An Introduction to Magnetic Properties of Ceramic Minerals: Part Three

In our previous two blogs, we provided an introduction to the problems that magnetic minerals can cause in ceramics production. We also took a close look at the five different magnetic properties that minerals can possess. In this blog, we will thoroughly examine the consequences of magnetic particle contamination in ceramics when magnetic separation is not used.

Consequences of Magnetic Particle Contamination

Magnetic particles can be found in mineral form or found as free-floating iron particles in ceramic body or glaze. Regardless of what form they take on, the presence of magnetic particles can cause a wide range of issues during the firing and processing of ceramic material. In the past, ceramics producers would disguise these visual flaws with colorful, ornate glazes and decorations. However, as solid color ceramics—particularly solid white ceramics—have grown in popularity, modern ceramics producers no longer have the luxury of relying on decorative patterns as a way of concealing flaws.

Magnetic particles in ceramics can cause a wide range of defects, including structural and surface defects as well as issues with color and brightness. It is very common to see visible black spots and glaze deformation in lower value mass-produced ceramics. It is also common to see slightly less visible black or brown particle specking. These defects occur as a result of magnetic minerals or free iron being present in the glaze during the firing process. When a ceramic item is being fired, the magnetic or free iron particle will absorb and retain heat, causing it to expand and darken in color. The particle may also “pop” at this point, resulting in defects in the surrounding glaze.

Another issue that can occur is altering of the final fired glaze color. This issue is particularly prevalent in white or off-white ceramics. Here, the presence of fine magnetic particles can alter the final fired glaze color and darken the overall brightness of the piece. This sort of design flaw is both highly visible and highly troubling.

Because ceramic tiles, tableware, and sanitaryware all require a perfect appearance, any defects will result in the item having to undergo either partial rework or be subject to total rejection and recycling. In plants that do not have sufficient magnetic particle removal systems in place, rejection rates can climb to higher than 10%. Needless to say, this is extremely costly in terms of both time and materials.

Even if there are no visual defects present, non-visible magnetic contamination can cause serious problems as well. Magnetic particles that remain in ceramic body prior to firing will experience the same reaction to heat as those mentioned above in the glaze. This results in the surrounding ceramic structure being weakened, causing fine cracks to develop over time that ultimately become more and more severe, often leading to total failure of the ceramic product.

Commonly, if a ceramic product fails due to fracturing, the failure can be traced back to magnetic particles which can then be seen along the fracture lines. Even a single magnetic particle can be enough to trigger a fracture that leads to catastrophic failure. For high-performance ceramics, this is a huge issue as contamination is not visible from the outset and failure can then only occur over time.

Magnetic separation is a highly effective way to remove magnetic particle contamination from ceramic processes. In our next blog, we will dive into more detail about how different types of magnetic separation can be effectively used in ceramics processing applications.

For more information about magnetic separation solutions for your specific application, contact Bunting today.