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E.Cesakova, Bohemia Glassworks, Podebrady

LEAD CONTENT IN EXTRACTS IN DEPENDENCE ON PROCESSING AND FINISHING TECHNOLOGIES USED IN THE MANUFACTURE OF LEAD CRYSTAL CONTAINING 24 % PbO. E.Cesakova, Bohemia Glassworks, Podebrady. Introduction.

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E.Cesakova, Bohemia Glassworks, Podebrady

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  1. LEAD CONTENT IN EXTRACTS IN DEPENDENCE ON PROCESSING AND FINISHING TECHNOLOGIES USED IN THE MANUFACTURE OF LEAD CRYSTAL CONTAINING 24 % PbO E.Cesakova, Bohemia Glassworks, Podebrady

  2. Introduction The paper is devoted to Glassworks Bohemia articles made of lead crystal containing 24 % PbO that come in contact with foodstuffs. The problems associated with the transition of lead into leaching liquids were investigated extensively in the 1990s and particular attention was paid to the glass composition (articles containing 6, 10 and 24 % PbO were tested), the physical properties of glasses (without taking the shape of the articles into consideration), the article size and the manufacturing technology. The modification of the glass composition based on the reduced alkali content was not accompanied by any suppression of the lead concentration in the extract. The possibility of reducing the lead content in the extract by dealkalisation of the glass surface by means of ammonium salts was investigated by Rybarikova at the Prague Institute of Glass and Ceramics. The lead content in leaching acetic acid has been determined by the laboratory of Bohemia Glassworks for the last 12 years. Significant relationships existing between the lead concentration in the extract and the glass forming and finishing technologies (in particular, the acid polishing) could be found during this period of time. Several types and shapes of articles were tested but special attention was given to those which pose some problems, i.e. to small hollow articles (saltshakers). Flatware was not subjected to testing and large hollow articles characterised by open shapes were tested only sporadically because their statistical evaluation showed that there were no problems associated with the release of lead (the values characterising the lead concentration in the extracts were below the permitted limits).

  3. The specified limit values of lead concentration in the extracts differ for hollow articles of different sizes: small hollowware containing less than 600 ml … the limit lead value is 1.2 mg/litre, large hollowware containing more than 600 ml … the limit value is 0.45 mg/litre. ISO standard 7086-2/2000 specifies larger values of lead concentration in the extract (1.5 mg/litre, respectively 0.75 mg/litre). The method used for testing the glass is the same for all standards. The values of lead concentration are determined in the leaching 4-per-cent acetic acid after its 24-hour contact with the inner (functional) surface of hollow glass articles coming into contact with foodstuffs. The temperature of the leaching liquid is 22 + 2 °C. The concentration of lead is determined in the works laboratory by means of the polarographic analyzer TRIBO II equipped with the pencil-type mercury electrode. The method of anode solution voltametry (ASV) was used for this purpose.

  4. The paper is structured as follows: 1 Composition of lead crystal subjected to the testing procedure 2 Shapes and characteristics of tested articles 3 Effect of finishing technologies on the character of the glass surface 3.1 Fire polishing 3.2 Acid polishing 3.3 Dealkalization 4 Conclusion

  5. 1. Composition of lead crystal subjected to testing The average chemical composition of lead crystal during the testing period was as follows: 24.4 % PbO 59.0 % SiO2 12.5 % K2O 1.8 % Na2O 1.5 % ZnO The PbO content ranged from 24.2 to 24.8 % PbO. The content of Na2O varied in the range from 1.7 to 1.9 % and that of K2O changed from 12.3 to 12.7 %. The chemical durability /hydrolytic resistance/ of the above glass was typical for the glasses belonging to the 3rd group HGB. The investigation into the influence of the variations in the chemical composition of lead crystal on the content of lead in the extract demonstrated that their effect was unimportant.

  6. 2. Shapes and characteristics of tested articles • Both open and closed shapes of lead crystal articles were tested. It was found that, in general, open shapes are characterized by smaller amounts of leached lead than the closed ones. This depends especially on the ratio between the inner (functional) surface of the article (S) and the volume of the leaching acetic acid (V). • Amount of lead leached from • OPEN SHAPES CLOSED SHAPES • is 3 times less than from • Large hollowware • Volume > 600 ml • is 6 times less than from • Small hollowware • Volume < 600 ml

  7. 3. Influence of finishing technologies on surface treatment 3.1 Flame polishing a) The polishing of crystal glass articles in a slightly reducing flame represents a sort of pre-polishing process and its objective is to reduce the costs associated with the acid polishing. The articles leaving the forming machine are polished with the aid of natural gas-air/oxygen burners made locally. About 1 to 2 cu. meters of oxygen are used for 1 cu. meter of natural gas. The exposure of the articles to high temperatures gives the possibility to obtain a smooth surface (pre-polishing) and, at the same time, to eliminate the formation of a uniform black layer of elementary lead. The required brightness of the finished articles is then obtained by acid polishing which is, however, substantially shorter. b) The polishing in the oxidizing atmosphere is carried out with the aid of so-called “capillary” burners supplied by the company Linde Technoplyn. The consumption of oxygen is two to three times larger (3 to 4 cu. meters of oxygen are needed for the combustion of 1 cu. m of natural gas). This type of finishing technology is expected to replace gradually the acid polishing because of its economic and ecological benefits.

