Volatile constituents can sometimes condense in the early preheat or early cooling in certain tunnel kilns. At times, the condensates fall on the product and cause dirt defects. Sometimes the condensates are so chemically reactive that it damages the kiln lining by fluxing the refractories in both the preheat and cooling zones. In either case, undesirable precipitation of volatiles can cause serious product loss or even kiln damage.

Sanitaryware

The use of Styrofoam as a placing cushion under the foot of sanitaryware pieces reduces handling breakage, but it comes at the expense of kiln dirt. Generally, the Styrofoam oxidizes (burns) as it is heated, and forms CO2 and H2O vapor. However, due to reactions of the gases in the kiln with various trace materials from the ware and glaze being fired, other complex hydrocarbons are also created. These gases have a relatively low temperature condensation point.

In the preheating zone of the kiln, the gases condense and attach themselves to various protrusions and other cool areas within the kiln, usually at temperatures below 400°F. As these materials collect, they gradually fall off the crown and onto the ware, causing defects.

The primary solution to this problem is relatively simple and direct. Sanitaryware kilns that I have worked with where no Styrofoam is used generally have no condensation problem at all. If, however, the setting Styrofoam is absolutely necessary, start by reducing the amount used to a bare minimum. After this has been done, care must be taken in the management of the preheat exhaust portion of the kiln. The volatile laden gases must be withdrawn upstream of the condensation point. While this means that the fuel consumption will be increased somewhat, elimination of the drip dirt problems can be greatly alleviated. Naturally, this adjustment technique can only be applied if the kiln has sufficient exhaust off-takes in the preheat zone.

Additional reduction of crown condensation can often be provided by better circulation. With stratification of gases, the highest concentration of volatiles tends to be at the crown. Cold air jets cool the crown and provide perfect sites for condensation. While condensation still occurs under higher circulation conditions, the reduction in concentration of the gases in proximity to the crown is decreased, leading to reduced crown buildup. Normally, preheat burners can be used to provide the increased circulation.

Dinnerware

A similar phenomenon occurs in dinnerware applications, due to volatile and corrosive glaze constituents. Several years ago, for example, a fine china manufacturer required direct fire kilns to fire leaded glaze products, and the available technology did not exist to handle the expected kiln lining attack from the vaporized lead.

In this application, Pb bearing vapors drifted into the preheating zone and precipitated on the kiln roof and walls. Since Pb is a powerful flux, it started to form complex Pb glasses with the lining refractories and severe erosion and subsequent dripping occurred, ruining both the kiln and ceramic ware.

To solve the problem, we developed a similar extraction system as explained above, with a few extra features. The condensation point in this case was around 760°C, so gases had to be extracted above this temperature. The exhaust ducts used were metallic, and had provision for dilution air to rapidly cool the hot exhaust products from the kiln. This caused rapid precipitation of the Pb vapors, and helped to minimize the gas stream pollution emitted into the atmosphere. The ductwork was cleaned periodically.

Summary

Control of precipitated chemicals on the interior of a kiln lining can cause product loss and even kiln damage. To solve this type of problem, the obvious cure—elimination of the chemical—cannot always be implemented. Accordingly, managing the flow and extraction of the exhaust gases can sometimes be achieved to avoid precipitation of these materials within the kiln.