Published Aug 10, 2020
When you pull a piece out of the kiln and there is a glaze defect on it, you might think there is something wrong with your glaze. But this is not necessarily the case. Many defects are caused by incomplete burnout of carbon and sulfur during the bisque firing.
In today’s post, an excerpt from the Pottery Making Illustrated archive, Steve Davis explains the causes of glaze faults and shares a bisque firing schedule for an electric kiln that will help you avoid them. –Jennifer Poellot Harnetty, editor.
Many clay and glaze faults in ceramic wares are caused by incomplete burnout (oxidation) of carbon and sulfur during the bisque firing. These faults are observed after a glaze firing, but the problems arise during the bisque firing. These initial problems can be attributed to a kiln operator’s lack of understanding about the chemistry that occurs during this first, critical firing. A good bisque firing schedule can help.
Many materials used in ceramics contain carbonaceous matter, including organic carbon and inorganic carbon from clays, whiting, dolomite, and talc. This carbon must be burned out (oxidized) during the bisque firing to ensure the best results possible in glaze firings. Bloating, black coring, pinholing, blistering, and poor color development are all the result of incomplete carbon burnout. To achieve the complete burnout of carbon, you need the following components: oxygen, time, and temperature.
Oxygen is the most critical component. Without sufficient oxygen in the kiln chamber, carbon in the clay will have difficulty forming carbon monoxide and dioxide gases that allow carbon to vacate the clay. If oxygen is in short supply, carbon will take oxygen from sources including red iron oxide (Fe2O3) that comes from ball clays, kaolins, fireclays, and particularly red clays. When carbon atoms strip oxygen atoms from red iron oxide (Fe2O3), the red iron oxide is converted into black iron oxide (FeO), which is a more powerful flux than the feldspars we add to clay bodies. The chemical equation representing the transformation from one form of iron to the other is: Fe2O3 + C g 4FeO + CO2 h. Starting at 1650°F (899°C), the walls of the wares become progressively sealed by the fluxing action of the black iron oxide. When this same clay is then fired for a second time in a glaze firing to maturation, the clay wall will be over-vitrified and soft from the fluxing action of the black iron oxide. Gases from carbon and sulfur that are trapped in the soft, sealed clay wall will expand to form pockets (bloating). In iron-bearing and black clay bodies, the bloats will be small to large pockets where gases have gathered together. In porcelain bodies, islands of trace iron exist that can form pimple-sized bumps in the clay wall.
In low-fire ceramics, temperatures are not high enough for bloating or melting to occur, but the carbon can cause faults such as black coring, pinholes, blisters, and poor color development in glazes and underglazes.
Time Proper carbon burnout requires time for the oxygen to penetrate the ware and form carbon monoxide and dioxide gas. Much thicker pieces, dense loads, and high-iron clays require substantially an extended bisque firing schedule for proper oxidation of the carbon. Sometimes the carbon content of the ware can be much higher than normal due to changes in raw materials.
Increased carbon content can cause problems that would not normally occur with established firing procedures that have been used for years, but now have to be planned for.
Temperature Organic carbon burns out (oxidizes) from 300–600°F (149–316°C). Inorganic carbon from clays and ceramic materials burns out (oxidizes) from 1292–1652°F (700–900°C). Sulfur in various forms will oxidize from 1292–2102°F (700–1150°C). Kilns must be well vented throughout these temperature ranges, especially from 1292–1652°F (700–900°C), and the firing should proceed slowly through this temperature range to allow oxygen time to oxidize all of the inorganic carbon and sulfur in the clay.
Here is the bisque firing schedule I recommend to avoid problems:
Bisque Firing Schedule for Automatic Kiln Controllers
(see above) Electric kilns with automatic controllers break firings down into ramps or segments of heat increase (or decrease), and the readout has abbreviations that correspond to different factors controlling the temperature rise per hour, top temperature in each segment, and thus the amount of time each ramp will take. A segment (SEGS) includes a rate (RA), a temperature (F), and a hold (HLD) setting. Alarm and Delay can be set after you input a program. Read your manual for details. The rate (RA) is the rate of temperature climb per hour. The (F or C) is the temperature that a segment will fire to. The hold (HLD) is how long the temperature will be held for that segment. Note: These same principles can be applied to manual kilns to some degree.
What’s your bisque firing schedule? Share it below in the comments.