Black Ceramic Dinner Plate Analyzed for Heavy Metals
Is there lead in this ceramic dinner plate? The ceramic dinner plate shown above is clearly composed of at least two different materials with different compositions. We want to analyze this ceramic material to determine if it has lead, cadmium, mercury, or other heavy metals in it. We will start with WD-XRF analysis, knowing that it will analyze some combination of the black glaze layer and the interior ceramic material, where how much of the signal comes from each material is determined by how energetic the x-rays are that it emits upon excitation by the x-ray source of the XRF spectrometer. The depth of detection for cadmium will be several times the thickness of the glaze layer and the detection depth for lead and mercury may be about twice that of cadmium. The plan was to establish whether lead, cadmium, mercury or some other health deleterious element was in one or the other of these materials. If so, then we would examine the surface with XPS surface analysis to see if those heavy metals were detectable in the surface where someone eating from the plate might be more likely exposed to them.
WD-XRF Analysis Results
The analysis by WD-XRF with the top or eating surface of the black glaze facing forward during the analysis of two pieces of the same plate are given in the table below. One of the pieces was reversed and analyzed with the black glaze of the bottom of the plate forward. The quantitative calculation assumes that each element is distributed homogeneously throughout the depth of analysis, which is not the case here given the glaze in front of the interior ceramic structure and their expected different compositions.
For oxide concentrations above about 0.03 weight percent (wt.%), the three analyses are very similar. Below that concentration, the signal-to-noise becomes a bit more of an issue, but the distribution of very low concentration elements can also be more variable. In a ceramic material, a given low concentration element may only be in a very few particles which were sintered to form the ceramic. These few particles may not be homogeneously distributed among the large number of particles of the more common constituent oxides.
No cadmium or mercury were detected in any of these three analyses. Lead in the ceramic dinner plate was detected at weight percentages of PbO of 0.0039, 0.0054, and 0.0027 wt.%. or 39 parts per million (ppm), 54 ppm, and 27 ppm by weight. There is lead in either the glaze or the underlying ceramic material or in both.
XPS Surface Analysis Results on the Eating Surface of the Glaze
While the WD-XRF results for the heavy metals can come from deeper than 2 millimeters into the material, those for XPS analysis come from the outer 20 nm of the surface. Material in this outer 20 nm may come into contact with food or may be leached out of the surface of the glaze. The table below provides the XPS analysis of the glaze surface:
XPS detects all of the elements except hydrogen and helium. Because it is so surface sensitive, it detects the hydrocarbons on the surface of the glaze even though the surface was washed with detergent, well-rinsed with water, and then further rinsed several times with isopropyl alcohol. Had there been 0.0003 atomic percent lead, XPS would probably have been able to detect the lead in the ceramic dinner plate. This corresponds to about 0.003 weight percent (30 ppm) lead on this surface. In the area of the surface of the black glaze examined (about 0.8 mm squared), the lead concentration was less than about 30 ppm by weight.
XRF Analysis of the Interior Ceramic Material with the Black Glaze Removed
To follow up on the issue of determining where the lead in the ceramic dinner plate resides, the hard black glaze on the eating surface of Piece 1 of the plate was removed by abrasion. The interior softer ceramic material was then analyzed with WD-XRF. The results in weight percentages of the oxides are given below:
Removing the black glaze layer somewhat reduced the silica wt.%, while increasing the alumina wt.%. This is consistent with expectations based on the XPS surface analysis of the black glaze surface. We can also conclude that most of the Ca, Fe, and Co oxides initially detected by XRF in the combined glaze and interior ceramic were in the black glaze layer, though neither the iron nor the cobalt were found on the surface itself in the XPS analysis. The iron and cobalt oxide particles may be coated with silica and alumina to encapsulate them. The PbO in this analysis decreased from 39 ppm to 29 ppm for this same area of the dinner plate upon removing the black glaze, but the difference of 10 ppm is within the overlap of the estimated errors of 8 and 7 ppm, respectively, for these measurements. I performed a second XRF analysis of this same piece with the glaze removed after remounting it and the PbO concentration was given as 27 ppm, so the repeatability of the measurement was very good.
Conclusions
The XRF analyses of the ceramic dinner plate through the black glaze into the interior ceramic material produced an average lead content as lead oxide of about 40 ppm by oxide weight. The individual measurements ranged from 27 to 54 ppm by oxide weight for lead in the ceramic dinner plate. The single measurement made with the top glaze layer removed gave a lead content in the interior ceramic material of 29 ppm by weight. The XPS analysis of a single surface area of less than one square millimeter concluded that that area of the surface had a lead content of less than about 30 ppm, which is again in the range of 27 to 54 ppm of PbO. XRF proves lead to be present and the XPS surface analysis cannot say that it is significantly below the XRF measurement in the surface. The lead content in the dinner plate may not be safely enveloped in a sheath of lead-free hard glaze. It is possible that any lead in the glaze is isolated from the surface, but we cannot be sure of that from the above analysis. We have to conclude that the user of this ceramic dinner plate may be exposed to the lead in the ceramic dinner plate.
