| Orton Firing Tip: Solving Glaze Defects, Blisters,
Craters and Pinholes |
Reprinted with the permission
of the Edward Orton Jr. Foundation
Blisters, craters and pinholes are related glaze surface defects.
They show up as a rough, grainy or bubbled surface on the ware and
appear after the glaze firing or decorating firing.
What Causes These Defects?
This family of problems can be caused by many different factors including:
- dust and contamination in the glaze
- air bubbles in the glaze
- air trapped in the slip
- improperly mixed slip
- a dirty kiln
What Happens During Firing?
Clays and glazes contain organic materials. When heated, these burn
out of the body, forming gases such as carbon, sulfur and water.
If the carbon in materials is not fully removed from the body, then
the gas will form during the glaze or decorating firing, forming bubbles
or blisters. These may pop to become craters or pinholes.
These defects can occur because:
- There was not enough air in the kiln during firing for the carbon
to properly burn out.
- Any combustion process requires air. Without air, oxidation
cannot occur.
- Carbon monoxide formed by oxidation of carbon has not been adequately
removed from kiln.
- If the gases produced during firing are not removed from
the kiln, they may deposit onto the glaze surface or affect
the glaze color.
- The kiln was heated so quickly that there was not enough time
for the carbon to burn out.
- Carbon which is only partially burned will continue to oxidize
during the glaze or decorating firing causing defects.
- There ware was underfired. That is, there was not enough heatwork.
- When the body is underfired, it is weaker and its expansion
may no longer fit the glaze.
How Do I Solve Glaze Defects?
To make sure that glaze defects do not occur, it is important to properly
mix glazes and slips and to use good pouring (slip) and application
(glaze) techniques. Proper housekeeping for the kiln and workplace
should be observed. Straining glaze through nylon often helps remove
any lumps (especially if glaze was not commercially prepared).
Most critical for good results are proper firing practices. We recommend
the following:
- Entering air into the kiln and make sure it circulates around
the ware especially during bisque firings:
- use setters and stilts to improve air flow around the warer.
- use half shelves to improve air flow through the kiln
- adequately vent the kiln
- position ware to take best advantage of air flow in the
kiln
Use a downdraft vent like the Orton KilnVent to bring a controlled
amount of air into the kiln and circulate it throughout the kiln.
This helps remove fumes and even out the temperature in the kiln.
- Control the firing.
- Fire slower, especially below 1200° F / 950° C.
Slow down the firing by adjusting switches to lower settings
or soak/hold at a temperature to allow carbon to burn out.
- Use an automatic controller to set heating rates and hold
times.
- Use witness cones to verify work.
- Underfiring can occur due to burn out heating elements or
improperly adjusted Kiln-Sitter, a controller thermocouple
which has changed or differences of heating within the kiln.
Witness cones give a true reading of the heatwork the ware
received.
- Witness cones placed throughout the kiln show differences
in heat distribution.
- Vent the kiln to remove gases and prevent them from redepositing
on ware. Only downfraft venting removes gases from the kiln.
If good firing and venting practices are observed during firing, problems
with glaze surface defects can be controlled.
Want to learn more?
Read more about glaze surface defects 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 |
|
