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How Does Heat Transfer Affect Temperature Uniformity In Kilns?
Orton has pyrometric products, electronic controllers and kiln vents for monitoring and controlling your electric and production kilns.
For most of the ceramic community, whatever happens inside a kiln as it heats our ware is a mystery. The kiln is not clearly understood. It is considered a black box that somehow takes our clay pieces and transforms them into something more permanent.
Heating is required for ceramic or glass products. The kiln provides the heat necessary to properly develop the properties - color, finish, durability, etc. Here, I will try to explain the concept of how an electric kiln transfers heat inside the kiln into the ware.
Heat can come from passing electricity through a wire or a heating element, which in turn gets hot and transfers heat to its surroundings.
The way that the heat is transferred occurs in three ways:
1.Convection 2. Conduction 3.Radiation
Convection is the first way heat is transferred in the heating process. As air is heated inside the kiln, it passes across the warming heating elements. Hot air will rise and cool air will be heavier and fall toward the bottom. As this occurs, air currents will begin to circulate and bring hot air to cooler places in the kiln. The temperature inside the kiln at this early stage of the firing will not be uniform unless the air is pushed through the kiln. Most low temperature cone firings, such as 022 to 016 depend more heavily on convection for heat transfer. These include firings for glass processing, decals, decorative lusters (mother of pearl, gold), and low fire bisque.
Conduction occurs when heat moves through a solid. An example would be how heat moves from a saucepan into the handle. This is a slower process of heat movement as the transfer of heat is dependent on the ability of the material to transmit heat. The measurement of how well a material transmits heat is defined by a term called thermal conductivity.
In a kiln, conduction of heat moves from the inside to the outside of the kiln and from the outside to the inside of the ware. Conduction is the main mechanism for getting the temperature uniformly distributed throughout the kiln. This is why soaking the kiln at a set temperature is so effective. This is a slow process and if the firing is too fast, the inside of the ware will not receive enough heat to fire properly.
Radiation starts to occur at the beginning of the firing, when the elements are the hottest part of the kiln. The heat from the elements radiates out like the sun warming us on a cool day. Eventually, the firebrick, kiln shelves, and the ware will also get hot and begin to radiate heat. As the temperature increases, more and more of the heat is transferred by radiation from the heating elements. For uniform heating, it is important that all surfaces of the ware be exposed to the heating elements, even partially. If large items are placed in front of smaller items, the large items will act is if they are trees shading us from the sun.
Temperature Uniformity within an electric kiln is best achieved by loading the ware so that the heat can penetrate all areas. The hot air and the items getting heated by radiation warm the surface of the ware. Sufficient time is needed to move the heat to the ware and into the ware.
Unless the heat is moved by mechanical means, the hot air will rise to the top of the kiln and the cooler air will fall to the bottom. A difference of two cone numbers can be seen if firing to a low temperature such as between cones 022 to cone 016. Use of a downdraft kiln vent will promote the distribution of the warm air at low temperatures when heat is moving by convection. Downdraft venting helps to counteract the effect of rising heat bringing the warmer air down to the cooler bottom of the kiln.
Soaking or holding the kiln at a temperature can help equalize heat, but truly uniform conditions will not naturally occur until higher temperature, where radiation is more effective. Electronically controlled kilns are capable of being programmed to include a hold at a desired temperature.
Heat moves through ware from the outside to the inside. It is important to try to uniformly heat all surfaces of the ware. How you load the kiln can help to promote even heating. Firing slower also allows for the heat to penetrate to the inside of the ware.
If the top of a piece is heated, but the bottom is touching a cool shelf, the item may crack or warp as one surface is heated more rapidly than the other. This is why it is important to use plate or tile setters for large flat objects.
The practice of nesting bowls or cups in bisque firings is discouraged, as this will prevent the inside of the ware from heating at all, and will restrict heat from moving into the inside of pieces, or the inside of the bottom piece. It is better to invert them and/or place these items rim to rim.
It is also important to make sure larger items do not block smaller ones from the heat source and that the ware is placed at least one inch away from the hot elements. Ware placed too close to the elements may distort or glazes and colors may not develop properly.
Placement of Orton Self-supporting Pyrometric Cones throughout the kiln will serve as an indication that the heat has penetrated the ware and by visual inspection of the degree of bend; will indicate the uniformity of the firing. Use of an electronic temperature controller, like the Orton Autofire®, will insure the kiln is firing on a controlled heating rate as well as provide the ability to hold the kiln at a temperature long enough to even out the temperature as heat moves by conduction. Coupled with the Orton Vent Master® downdraft vent, and employing the knowledge of how heat is transmitted through the ware, temperature uniformity can be achieved.