Orton AutoFire Plus User Manual page 29

AutoFire® Kiln Controller
Hi Fire
The firing range of higher temperature bodies, such as stoneware and porcelain varies between Cone 4 and Cone
10. These bodies are fired nearly to vitrification and can shrink up to 16%. Since stoneware is formulated from
materials such as ball clay feldspar, silica, and kaolin, the firing must be controlled to permit proper burn-out of
organic materials and to prevent cracking. As with earthenware bodies, water, carbon, and sulfur are also
potential burn-out materials for stoneware. Due to the types of materials used, the color of the fired product is
affected by the amount of oxygen present during the firing process.
With the presence of silica, care must be taken during the silica phase change near 1063°F (573 °C) to prevent
cracking. Since the fired properties of density and porosity are critical, the firing profile during the last 210°F
(100°C) is important for temperature uniformity and product maturity. It is during this time that those critical
properties develop. Thick pieces of ware should be treated similar to heavy loads.
Porcelain bodies are formulated to produce very distinct colors and finish. Fired properties such as hardness, no
water absorption, uniform color, and density require detailed attention to the firing process. Normally the
maturing range is very small and may be less than half of a cone.
Typical porcelain bodies are formulated from kaolin, feldspars, silica, and ball clays. The weight loss during
firing can be around 10 to 12 percent and shrinkage can approach 20%.
To obtain the desired fired properties, the firing must be controlled to properly burn-out all organic materials. If
this is not done, imperfections can occur or show up in later firings. This normally requires slowing up the
heating rate for proper burn-out of binders, release of chemically combined water, and to burn-out carbon.
Sufficient oxygen is mandatory to accomplish this, so venting is important. Cracking can also occur if a rapid
heating or cooling occurs around 1063°F.
Porcelain bodies require very good temperature uniformity at their final firing temperature. If a slight over-fire
occurs, the body will normally warp or blister. On maturing, the body becomes its own "glaze." Some hold time
is usually desirable for best results.
The preset firing program for Hi Fire is shown below. The standard firing time is 8 hours, 40 minutes.
360 °F/hour (200°C/hr) for 2 hours, 40 minutes
180 °F/hour (100°C/hr) for 50 minutes
216 °F/hour (120°C/hr) for 3 hours, 10 minutes
108 °F/hour (60°C/hr) for 2 hours
The firing speed based on load size adjustment will vary the firing times from about 6 hours, 20 minutes to
about 10 hours, 50 minutes.
User's Guide
27