42 



NA TURE 



[May lo. 1900 



POTTERY AND PLUMB ISM. 

 T\ R- T. E. THORPE, F. R.S., gave a lecture on Friday evening, 

 -*-^ May 4, at the Royal Institution, on the results of an 

 experiniental inquiry which he has made, at the instance of the 

 Home Office, on the hygienic questions involved in the use of 

 lead compounds in the manufacture of pottery. 



After explaining how lead poisoning occurs in connection 

 with pottery manufaciure, he described ihe conditions which a 

 perfect glaze must fulfil, and named the various forms in which lead 

 compounds enter into the composition of the glazing material as 

 ordinarily employed. He pointed out that experience amply de- 

 monstrated, both in this countryand on the Continent, that " raw " 

 lead is more generally mischievous in its action than " fritted " 

 lead, that is, lead in the form of a complex silicate associated 

 with alumina, lime, &c. This depends on the more ready 

 solubility of the various modifications of raw lead in the 

 animal secretions, and more particularly in the gastric juice. 

 This fact, indeed, is now generally recognised, and in the 

 inquiry which was instituted by the Home Office in 1893, 

 manufacturers whose names deservedly carry authority in the 

 pottery districts strongly urged the substitution of fritted lead 

 for raw lead in all glazes. Unfortunately, however, this recom- 

 mendation was not enforced. This may have been due, partly 

 at least, to the circumstance that cases of plumbism occurred 

 from time to time in works where fritted lead was exclusively 

 used. The fact is there is fritted lead and fritted lead. 



Dr. Thorpe then proceeded to explain the results of a recent 

 inquiry into the conditions which determine the ease with which 

 lead may be dissolved out from a fritt by dilute acids such as are 

 present in gastric juice. In the first place, it was found that, 

 speaking generally, English fritts yielded a far larger amount of 

 lead to solvents than those made in Holland, Belgium, 

 Germany or Sweden. Indeed, some English specimens of 

 fritted lead were found to be hardly less soluble than raw lead, 

 as shown by the following numbers : — 



Lead silicate, Specimen I. 

 11- 

 Glaze A, made with lead silicate 



White lead 



raw " lead 



Lead oxide dissolved, 



expressed as per- 

 centage of total lead 

 oxide present. 



... 99-6 



... 99-6 



... 99-2 



... 99-2 



... lOO'O 



.... loo-o 

 ... loo-o 



Next, the inquiry showed that there was no necessary relation 

 between the amount of lead oxide in a fritt and the extent to 

 which it would yield lead to solvents comparable, as regards 

 their action, with animal secretions. Some of the compounds 

 richest in lead were, in fact, among those least attacked by 

 solvents. This is illustrated by the following series of 

 numbers : — 



I. Solubilities practically the same, amounts of lead oxide in the 

 fritt very different. 



II. Solubilities very different, amounts of lead oxide in fritt 

 practically the same. 

 English fritt, D ... 37-9 ... 28-0 



., ,, E ... 36-2 ... 1-4 



Swedish fritt 



45 •« 

 44-1 



io'8 



2-1 



Further inquiry elicited the fact that the extent to which 

 the fritt gave up lead to the solvent depended upon two 

 conditions :— 



(i) The existence of a definite numerical relation between the 

 basic and acidic oxides in the fritt, and 



(2) Complete chemical union. 



The definite numerical relation thus alluded to may be stated 

 in the following terms : — If the sum of the equivalent percentages 

 of basic oxides, expressed as lead oxide, is not more than double 



NO. 1593, VOL. 62] 



the sum of the equivalent percentages of acidic oxides, expressed 

 as silica, the solubility of the fritt, as regards lead, is rarely more 

 than 2 per cent. Any increase in this ratio is attended by an 

 increase in the amount of lead dissolved, and the amount of 

 soluble lead increases very rapidly with even a slight increase in 

 the ratio. The following figures serve to illustrate this fact : — 



Dutch fritt, No. i 

 Belgian fritt. No. 

 Dutch fritt. No. 2 

 Belgian fritt. No. : 

 Swedish fritt 

 English fritt . . . 



It was further found that, provided the ratio of acids to bases 

 is below 2, the nature of the basic oxides has little or no effect 

 upon the amount of the lead oxide dissolved. This may be 

 illustrated by the following numbers : — 



Dutch fritt 

 English fritt 

 Swedish fritt 



Lead 

 oxide. 



I9'0 

 l6-2 

 44-1 



8-1 

 IO-3 



Lime. 



8-5 

 0-9 



Alkalis. 



Solubility 

 per cent, 

 on fritt. 

 4-9 ... 1-2 

 9-2 ... 17 

 3-4 ... 2-1 



Further evidence of the fact that the insolubility of a complex 

 silicate is determined by the ratio of acids to bases, and is inde- 

 pendent of the specific nature of the bases, is afforded by the 

 case of flint glass, which consists essentially of a silicate of alkali 

 united with a silicate of lead. Separately, these silicates are 

 readily attacked by dilute acids. When united, as in flint glass, 

 the compound is only very sparingly soluble. Merely to flux 

 together the ingredients of a fritt, with no regard to its composi- 

 tion as a definite chemical compound, and with no regard to the 

 time or temperature needed to complete the chemical changes, 

 is not the proper way to make a fritt. 



In the course of the inquiry it was found that the Continental 

 fritts, which conformed to the above ratio, and were distinguished 

 by their comparative solubility, were very difficult to break up 

 by the action of acids, and yielded only minute portions of 

 soluble matter (much of which, however, consisted of lead) to 

 solvents, whereas the English fritts were, for the most part, 

 very easily decomposed by the same treatment, and gave up the 

 greater part of their lead to solution. This led to the surmise 

 that the Continental fritts consisted, in the main, of comparatively 

 stable chemical compounds, the minute quantity of lead dissolved 

 being due to some lead compound — oxide or silicate — in a state 

 of incomplete chemical union. Experiment showed that this 

 surmise was correct. By treating a Iritt, compounded so as to 

 be within the limiting ratio, with dilute acid, by far the greater 

 portion of the soluble or incompletely fixed lead may be re- 

 moved, and a highly insoluble complex lead silicate is obtained. 

 A fritt, for example, containing upwards of 53 per cent, of 

 oxide of lead, and of which the limiting ratio of acids and bases 

 was about 2, had this ratio lowered to i "8 and the solubility 

 diminished from 2 per cent, to four-tenths of a per cent., the 

 amount of lead oxide in the product so treated being upwards of 

 52 per cent. 



A number of manufacturers and professional fritt makers, 

 acting in conformity with the suggestions which have been put 

 forward, and in response to the invitation of the Home Secretary 

 to have their glazes tested in the Government Laboratory, are 

 now producing lead fritts having a solubility which is even below 

 the standard provisionally suggested in the Home Office 

 Circular of December last. 



Although measures based upon the above facts will no doubt 

 largely minimise the evil of lead poisoning, Dr. Thorpe stated 

 that he was not sufficiently .sanguine to suppose that they would 

 altogether stamp out plumbism in the Potteries. It must be 

 clearly understood that complete immunity from lead poisoning 

 can never be obtained so long as lead compounds continue to be 

 used. The true solution is to be found in the more general 



