspect you are firing at too fast a rate for your ware to properly
absorb the heat, the following is a useful check. Place a cone on a shelf in the middle of the load. Put a bisque bowl over
it, and fire as usual. Afterward compare this cone with the witness cone you used for the firing. If it is not deformed to
the same degree, you are firing too rapidly for the weight of the load.
• Incompatible clays and glazes. Check with your dealer for compatible materials. Crazing may often be minimized by
refiring slowly to a slightly higher temperature than that to which the ware was previously fired.
Delayed crazing (crazing which does not develop immediately but may appear months after firing) is the result of internal
stress, usually caused by too rapid cooling (which may seem very slow to the hobbyist). Permanent internal stress combined
with a small jolt, or vibration, can crack the piece months after firing. If delayed crazing is a problem, try refiring to the proper
temperature and cooling very slowly. Do not open the lid or peepholes during the cooling period.
To Next Page
Loading...