  8. Testing of articles polished in the oxidizing atmosphere Analysis of extracts TABLE I. – Articles used for testing after fire polishing

  9. TABLE I. – Continuation Conclusion Articles with open shapes characterised by a smaller S-to-V ratio release less lead into the extract.

  10. 3.2 Acid polishing The process takes place in a revolving polypropylene drum containing 16 boxes with articles. The partition walls inside the boxes are arranged in accordance with the article size. The articles are usually immersed once or twice into the polishing bath. The composition of the acid bath: 60 - 64 % H2SO4 1 - 2 % HF 0.5 - 2 % H2SiF6 The temperature of the polishing bath is 60 °C. The articles are rinsed in water containing usually 5 to 10 % H2SO4 and 0.1 to 0.2 % HF. The articles are arranged in the boxes in such a way so that both the polishing bath and the rinsing water could act uniformly on the surface of glass articles while the drum is rotating.

  11. TABLE II – Open-shape articles used for testing after acid polishing

  12. TABLE II. – Continuation

  13. TABLE III. – Articles with closed and irregular shapes used for testing after acid polishing

  14. TABLE III. – Continuation The articles mentioned in above Tables meet the ICF limit values of lead concentrations in the extracts: Large hollowware, volume superior to 600 ml, Pb = 0.45 mg/l. Small hollowware, volume inferior to 600 ml, Pb = 1.20 mg/l. It is obvious that all the articles given above exhibit values of extracts that are far below the ICF limits.

  15. Evaluation of the data given in Table II and III Larger lead concentrations are typical, in particular, for rectangular/irregular shapes of saltshakers or small articles with a little pouring opening (saltshakers again, little liqueur bottles). In this case, the articles should be arranged conveniently in the handling boxes and, furthermore, the last rinsing process should last 4 minutes. A two-minute rinsing is sufficient for articles that may be emptied easily (bowls, boxes, pitchers, etc.). The lead content in the extracts can be reduced substantially by observing carefully and rigorously the technological process of acid polishing. On the contrary, the lead concentration in the leaching liquid may increase if the process is not managed well. All the values indicated in above Tables have been obtained atthe optimum rinsing times and for the optimum article arrangement.

  16. 3.3 Dealkalization The transition of lead from the inner surface of glass articles to the leaching liquid was achieved by using ammonium sulfate (NH4)2SO4. Ammonium sulfate was introduced into the articles at a temperature of 460 °C in front of the lehr inlet. The results showed that the amount of lead passing into the leaching liquid could be reduced substantially. However, it is interesting to note that the penetration of this chemical agent into the surface of the glass is not very deep because the subsequent acid polishing eliminates completely this favorable effect (about 100 µm of glass are removed from the glass surface during the acid polishing). TABLE IV.– Lead concentration in the leaching liquid after the dealkalization of the inner surface (mg/l, ppm)

  17. 4. Conclusion Assesment of the influence of finishing technologieson the lead content in the extracts • The flame polishing under stabilized technological conditions is particularly dependent on the ratio of the inner (functional) surface of the article to the volume of the leaching liquid (acetic acid), i.e. on the S-to-V ratio. This may be a disadvantage for crystal manufacturers producing articles characterised by an unfavorable S-to-V ratio because only the critical volume of 600 ml is of importance as regards the permitted limit values of lead. • The following factors are of paramount importance during the acid polishing of articles: 1) The time of exposure of the inner (functional) surface of the articles to the action of the last rinsing bath (2 minutes, respectively 4 minutes in case of small or closed articles). 2) Correct arrangement of articles in the polishing drum to ensure a thorough washing of the functional surface of glass articles in the rinsing bath. Care should be taken not to polish together articles of different sizes and shapes in a large - capacity polishing plant (eg. saltshakers of intricate shapes and large pieces) because the flow pattern in the baths may be affected substantially.

  18. The release of lead from the surface of the articles can be suppressed significantly regardless of the shape dependence (the S-to-V ratio is not important any more) if the above principles are observed. However, it may happen that the articles do not meet the required limit values. In that case, it is sufficient to subject them to an additional short polishing in the acid bath followed by a 4-minute rinsing. Of course, the articles should be arranged correctly in the boxes (the pouring openings facing the box bottom). • Should the trend characterized by the use of flame polishing prevail in the future and the permitted limit values of lead content in the extract fall even further, then it may be necessary to dealkalize the inner surface of flame-polished articles. This process would require a temperature of at least 400 °C. The laboratory tests of dealkalization carried out at a temperature of 300 °C were not efficient enough (the lead content in the extract was about 3 to 5 times larger than the values obtained for articles exposed to 460 °C). The exposure of the articles to acetic acid may represent another possibility of reducing the lead content in the extract.

  19. References Possibility of suppressing the amount of lead leached into alcoholid beverages from lead crystal glass. Technical report. 1991. Czechoslovak Academy of Sciencies, Institute of Vitreous and Ceramic Materials. Part I and II. Modification of low - lead glass. Technical Report. Research Institute of Glass, Hradec Kralove, 1991. Modification of the composition of 24-per-cent lead crystal aimed at reducing the lead content in the extract. Technical Report. Research Institute of Glass, Hradec Kralove, 1991. Sklar a keramik, p. 354, 1992. Sklar a keramik, p. 362, 1992. Silika, p. 106, 1996. Silika, p. 96, 1991.

  20. THANK YOU FOR YOUR ATTENTION

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