Quick Color Test to Detect Lead Release From Glazed Ceramic and Enameled Metal Ware

The present invention relates to a test method to be used for quickly identifying glazed ceramic and/or enameled metal ware which releases excessive amounts of lead (Pb). More specifically, the invention is directed to a method of using a filter paper containing a solution of citric acid to extract Pb from the ware so that a Pb sensitive chromogen indicates the presence of lead on the paper.

Lead components have long been used to impart many desirable properties in glaze and enamel compositions for dinnerware. Unfortunately, improperly formulated or improperly fired lead glazes or enamels allow hazardous levels of Pb to be released from the ware's surface. The Food and Drug Administration (FDA) has established action levels for maximum Pb release from earthenware (Table 1). Ware determined to have a lead content at or above these levels will be subject to possible regulatory action by the FDA. The FDA monitors both domestically produced and imported dinnerware on a routine basis. The AOAC (Association of Official Analytical Chemists) or equivalent ASTM (Annual Book of ASTM Standards (1987) 5.02, 314-316) test methods for Pb are used in the United States. Both of these methods require extraction (leaching) of 6 pieces of ware for 24 hours with 4% acetic acid at room temperature. This leach solution is analyzed for Pb by flame atomic absorption spectroscopy.

A representative test technology is exemplified by U.S. Pat. No. 3,809,537, of which Example 17 is typical. The ware being tested is leached by a solution of acetic acid in water (or as in the patent, white vinegar) for various periods of time ranging from 30 minutes to 24 hours. The process of this patent requires a 24 hour leach period which saves no time relative to the Official Methods of Analysis, sec. 25.024-0.027, 14th ed. (1984) pub. by AOAC) and is qualitative at best. Other methods claim results after only 30 minutes of contact. (See Official Methods of Analysis, sec. 25.028-25.030).

The short-term leach methods require extensive chemical operations, such as the adjustment of hydrogen ion concentration, the extraction by an immiscible solvent containing the complex, and an estimation of the depth of color produced. Some of these methods are viewed as being quantitative. These extensive operations are required because the large volumes of acetic acid solutions required to fill the test vessel dilute the lead released and for that reason, a very sensitive and complex method is required.

An object of the present invention is to provide a method to determine within thirty minutes or less if a glazed ceramic or an enameled metal surface is releasing excessive amounts of toxic lead.

Another object of the invention is to provide a simple test to determine if a glazed ceramic or an enameled metal surface is releasing excessive amounts of toxic lead, so that the test can be applied in shipping sheds, in warehouses, at dock-side, or in the laboratory to preselect ware for a full-scale 24 hour leach test.

Still another object of the invention is to develop a prescreening procedure of lead-releasing glazed ceramic or enameled metal surfaces so that only those wares initially determined to have a greater likelihood of being unsafe are subjected to a full-scale 24 hour leach test.

Yet another object of the invention is to provide a method to test specific parts of decorations and colored glazes separately so as to pinpoint the exact source of lead that is being released by separate areas of the ware without increasing the time needed to perform the test.

A further object of the invention is to provide a simple method for testing lead release in the home from ware in everyday use.

These and other objects of the present invention are accomplished by providing a slip of filter paper which is wet with a solution of citric acid or formic acid. This filter paper is contacted with a glazed ceramic or enameled metal ware surface so as to extract soluble Pb from the ware. After approximately thirty minutes, the filter paper is spotted with a chromogen. The immediate formation of a rose to rose/red stain indicates the presence of excessive Pb.

The filter paper thus can be used in a method for determining the presence of excessive amounts of lead on an area of a glazed ceramic or an enameled metal ware, which method comprises extracting lead from the ware by contacting the ware with a filter paper containing formic and/or citric acid, spotting the filter paper with a chromogen, and detecting a change in color of the filter paper.

The method can further comprise subjecting the ware to a full scale leaching procedure.

The filter paper can contain a solution of citric acid or formic acid. The concentration is preferably in the range of about 3 to about 5% V to Wt. or V to V as monobasic acids, and more preferably, the concentration is about 4% V to Wt. or V to V as monobasic acids.

The filter paper is preferably spotted with the chromogen from about ten minutes after the filter paper has contacted the ware to about thirty minutes after the filter paper has contacted the ware.

The quick color test (QCT) of the invention is sufficiently simple so that it may be applied to screen ware in shipping sheds, warehouses, or at dock-side as well as in a laboratory. Optimal preselection of ware for analysis by the 24 hour AOAC/ASTM method utilizes laboratory and investigative resources more efficiently by limiting the rigorous 24 hour AOAC/ASTM examination to ware which has a greater likelihood of being unsafe.

