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While praseodymium is typically green in most salts, it is bright yellow in solid solution in a zircon crystal. It derives its color by being incorporated into the structure of the zircon crystal, which can be seen in the formula:
Praseodymium zircon (Pr-Zr-Si) yellow has been used extensively as one of the triaxial colors—along with vanadium zircon blue and iron zircon coral—for wall and floor tile, as well as dinnerware and pottery. However, due to perceived stability issues, it has been used to a lesser degree in high-fire sanitary glazes, where a zirconium vanadium (Zr-V) pigment is more common. The reason for this shift is illustrated in Figure 1, which shows the spectral reflectance curve for Pr-Zr-Si yellow (yellow line) versus Zr-V yellow (blue line) in a low-fire opacified wall tile glaze. In this application, the Pr-Zr-Si yellow has a stronger absorption at 450 nm, as well as a sharper absorption band, yielding a stronger, brighter color than the Zr-V yellow. However, this strength advantage disappears when the comparison is done in the sanitary glaze, with the absorption at 450 nm being equal. Because Zr-V yellows are more temperature-stable, these pigments are often preferred in these high-temperature glazes.
The general belief is that Pr-Zr-Si pigments dissolve in the glaze at these higher temperatures, with a resultant decrease in color. Researchers at Ferro Corp. decided to perform a number of experiments on Pr-Zr-Si pigments to better understand this phenomenon.
ExperimentsA Pr-Zr-Si yellow pigment was made in the laboratory and was formulated with 2, 4 and 6% praseodymium and reacted at temperatures of approximately 900 degrees C. After calcination, the pigment was washed in a hydrochloric acid solution and filtered to remove any unreacted praseodymium. The washed pigment was then reheated to temperatures ranging from 1000 to 1400 degrees C with a two-hour soak. For comparison, the same heating cycle was performed on a zirconium vanadium yellow pigment. By thermally treating the pigments before they were put into the glazes, researchers were able to separate the effect of temperature on the pigments from the effect of dissolution on the glazes.
After each pigment was reheated, it was tested in three glazes: a cone 1 opacified fast-fire glaze, a cone 01-1 opacified wall tile glaze, and a cone 10 sanitaryware glaze. Peak firing temperatures for these glazes were 1120, 1090 and 1230 degrees C, respectively. The colors resulting from these trials were measured and described using the CIE Lab system, in which a higher b* value represents the most yellow pigment. In addition to color value, color strength was also calculated from a reflectance curve.
Even more important to note is that the pigment strength can be explained by a series of linear relationships that correlate to the temperature at which the pigment was soaked. No significant change in strength occurred until the pigment reached 1100 degrees C. At this temperature, the color strength linearly declined until it reached 1350 degrees C, where it lost almost all of its pigmentary properties. The 4% praseodymium reached this line at 1180 degrees C, while the 2% pigment reached this line at 1270 degrees C. Beyond these temperatures, all of the pigments decreased in color strength together and were essentially undifferentiated.
Results and Further AnalysesThese trials make a strong case that it is not dissolution that causes the Pr-Zr-Si yellow pigments to lose color at high temperatures but an instability in the pigment itself. Although these pigments do decrease in color with longer soak times, temperature is the most important variable.
A quick study was done to better understand this instability. X-ray diffraction showed that no changes occurred in either the phases or crystallinity of the pigment after reheating. Scanning electron microscopy showed that no significant change occurred in the zircon crystal morphology after thermal treatment (see Figure 6). Optical microscopy also showed little difference in morphology; however, it did show that the pigment was essentially colorless after the 1400 degrees C treatment.
So how did the crystals lose all their color while retaining their size, shape and crystal structure? Two possibilities exist—either the zircon pigment lost the praseodymium, or the chemical state of the praseodymium changed. To determine whether a loss of praseodymium was the culprit, researchers performed an extraction by washing untreated and thermally treated pigments with hot hydrochloric acid. If the pigment was not reheated, very little praseodymium was extracted. In the case where the pigment was heated to 1400 degrees C, 1.8% praseodymium was extracted with the acid washing. From these results, it can be concluded that praseodymium leaves the crystal at higher temperatures and is therefore unable to contribute to its yellow color. In this case, dissolution of the crystals in the glaze plays a minor role.
Maintaining StabilityThis study reveals that praseodymium zircon yellow pigments begin to lose strength at approximately 1100 degrees C, going almost colorless by 1350 degrees C, while zirconium vanadium yellow pigments exhibit little change during reheating—both as a powder and in a glaze—at these high temperatures. However, in the opacified glazes used in this study, the lack of stability can be explained by a color change in the crystals—not dissolution of the crystals in the glaze. Evidence exists that at least part of this decrease in strength is due to the migration of the praseodymium out of the praseodymium zircon crystal at temperatures above 1100 degrees C.
Although more research is needed to explain the mechanism of the color loss of Pr-Zr-Si yellow pigments, it is important to show that the pigment itself is unstable at higher temperatures. Because this loss in color is clearly related to temperature, a change in process temperatures when firing between 1100 and 1350 degrees C will yield a change in color of the Pr-Zr-Si yellow pigment. Reformulation of an opacified glaze would probably have little effect.
Despite their stability issues, Pr-Zr-Si yellow pigments continue to be a valuable addition to the glaze pallet. They are the most cost-effective means of producing yellow colors in ceramic glazes at most firing temperatures—in fact, using Pr-Zr-Si yellow pigments is the only way to produce a strong, vibrant yellow color without the use of cadmium. As a result, these pigments are commonly used in the hobby, dinnerware, ceramic wall and floor tile, and pottery industries. They are also used in the sanitaryware industry, even though the Zr-V yellow pigments are more stable at elevated firing temperatures.