Question:
How do I fire Decals?
Thickness
Thickness of the piece.
Air has to penetrate through the entire thickness of the piece and the gases have to escape the same way. It takes longer for carbon to burn out of a thicker piece of ware.
How Carbon Burns Out
How Carbon Burns Out
During heating (firing) the carbon reacts with oxygen to form carbon dioxide and carbon monoxide gases. The carbon leaves the body as a gas. Binders are burned off at a relative low temperature: 300şF to 500şF.
Naturally occurring carbon in clay burns off (becomes gases) at higher temperatures: up to 1200şF - 1400şF.
Precipitating burnout
Slow Down Firing
Be sure the kiln is vented adequately so there is sufficient oxygen.
Load the kiln with burn out requirements in mind.
Leave plenty of space between ware and shelves. Do not stack ware. Use tile and plate stackers and invert pieces on top of one another to help conserve space and insure proper burnout.
Measuring Heat Distribution
Differences in heat distribution from top to bottom in the kiln are usually far more noticeable for cooler firings like decals. A 2 or 3 cone difference at 022 may only be a 1 cone difference at cone 6. This is because at higher temperatures radiation heats the kiln more effectively. Slowing the first half of the firing can help heat distribution problems. This also helps by allowing more time for air to enter the kiln and burn out organics and for carbon monoxide to leave the kiln. Use a controller to set heating rates and soaks for more precise firings.
Carbon burnout rate
The rate at which this carbon burns out is related to:
The amount of carbon present (that is, the amount of natural contaminants in the body). Some bodies have more contaminants than others, such as red clays. This needs to be considered when planning the firing.
Amount of air available (air provides oxygen for burnout) Air needs to be considered when planning the firing.
This is impacted by several factors. A load that is fired very quickly will not allow enough time for the oxygen to react with the carbon, form gases and leave the ware. If ware is stacked during bisque firing, oxygen may not be able to penetrate all surfaces of or inside all the pieces. Also, if gases are not removed from the kiln and replaces with fresh air, then there may not be sufficient oxygen to burn out the carbon.
Time and temperature
Time and temperature profile during the burn out period. Both time and temperature are important for proper burn out of the carbon. Some carbons require much higher temperatures that others. Oxidation should be completed below red heat (1400şF). Carbon burns out from the surface first. As more oxygen penetrates the body, them more carbon is reacted to form the CO or CO2 gas and the burn out process continues. If there is sufficient time, temperature and oxygen, then complete burn out occurs. If these conditions do not exist, the resulting incomplete burn out is referred to as black coring (where the center of the piece has black or gray cast).
Measuring Heat Distribution
Measuring Heat Distribution
Differences in heat distribution from top to bottom in the kiln are usually far more noticeable for cooler firings like decals. A 2 or 3 cone difference at 022 may only be a 1 cone difference at cone 6. This is because at higher temperatures radiation heats the kiln more effectively. Slowing the first half of the firing can help heat distribution problems. This also helps by allowing more time for air to enter the kiln and burn out organics and for carbon monoxide to leave the kiln. Use a controller to set heating rates and soaks for more precise firings.
Measuring Heatwork
Measuring Heatwork: Heatwork is another critical factor in the colour development of decals. Fading, Shifting and dullness are signs of too much or tool little heatwork. This is also true when decals rub off after firing. (White or blank spots or burned off areas are generally related to application, not firing.) Use witness cones to measure heatwork and to check the heat distribution in the kiln. Firing to a temperature or firing to a Kiln-Sitter cone may not give the same results as found with a witness cone next to the ware.
Determining Firing Range
Determining Firing Range
Because the colours on decals can so easily be affected by the amount of heatwork they receive, we recommend test firings to determine the best firing range.
Use a series of witness cones to fire samples of the decals on tiles or blanks. Make several firings and then select the fired appearance which looks the best.
Incomplete Burn Out
Incomplete burn out can result in several firing problems including: Bloating of the ware - if the temperature is hot enough, the outside of the piece will seal up before all the gases can escape. As the body becomes plastic due to glass forming, gases trapped inside the body expand with heat and cause bloating and sometimes cracking of the ware.
Glaze defects, such as pinholes - the escaping gases will push through the glaze surface and cause bubbles which pop. If these do not heal, then pinholes will result. Appearance of fired bisque - where carbon burn out is incomplete, the piece will have a grayish cast (white bodies) or may have greenish cast (red bodies). The body will also be more porous and weak.
Carbon burnout
Carbon burnout: Materials used in ceramics contain naturally occurring impurities that can affect the colour, appearance and maturing temperature of the product. Carbon, found in most clays, is normally considered one of these impurities. Carbon can also be present in the additives and binders which make up clay bodies, slips, decals and lusters.
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| Guru Spotlight |
| Patricia Walters-Fischer |