The first step of the present process involves extracting soluble Pb from test glazes and/or enameled surfaces. This result is achieved by contacting the test glaze and/or enameled surface with a slip of filter paper wet with a solution of citric acid or formic acid. After this, preferably from about ten to about thirty minutes later, the paper slip is spotted with a chromogen. (See F. Figel, "Spot Tests in Inorganic Analysis" pp. 73-74 (1958)). The instant formation of a rose to rose/red stain signifies that the test piece contains an excessive amount of lead. Consequently, these test pieces will have a greater likelihood of containing dangerous amounts of lead as judged by a full-scale 24 hour leach test. The present invention provides an efficient and simple preselection procedure whereby only those test pieces having a greater lead content are subjected to the full-scale leach test.

This proposed test method concentrates the lead ions that are released from the test surface under acid conditions in about 30 minutes or less into paper slips and not into a large volume of solvent. The final concentration in the paper is dependent only upon the rate of release of the lead ions. The final color developed in these test slips is qualitatively dependent upon the quantity of lead ion present. The various decorations, or parts of decorations and individual glazes, colored or clear, that might be present on the food-contact surface can then be sampled individually so as to determine the exact source of any lead that might be released without materially increasing the total time for the test. The area resolution of this test and the colored complex formed have been found to be sufficiently precise that the exact shape of the lead releasing source can be determined. This visualization can enable ware manufacturers to survey their products and eliminate all sources of soluble lead on the finished food-contact surfaces.

The rapidity and the ease with which this test can be conducted permits it to be applied at the repositories of commercial ware, such as shipping sheds and dock-side warehouses. Such applications would eliminate the current practice of randomly selecting ware for removal to analytical laboratories for testing by designating at once that ware that is of questionable safety. This preselection, therefore, will make the entire analytical laboratory operation much more efficient.

All reagents used in the present method are ACS Reagent Grade. Preferably, a 1.3% citric acid solution is used, and this solution is prepared by dissolving 1.33 g of citric acid in 100 ml of distilled water. The preferred chromogen solution is in a 0.2% solution, prepared by dissolving 0.05 g rhodizonic acid dipotassium salt (#2942 of Eastman Kodak Co., Rochester, N.Y., or any equivalent) in 25 ml of distilled water. This solution should be stored in the dark. A new solution should be prepared every three days or when the solution no longer gives a bright yellow stain when a drop is applied to neutral filter paper. The filter paper can be Whatman #1 chromatographic paper, or Schleicher and Sctuell white ribbon #589, or any equivalent. The filter paper is preferably cut into 2.times.2 cm pieces.

The samples to be tested are prepared by cleaning each piece of ware with a detergent wash. The detergent wash is formulated by adding 15 g of alkaline detergent (for example, Calgonite of Calgon Corp., Pittsburgh, Pa. or an equivalent) to 1 gallon of distilled water. The samples can also be wiped with a clean cloth, as this may be all that is available at the warehouse.

One or more filter paper slips is placed on a clean, dry, smooth, horizontal, food contact surface of the ware to be tested. Each slip is wetted with about 75 .mu.l of citric acid solution. The paper must be soaked, not just moist, but there must be little or no excess solution present. The slip must be in complete contact with the surface (i.e., no ridges or bubbles). All colors that would be accessible to the AOAC acetic acid leaching should be tested.

After the wet paper slip has been in contact with the ware for about 30 minutes (or sooner if the slip becomes dry), it is removed from the ware and placed on a clean white surface. The slip is then spotted with 5 .mu.l of chromogen solution. The appearance of a rose to a rose/red stain on the paper slip indicates the presence of Pb. The color of the Pb complex is best viewed against a white background such as a porcelain plate, a teflon dish or other white water-proof surface.

Both formic and citric acid can be used. Citric acid is preferred because it is nonvolatile and thus is less affected by drying if the application of the chromogen to the paper slip is delayed.

The present test method is not sensitive to cadmium, the other toxic element determined by the AOAC/ASTM leach method. However, the presence of cadmium should be presumed if the glaze, enamel, or decorations are yellow, orange, or red in color. There are other pigments which can produce these colors, but ware having these colors should be analyzed by the AOAC/ASTM method for Cd independent of the outcome of the present QCT for Pb.

The sensitivity of the present test was determined by pipetting Pb standard solutions (in 4% citric acid) onto 2.times.2 cm filter paper slips and spotting the slips with chromogen solution. The minimum detectable amount of Pb was 0.25 .mu.g/cm.sup.2. The maximum amount tested was 5 .mu.g/cm.sup.2.

Of all the ions reported by Feigl and Suter (Ind. and Eng. Chem. Anal. Ed. (1942) 14, 840-42) which produce colored complexes that might be confused with the Pb complex, only Ba and Cd are found in glazes, enamels, or decorations. These ions give red-brown and brown-red complex colors, respectively, in dilute acid media. Both Ba and Cd were dissolved in citric and formic acid solutions and applied to paper slips at a concentration of 1.25 .mu.g/cm.sup.2. No color developed when these dried slips were treated with the chromogen solution.

