Adventures in Fast Firing
While fast firing can offer multiple benefits, several key factors need to be taken into consideration to help ensure success.
For the purposes of this discussion, a “fast firing” is defined as a firing duration from ambient to ambient within 180 min or less. The primary benefits of implementing a fast-firing protocol include: significantly reducing energy consumption per payload, reduced scrap and re-fire, lowered labor costs, shorter production lead times, and more reliable product consistency.
The firing of 180 min or less encompasses all firing categories, including decorating soda glass and fine crystal (75-90 min at 605°C), stoneware and porcelain bisque (180 min at 1,010°C), high-fire glaze and in-glaze ceramic decals (150-180 min at 1,180-1,220°C) and on-glaze ceramic decal firings (120-180 min at 840-880°C). The damper settings, cooling vent settings, tachometer speed of the rollers, gas pressure settings, burner butterfly valve settings, air pressure settings, temperature controller set points and percentages of output are based on the controls that are available with our 110-ft Noritake roller hearth kiln (RHK). My firm purchased the RHK in 2004. Figure 1 illustrates a successful curve for firing to 1,185-1,220°C in 2.5-3 hrs.
Perhaps one of the more important components needed to manufacture successful fast-fired products are clay bodies and glazes specifically formulated for fast firing. We purchase the majority of our ceramic materials from Laguna Clay Co. in the City of Industry, Calif. Their laboratory has been very helpful in providing raw material specifications and behavior information as I developed my fast-fire procedures. I have also worked in conjunction with Joel Veach of Hawaii, who is perhaps one of the most knowledgeable people I know when formulating glazes for every temperature application and firing technique. Although I genuinely believe that 95% of first-quality ceramic products can be attributed to various production techniques, a properly engineered batch recipe does play a significant role in the fast-firing arena.
Generally, the introduction of calcined materials in lieu of any material that has a relatively high loss on ignition (LOI) is paramount. It is also important to use raw materials that naturally have relatively low LOI properties. The introduction of low-LOI materials includes both the glaze formulas and clay body formulas. Due to the fact that calcined materials are generally more expensive than raw materials, it is an economic consideration when dialing in the suitable percentages of calcined materials and low-LOI raw materials. The goal is to motivate all volatile substances to escape during the ramp until the mass of the clay and glaze no longer changes.
In addition to paying attention to low LOI, it is equally important to develop and implement the correct temperature and duration of the bisque firing. Good bisque firing practices will mitigate the LOI for the clay body.
Clay Design bisque fires to 1,010°C from ambient to ambient in 180 min. Of all of the various set points used to create fast-fire finished stoneware products, the bisque firing is the most challenging and plays a significant role in the quality of the finished fired glazes.
In order to eliminate thermal shocking during the cooling of the bisque ware, the selected bisque set point should be high enough to create the beginnings of a glassy matrix in the clay body. The formation of the beginnings of a glassy matrix helps the bisque ware resist rapid and uneven thermal contractions and subsequent thermal shocked shards. In our case, the bisque ware is borderline too tight to dip in glaze, and the flash time for the glaze application is slightly retarded. Flash times will vary with different clay and glaze formulas. We prepare our glazes in a 600-lb-capacity ball mill with a specific gravity of 1.47.
Forming and drying the stoneware pottery also plays an important role in productive fast bisque firing. At Clay Design, we press our open shapes (e.g., platters, plates and open bowls) using deflocculated casting slip that has been screened at 100 mesh and filter pressed at 80-100 lbs of pneumatic pressure. After approximately 4 hrs in the filter press, we run the “cakes” through a stainless steel pug mill with a vacuum chamber set at 29.7 in. of HG. Pressing a deflocculated plastic clay body removes nearly all of the mill’s auger memory and delamination defects. With the reduced percentage of water, warpage during drying is virtually eliminated.
The correct amount of clay used to charge the dies is also very important. Too little material in the die will cause delamination stress cracks to reveal themselves after the glaze firing. Fractures from delamination are the most expensive defect in our production. The stress fracture is set up in the plastic state and when the piece is formed. It does not reveal itself until the piece has been processed through all of the labor-intensive steps of manufacturing and then finished glaze fired. We slip cast all of our hollowware, and delamination is not a factor in this forming technique.
Reducing the amount of organic and inorganic gas that is released from the clay and glaze during the sintering of the glaze is critical. The gases released are primarily steam, oxygen and nitrogen, as well as organic compounds that are found in the raw materials. During the relatively short duration of the soak time in a glaze firing, these gases can cause the glaze to pinhole, boil and generally fire poorly, revealing many glaze defects. Calcined materials and proper bisque firing protocols tend to mitigate the exposure to the release of gases during the sintering of the glaze and the maturity of the clay body.
Another important factor is the duration of the glaze’s molten state, coupled with a neutral atmosphere in the ramping tunnel and the hot zone chamber. Note in Figure 1 the absence of a steep or pointed set point in favor of a more gradual ramp curve that reaches the set point at a nearly horizontal plane. The longer we can keep the glaze molten, the better the quality of the glaze will be.
One must consider the temperature ramp and the cooling cycle so that a prolonged hot zone doesn’t overheat the entrance and the exit zones in the RHK. Adjusting the exhaust and waste heat dampers is the most effective technique to control these areas within the kiln’s tunnel. If the entrance ramp is too abrupt, the glazed bisque ware, as well as green ware, can fail. If the exit’s cooling curve is too abrupt, the ware will thermal shock.
This firing curve maintains a neutral atmosphere, as opposed to a highly oxidizing atmosphere. Depending on the formulas, highly oxidizing atmospheres will aggravate glazes and cause them to be susceptible to what I call “cat’s eyes,” as well as everyone’s favorite defect—pinholes. Cat’s eyes are small divots in the surface of a glaze that look similar to the surface of a golf ball. They are caused by tiny bubbles that form, rise up and pop. As they settle back down, a divot is created. In a periodic kiln, the soaking duration at set point allows these blemishes to heal. The soaking time in a fast fire is dramatically reduced. If bubbles form, then cat’s eyes are not avoidable. There is not enough time during the molten period for the glaze to puddle and settle. The combination of a neutral atmosphere and the nearly horizontal firing curve at set point will produce brilliant glazes with no glaze blemishes created by the fast firing.
Loading the kiln setters with equal mass, the use of spiders and heat sinks are another critical aspect of the successful fast firing of stoneware. Generally, we do not load anything within an inch of the outside edge of the setter. We keep the approximate weight of the payload per setter from 11-13 lbs. Unequal mass will cause the kiln’s temperature controllers to vary their output and reduce the yield in the cooling zone at 585°C. Even moderate fluctuations in the cooling zone at quartz inversion will cause thermal shock.
Fast firing is far more complicated than this somewhat limited perspective; I have attempted to outline the most important factors that contribute to success. The benefits of increased efficiency and reduced fuel consumption are real and verifiable. It is my hope and desire that my experience can benefit everyone—from the backyard potter firing a homemade catenary arch kiln to the most sophisticated automated facility run by ceramic engineers.