In ceramic production, glaze black spots (also known as black specks, black dots, buckwheat spots, or pinhole-like defects) remain one of the most frequent and frustrating surface flaws. They ruin the visual perfection of the product, drastically reduce the first-pass yield, and drive up scrap costs. Many factories repeatedly adjust firing schedules and invest heavily in manual iron removal — yet black spots stubbornly persist.
The real root cause is hidden in the raw materials.
The primary origin of glaze black spots lies in trace impurities such as iron (Fe?O?) and titanium (TiO?) present in the raw materials. Feldspar and nepheline are the main fluxing agents in most glaze formulations, typically accounting for 10–35% of the batch. When these minerals contain elevated iron levels, especially under high-temperature firing (particularly in reducing atmospheres), ferric iron is reduced to lower-valence states or even metallic iron/black oxide compounds. These then melt and diffuse within the glaze layer, forming visible black specks, spots, or starry points.
Even conventional wet magnetic separation or manual magnetic-bar removal often fails to eliminate ultra-fine iron-bearing minerals (such as biotite, limonite, magnetite). Secondary contamination from processing equipment, infrequent cleaning of magnetic bars, or incomplete screening further introduces iron, creating a vicious cycle: “incomplete iron removal → reduction during firing → black spot proliferation”.
Clean raw materials create clean glaze surfaces.
LSAK Minerals addresses this long-standing industry pain point with high-gradient magnetic separation + multi-stage ultra-strong magnetic purification technology, delivering truly ultra-low iron, high whiteness, and ultra-clean feldspar and nepheline .
– Ultra-low iron feldspar powder / magnetically purified feldspar：Fe?O? content significantly lower than typical industry grades, higher whiteness, uniform particle size, and no visible iron specks.
– High-whiteness nepheline ：Lower melting point and excellent fluxing performance, while undergoing rigorous multi-stage magnetic purification to minimize iron and other impurities — dramatically reducing the overall iron risk in glaze formulations.
Customers who switch to our materials report a sharp reduction in glaze black spots, black specks, pinholes, and other defects, along with noticeably improved surface gloss, smoothness, and overall appearance. Many have shared feedback such as:
“After switching to LSAK feldspar and nepheline, the persistent black-spot issue that previously required rework almost disappeared, and our first-pass yield increased by 8–12%.”
Don’t let cheap raw materials cause expensive defects.
Investing in high-purity ceramic raw materials is far more cost-effective and reliable than endlessly troubleshooting firing conditions or adding extra iron-removal steps downstream.
LSAK Minerals — Stable Supply of Premium Ceramic Raw Materials
We own our mining resources and operate modern processing facilities, ensuring long-term, stable supply of ultra-low iron feldspar powder, high-whiteness nepheline , and other top-grade ceramic fluxes with consistent batch quality and reliable delivery.
If you are struggling with causes of glaze black spots, how to prevent glaze black specks, ceramic glaze formulation optimization, or similar challenges, feel free to reach out!
We are happy to provide free samples for you to test and compare directly in your production line.
Let every piece of ceramic leave your factory with a flawless, spotless glaze surface — starting with truly pure raw materials.
Contact us today to request your free sample and take the first step toward perfect ceramic quality!