Stannous ion is a third possible interference which gives a violet complex color. Stannous ion is probably not present in a finished coating because it is converted to the stannic ion by the firing process. Stannic oxide is often added as an opacifying agent to glazes and decorations but its solubility is expected to be very low in the dilute acid medium used to extract the surfaces.

The influence of the contact time between the paper slip and the ware on the intensity of the complex color was studied by applying 3 QCT paper slips to each piece of ware and successively developing the color on the slips after 5, 10, and 30 minutes, respectively. (Each paper slip was wetted with about 75 .mu.l of the citric acid solution as discussed above). Each of the 3 slips was applied to the same type of area (or decoration) on the ware. Thirty-three different sets (6 pieces/set, except one 3 piece/set) of ware were found to form a positive QCT at least after the maximum 30 minute contact period. Of these sets, 30 were decorated ware (177 pieces) and 3 were plain ware (18 pieces). The average subjective intensity of the complex color was estimated on a scale from 0 (no color) to 5 (very deep rose/red). Using these values, the average increase in color intensity over the 5 minute contact time was calculated for each piece of decorated and plain ware. The decorated ware's average increase in intensity over the 5 minute test was 17% and 38% for the 10 and 30 minute periods, respectively. The plain ware's average increase in intensity was also 17% and 37% for the 10 and 30 minute periods, respectively. Due to the greater color intensity obtained, a 30 minute contact time is preferred. However, a contact time of 10 minutes may be sufficient for testing of decorations on ware.

One requirement of a screening method is positive identification of those items requiring more thorough testing. It is extremely difficult to correlate qualitative results from the invention with quantitative results of the AOAC/ASTM method. Metal ion release is a function of both time and the releasing solution which are different for these test methods. The present test measures Pb released from a small area into a small volume of citric acid in a short period of time, whereas the AOAC/ASTM method measures Pb released from a large area into a large volume (relative to the strip) of dilute acetic acid over a long period of time.

Another difficulty arises from Pb containing decorations on the food contact surface of the ware. Painted decorations, decals, or colored glazes added to finish the ware require firing of the ware a second or even a third time, and usually at a lower temperature than was used to first fuse the background glaze. If this low temperature firing is improperly performed, such decorations tend to release Pb more readily. The contributions of Pb from such decorative areas are functions of the manufacturing practice and the total area the decorations occupy. The AOAC/ASTM method will leach all decorations on the food contact surface whereas the present method will only test decorations to which it is applied. In addition, the present method may indicate high Pb release from a particular decoration but the AOAC/ASTM method may not confirm Pb excessive due to its larger dilution volume. An advantage of applying the present test to decorations is the identification of Pb releasing decorations; occasionally an exact colored complex image of the Pb releasing portion of the decoration will appear. Generally, the process of the invention will not deface good quality ware.

In view of the above, a purely empirical means of proving the reliability of the QCT was obtained by testing ware with both the QCT and the AOAC/ASTM methods. In order for the QCT to be useful, a positive QCT finding must indicate that the ware releases Pb near or over the FDA Action Level when tested by the AOAC/ASTM method. In addition, and more importantly, a negative QCT finding must indicate that the ware does not relase Pb above the Action Level.

The present invention will be exemplified by the presentation of the following examples, but should not be construed as being limited thereto.

The present QCT and AOAC/ASTM tests were applied to 67 sets of ware (6 pieces/set, except two 3 pieces/set). The color intensity of the complex formed was estimated on a scale ranging from no color to a very strong rose/red color. A 5, 10, and 30 minute contact time was used for this evaluation. The QCT paper slips were applied to decorations when they occurred on the food contact surface. If no decorations were present, the slip was applied to the glaze or enamel of the food contact surface. After the QCT, each piece of ware was analyzed by the AOAC/ASTM method.

The present test slips were cut into 2.times.2 cm pieces, and they were wetted with 75 .mu.l of citric acid solution, as discussed previously. Each slip was spotted with 5 .mu.l of the chromogen solution as discussed above.

The QCT and the AOAC/ASTM methods were performed on 51 sets of decorated ware. The QCT produced 32 positive findings and 19 negative findings. The 19 negative QCT sets were also negative (i.e., average <1 .mu.g Pb/ml) by the AOAC/ASTM method. By the AOAC/ASTM method, 14 of the 32 positive QCT sets had Pb levels above the appropriate Action Level. Nine of the remaining 18 positive QCT sets had average Pb levels ranging from 0.6 to 4.4 .mu.g Pb/ml but all were below the appropriate Action Level due to the influence of the ware's shape and the relationship of surface area wet to volume. All 9 sets had at least two pieces in the set with levels .gtoreq.1 .mu.g Pb/ml, and 4 had levels .gtoreq.2.5 .mu.g Pb/ml. The remaining 9 QCT positive sets were <1 .mu.g Pb/ml hence negative by the AOAC/ASTM method. This was probably due to the Pb release from a localized decoration.

