| Orton Firing Tip: Firing Red Glazes |
Reprinted with the permission
of the Edward Orton Jr. Foundation
Red glazes are among the liveliest, brightest colors we can use, but
unfortunately, red glaze problems are legendary. Many of us simply
give up using reds or accept whatever results we can get, including
the problems.
Common Red Glaze Problems
- improper color development- dark bluish or
purple cast to the glaze
- color loss- glaze looks gray, white
- poor surface texture- a rough matte finish
and/or visible surface defects
- "The Strawberry Effect"- tiny black
dots or spots in the fired glaze
- crazing- a crackled or cracked appearance in
the fired glaze
Some of these problems relate to the preparation of the piece and
application of glaze, but many defects are the result of improper
firing practices.
Preparation and Application
- ware must be clean and free of dust
- do not apply red glaze to greenware
- apply only to properly fired bisque (use witness cones to verify
firing)
- work area and tools should be kept clean and free of contamination
- no eating/smoking in glazing area
- glaze away from cleaning areas
- apply adequate coats of glaze - four is often recommended
- allow each coat to dry
How Colors Develop
Many ceramic glazes need to be fired in an oxidizing (air) atmosphere
for best results. Red, orange and yellow glazes in particular are
very oxygen sensitive. This means they require sufficient air during
the firing to bring out the colors to their fullest and to prevent
surface/ finish defects.
Firing reds requires us to control the firing rate and properly vent
the kiln.
Controlling the Firing Rate
Nearly all ceramics fire better when fired slowly below red heat. Slow firings have the advantage of allowing the necessary physical and chemical changes to occur in the ware. Slower firings also permit time for sufficient air to enter the kiln and displace the carbon monoxide. This is true for both bisque and glaze firings.
Firing rate can be controlled using the settings on an automatic kiln, programming an electronic controller or by adjusting the switching. Control or slowing of the firing rate is most important in the early stages of the firing when most of the reactions are occurring and when air is needed to burn out the organics in ceramic materials. Near vitrification (the end of the firing) a faster rate is desirable and can usually be applied.
Venting for Proper Air
It is most important that enough air gets into the kiln in the early stages of firing. This is when the organic materials are burning out of the ware and air reacts with carbon to form carbon monoxide. Kilns can be vented manually or with an automatic venting system.
Manual Venting
Manual venting lets the fumes out of the kiln, but is only somewhat
successful at letting air into the kiln. For manual venting, the top
lid should be propped and the peephole plugs out for at least the
first hour and a half. Slower firings require additional time. When
the kiln reaches red heat, the lid can be closed and peephole plugs
replaced. Leaving the peephole plugs out for the whole firing is not
recommended since it can cause cold spots in the kiln.
Manual venting works better with a smaller load. Also, using split
level shelves allows air circulation and helps ventilation.
Manual venting is recommended whenever a downdraft vent is not available.
When venting manually, it may be desirable to locate red glazes on
the top shelf to assure sufficient air.
Automatic Downdraft Venting
A downdraft automatic venting system like the Orton KilnVent efficiently
brings the proper amount of air into the kiln and removes the fumes
for exhausting. The kiln lid and peepholes remain closed the entire
firing. Using the Orton Vent, test have shown reds can even be fired
with other colors with good results.
Firing to Proper Cone Number
Using witness cones on the kiln shelf to verify results is important to good results. Many problems occur when red glazes are not fired to the proper cone number. Blistering can occur if underfired and loss of color if overfired. Glaze on underfired bisque may craze. Firing lead free glazes to the proper cone number is especially important.
Firing reds can be a challenge, but by following good preparation, application, firing and venting practices, and by firing to the proper cone number, most problems can be eliminated.
Want to learn more?
Read more about Firing Reds in the Orton Firing Line and Technical
Tips publications. Each issue is packed full of articles to help you
learn more about firing. Members of the Orton Firing Institute receive
these publications at no charge. Single copies are available to non-members
at a per issue rate. Orton's 80 minute video, Key Principles of
Successful Firing, is also an excellent resource on firing.
For information on Orton products, see your Orton dealer or distributor.
For information on the Firing Institute or publications, contact:
Orton Firing Institute
PO Box 460
Westerville, OH 43086
(614) 895-2663 |
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