Choosing the right solution to a firing problem can save your plant a great deal of time and money in the long run.

Of all of the problems experienced in a typical ceramic manufacturing plant, firing problems usually top the list. Companies are often unhappy with the amount of second-quality ware or rejects that emerge from their kiln, and are unhappier still with the idea of having to invest valuable capital to find a solution. But as with any situation, there is usually more than one way to solve firing problems. In some cases, simply adjusting the product formulation and/or firing curves may be enough to eliminate the problems. In other cases, new equipment may be needed to ensure that the plant runs as efficiently as possible. In either case, choosing the right solution can save a great deal of time and money in the long run.

Making Adjustments

One manufacturer in northern New Jersey, for example, was experiencing an intermittent problem with cracking, blistering and black coring on its extruded and machined steatite ceramics. The products were fired in an older, small-cross-section shuttle kiln. The problem had been occurring for over 25 years, but no one had been able to solve it.

The company finally enlisted the help of a consultant to discover the source of the problem. Since kilns with small cross sections usually yield good results, the consultant determined that the product and/or firing curves were probably the culprit.

As in many cases, this was not a single problem but a collection of several problems. As one problem was solved, losses continued to occur as a result of the remaining unsolved problems. The consultant knew from experience that the blisters were coming from bentonite in the ceramic body, which was being added to enhance extrusion properties. However, the company decided that the bentonite was too important to be removed from the formulation, so another solution had to be found.

The company decided to try heating the parts in the kiln to no higher than 900?F before a fast temperature rise was allowed. As different firing curves were applied, the blisters disappeared first, and additional adjustments to the firing curve eventually led to the end of the black coring. However, the company then began experiencing problems with white coring, and these problems were even harder to find. Two pieces placed side by side after the first firing would both seem to fire well, but one would crack apart several days after glazing. When the good pieces were cut in half, they were the same color throughout, but the cracked pieces showed a central core that was whiter than the outer part of the piece.

The company tried a number of different firing curves, but these only produced minor improvements. Finally, the piece was sliced thinly, and a separate section was also ground. Both the powders and the thin slice were evaluated with X-ray diffraction. The X-ray showed that the white coring was a sub-oxide, meaning that it was not fully oxidized in the later stages of firing. The firing curve was slowed again, this time between 1800 and 2000?F.

The problem did not show up again for a long time. When it did, the curve was once again adjusted slightly. The white coring has not occurred since.

This company’s firing problems were solved by judicially applying firing and analysis techniques. However, the process required over a year of various tests and trials with the assistance of a consultant. A great deal of patience and perseverance is often required when pursuing an adjustment-based solution to firing problems.

Purchasing New Equipment

A ceramic manufacturer in Ohio had a different kind of problem. In an effort to expand its market share, the company had taken on a new product line. Unfortunately, the product required the use of an organic die lubricant, which smoked badly inside the tunnel kiln when the product was fired. This created an air pollution problem and raised health concerns for the employees inside the plant.

The company added an afterburner to the chimney, but a great deal of smoke still came into the building through leaks in the kiln’s structure. The company could not afford to build a new tunnel kiln, so it began investigating alternative options. Any new equipment that was purchased would need to fire the same amount of product per week as the existing tunnel kiln without creating air pollution problems. The firing temperature also needed to remain very uniform so that the products could be fired quickly.

The solution was to design a new periodic kiln that would fire uniformly with the least amount of fuel possible. Since the load was larger and denser than in the tunnel kiln, the temperature uniformity had to be much better than normal to achieve the required fast firing cycles.

The new kiln was based on a traditional downdraft design, which allowed the flue gases to flow evenly over the entire load of setters. To enhance this flow, the burners were placed alongside the load in a staggered pattern. Only six burners were used, and each was connected to its own control loop. Individual control loops enabled the controller to maintain an even temperature from each of the six burners, ensuring that the hot gases flowing over the load remained uniform throughout the firing process. Figure 1 shows a picture of the loaded shuttle kiln car after installation.

The car holds the same number of saggers as the tunnel kiln throughout its length. The shuttle kiln fires in the same cycle time as the tunnel kiln, yielding the same amount of fired product per week, and an afterburner consumes the smoke generated by the organic lubricant. The design, construction, delivery and installation took nine weeks from the time the company placed the order to the first burn, satisfying the company’s need for a fast turnaround.

In this case, the existing kiln system could not be used to provide a solution. A new, custom-designed shuttle kiln was required to eliminate the air pollution problem without hampering the company’s production capabilities.

Looking Past the Purchase Price

While most companies would prefer to save their capital and solve their firing problems by making adjustments, that isn’t always the best option. In fact, equipment can be the cheapest part of a plant—if it is chosen correctly. Additionally, more expensive equipment will often pay dividends in long-term operational benefits. A difference of 25% in the up-front cost of a piece of equipment may seem like a lot, but it often disappears in the overall cost of operating a plant. For this reason, it would be foolish to buy an inadequate kiln just to save a few pennies. This does not mean that you have to spend more—you just shouldn’t be afraid to do so to obtain the right equipment.

For example, assume that a kiln with cars, start-up and installation costs $120,000. The kiln holds 2000 lbs of ware and will produce 40,000 lbs per month. Amortized over ten years at 8% interest, the cost of this kiln per month is approximately $1700.00. Dividing that amount by 40,000 lbs per month equals $0.0364/lb.

The labor required to load and unload the kiln—eight hours per burn at an overhead rate of $25.00 per hour—equals $200.00 per burn. Dividing that amount by 2000 lbs/burn equals $.10/lb.

The kiln requires approximately 2000 cubic feet of fuel per burn at $5 per thousand cubic feet, which equals $10/burn. Dividing that amount by 2000 lbs/burn equals $0.005/lb.

The space required for the kiln is about 50 sq ft, including a car outside of the kiln. Assuming $50/sq ft for construction spread over, say, seven years at 8%, the cost of $2500 is about $39 per month. Dividing this amount by 40,000 lbs/month equals $0.000975/lb.

Assume that maintenance costs $100 per month. Dividing that amount by 40,000 lbs equals $0.0025/lb. Spare parts cost about $1200.00 per year or $100.00 per month on average, for a cost of $0.0025/lb. Major rework of the kiln lining, etc., every 500 burns (a very conservative estimate) at a cost of $25,000 equals $50/2000 lbs., or $0.025/lb.

Table 1 compares the per-pound operating costs of this kiln with the operating costs of both a less expensive and a more expensive kiln. As you can see, less than a $.01/lb difference exists in the operating costs of all three kilns. Furthermore, the labor is the single largest cost involved, by far, and this example uses very little labor time to load and unload. Much more would probably be required.

Choosing the Right Solution

Every situation is different. Buying a new kiln is not always the best solution—sometimes a little hard work and a lot of patience will enable existing kilns to produce much more ware by reducing or eliminating losses. But sometimes a new piece of equipment is needed to solve firing problems. If you’ve already tried making adjustments to your product formulation and firing curves and the problems still exist, a new kiln might be required.

When purchasing new equipment, price is not nearly as important as many people think. In some cases, a kiln that costs a little more money up front will significantly reduce the amount of refires and losses, saving money in the long run. In many cases, an experienced consultant can help you determine the right solution for your plant.

For More Information

For more information about solving firing problems, contact Cameron Harman Jr. at Ceramic Services, Inc., 1060 Park Ave., Bensalem, PA 19020; (215) 245-4040; fax 215-638-1812; e-mail kilns@kilnman.com; or visit http://www.kilnman.com.