All sets of ware identified by the AOAC/ASTM method to be above the Action Levels were positively identified by the QCT. Overall, the QCT correctly categorized 65% of the sets of ware as being above or below the Action Level. All incorrectly categorized sets were false positives. The QCT false positives are thought to occur because of the localized release of Pb by the decorations and dilution of Pb to below the Action Level.

One of the QCT positive findings resulted in a very weak color even though the AOAC/ASTM finding was relatively high. This ware was highly decorated with many different colors of glaze but the QCT paper slips had been applied to only one of the colors. This suggests that the QCT was applied to a decorated area that was not releasing the bulk of the Pb. Therefore, for the QCT to reliably detect excessive Pb release, all colored food contact areas of multi-colored ware should be tested.

The QCT and the AOAC/ASTM methods were performed on 16 sets of plain ware (6 pieces/set). The QCT produced 3 positive findings and 13 negative findings. The 3 QCT positive findings were also positive by the AOAC/ASTM method. The 13 negative QCT sets were also negative by the AOAC/ASTM method. With respect to the Action Levels, the QCT correctly categorized 100% of the sets of undecorated ware. One QCT negative finding gave an AOAC/ASTM finding of about 0.8 .mu.g Pb/ml (volume of ware=300 ml). This Pb level is below the quantitation limit of the AOAC/ASTM method (without applying the concentration option) and may also indicate the lower limit of the QCT for small hollow ware (<300 ml).

It has been observed that the moistened QCT must be in good contact with the surface in order to extract the Pb properly. QCT results on rough or textured surfaces may not be reliable and must be leached by the AOAC/ASTM method.

Because the Action Levels are based on the findings of 6 pieces of ware, some information on the accuracy of the QCT is lost when solely comparing the QCT findings to these sets. To more precisely evaluate the analytical ability of the QCT, the QCT findings of each individual piece of ware were compared to its corresponding finding by the AOAC/ASTM method.

Analytical findings from the contact time experiment discussed above were used for this evaluation which allowed comparison of 5, 10, and 30 minute contact times. One slip of paper for the QCT was used for each timed contact on each 96 pieces of plain ware and 300 pieces of decorated ware. Since 2.5 .mu.g Pb/ml is the lowest Action Level, a correct QCT finding was assumed to occur when the QCT gave a colored complex and the AOAC/ASTM finding was .gtoreq.2.5 .mu.g Pb/ml. A correct QCT comparison was also assumed when the QCT was colorless and the AOAC/ASTM finding was <2.5 .mu.g Pb/ml.

For the decorated ware, the QCT gave a correct response in 75, 74, and 71% of the cases for the 5, 10, and 30 minute contact time, respectively. For the plain ware, the QCT gave a correct response in 99, 98, and 98% of the cases for the 5, 10, and 30 minute contact time, respectively. There appears to be little difference in reliabiity of the QCT between the different contact times for plain ware even though the color intensity was greater after 30 minutes, as discussed earlier. However, there is less reliability in the QCT for decorated ware than plain ware. Many of the incorrect QCT positive findings for decorated ware had Pb levels below detection by the AOAC/ASTM method. As mentioned earlier, these QCT false positive findings are probably due to the Pb released by a decoration being diluted by the relatively large volume used by the AOAC/ASTM method.

The quick color test of the invention has been shown to reliably indicate the excessive release of Pb from glazes and enamels in less than 30 minutes. The quick color test is sufficiently sensitive to permit the preselection of ware for the full 24 hour AOAC/ASTM leach test with the expectation that Pb will be found in the leach solution. Application of the QCT to decorated ware requires that a sufficient number of slips be applied to each piece to test each decoration and each color of glaze. Plain, undecorated ware can be tested with one or two slips on the glaze. The QCT should be applied to the food contact surface for 30 minutes although 10 minutes is usually sufficient to test ware decorations. The test appears to be sufficiently simple that it can be applied outside the laboratory (e.g. in warehouses) to preselect ware for the 24 hour AOAC/ASTM test.

A QCT kit could be prepared using premeasured chemicals and precut paper slips. Such a kit could be used outside the laboratory, including the home. The home use of the kit would be valuable in testing ware already in the hands of the consumer.

The QCT is not sensitive to Cd, the other toxic element routinely tested for excessive release from glazed ceramic and enameled metal ware. Cadmium based glazes, enamels, or decorations are either shades of yellow, orange, or red. Therefore, ware with these colors should be tested by the AOAC/ASTM method for excessive Cd release.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.