06-07 molded clay pots

[Image above] Credit: LensEye Media, Wikimedia (CC BY-SA 4.0)

When conducting experiments, sometimes the smallest change in procedure can have outsized effects on the final product. As such, documenting and publishing lists of these effects can help other scientists optimize their research process.

In May, we looked at the effect of forming method on the properties of flash-sintered ceramics. Today, we look at the effect of drying procedure on the behavior of different clays.

Forming ceramics through extrusion requires high levels of moisture, which must be removed from the final product. Drying the ceramic leads to loss of mass and contraction that can crack the ceramic if done too quickly, so researchers must be careful to avoid reducing the product quality.

The first step in optimizing the drying process is understanding that it consists of several phases.

  1. Initially, the water within a material is transported to the surface by capillarity, where it evaporates. At first, the drying rate is constant because the amount of water that is evaporating is equal to the amount of water supplied to the material’s surface.
  2. As transport of liquid water from inside the material to the surface slows, the drying rate decreases as well.
  3. When the water inside the material reaches a critical moisture content, the drying rate effectively approaches zero and the drying process ends.

To manage these different phases, manufacturers process ceramics in three designated “zones.”

  • The wet zone is where the piece is heated and worked in a saturated environment to prevent evaporation.
  • The neutral zone is where drying of the ceramic begins and critical water is eliminated.
  • The dry zone is where porosity water is eliminated and temperature becomes the main drying agent, with no more probability of cracks.

Determining optimal parameters for the drying process is difficult due to the many material characteristics of clay that affect how it will respond to moisture loss and contraction, including different mineralogical and chemical compositions and particle size distributions. “Therefore, it is crucial to know how to reproduce the characteristics of specific raw materials in the drying of forced cycles,” researchers write in a recent open-access paper.

The researchers come from Universidade do Extremo Sul Catarinense, Federal Institute of Santa Catarina, and Parque Científico e Tecnológico in Brazil. In their study, they looked to further clarify the behavior of clay during different phases of the drying cycle and match the raw material characteristics with the final quality of the pieces.

The researchers selected three natural clays from different geological conditions: a fine-grained clay from lagoon river deposits with high plasticity content; a sandy clay from the Rio do Sul; and claystone from the Imarui-Capivari Granites formation.

They measured the clays’ chemical, mineralogical, and particle size characteristics and then developed 10 formulations that were processed using vacuum extrusion. Samples of each formulation were subjected to forced drying cycles of 180 min, varying the temperature from 30°C to 90°C and air speed from 1.5 m/s to 4.0 m/s.

Table 1. The 10 ceramic formulations considered in this study. Credit: Zaccaron et al., Journal of King Saud University – Engineering Sciences (CC BY 4.0)

Raw MaterialMixture (%)
Plastic clay100005050033.366.616.716.7
Sandy clay010005005033.316.766.616.7

The researchers found the plastic clay showed greater sensitivity to drying and a higher percentage of loss during forced drying. For example, formulations with up to 33% of the plastic clay showed losses greater than 50%, while formulations with up to 16% presented losses of ∼25%. In contrast, the sandy clay and claystone showed low sensitivity to drying and minimized losses.

Most importantly, the researchers found that the drying sensitivity coefficient (k-factor), which was determined by using a graphical tool called Bigot’s curve, was a determining factor associated with loss content of the formulations.

“Therefore, it is believed that the k-factor can be extracted by employing analytical tests, accelerating the investigation of a particular formulation and optimizing the research process, saving time in decision making and minimizing the possibility of losses when the application is effectively carried out in the industry,” they conclude.

The open-access paper, published in Journal of King Saud University – Engineering Sciences, is “The behavior of different clays subjected to a fast-drying cycle for traditional ceramic manufacturing” (DOI: 10.1016/j.jksues.2022.05.003).

Interested in learning more about drying of ceramics?

The ACerS online course “Drying of Ceramics,” taught by Denis Brosnan, explores the why and how to control elements of drying operations. Learn more about the course here, which is conveniently available in prerecorded format.