5o6 



NATURE 



[October 6, 1923 



glass is not tlia- lo Hh presence of lichens but the 

 undoubted gn>\stli >i In hens on it is due to, and 

 subsequent to, the glass being decayed. 



The immediate cause of decay and the formation 



of thf ' '• trristic pit holes is snrrlv fhio to chomicftl 



and \ ilecompositioii, .mil it i-, (.nl\' when tlic 



glass tdvanced state of ilccay that the hcheiis 



find 111 till disintegrated glass accumulated in the 



Fits a soil suitable for their growth, (For details 

 would refer to an article in Nature of May 2, 1907.) 



One finds, in fact, that the degree and character 

 of till corrosion is determined by the chemical 

 coiniinsiiion of the glass. The statement that the 

 glass of the twelfth to the fifteenth century shows 

 a slower rate of alteration than that used later needs 

 some modification. The glass of the twelfth century 

 was of good quality and shows little decay, but there 

 was steady deterioration from the thirteenth to the 

 beginning of the fifteenth century ; the glass of this 

 latter period shows the most pronounced decay. 

 After this time the composition of the glass in general 

 steadily improved. 



The point I would particularly challenge, however, 

 is the suggestion that windows should be treated with 

 a liquid mastic to prevent the growth of lichens. 

 I am not quite sure if this is intended to apply to 

 new or old windows. If the latter, surely the remedy 

 is a thousand times worse than the disease. If the 

 former, I suggest that the proper way to prevent 

 the growth of lichens is to prevent the decay of the 

 glass which enables them to gain a foothold. That 

 can be done only by ensuring that glass of a com- 

 position which ensures durability is used in new 

 windows. As a matter of fact the glass used nowadays 

 as a rule leaves little to be desired in this respect. 



One further point occurs to me. I have made 

 many analyses of medieval stained glass and I 

 invariably find phosphates as a constituent — 

 particularly in glass of the fourteenth century. As 

 the glass decays this would presumably be deposited 

 as calcium phosphate in the corrosion pits. Would 

 this encourage the subsequent growth of lichens and 

 account in some measure for the prolific flora described 

 by Dr. Mellor ? No£l Heaton. 



81 Queen Victoria Street, E.C.4, 

 August 29. 



The article referred to by IVIr. Noel Heaton 

 describes the results of " one of several possible lines 

 of research " ; it shows that lichens accelerate the 

 chemical change of the glass and lead, and exert a 

 mechanical action on the altered glass. 



Certain species of lichen are found only on un- 

 altered glass ; they do not persist, and on disappearing 

 leave a roughened surface conformable to their own 

 shape. On deeply corroded glass, lichen debris, not 

 the plant, is the more frequent. Lichen physiology 

 is a controversial subject, but the probability is that 

 neither the calcium phosphate nor the " soil " 

 mentioned by Mr. Heaton accounts for the flora. 



References to the presence of three species of lichen 

 on the windows of two churches are made by Fries 

 and Nylander, and reproduced by a few lichenologues ; 

 there has been, to my knowledge, no scientific 

 investigation of the lichen flora on church windows 

 or of its relation to the deterioration of glass until 

 three years ago when the research was undertaken 

 at the Sorbonne. I cannot therefore appreciate Mr. 

 Heaton's statement that it "has frequently been 

 suggested " that " the decay of ancient stained glass 

 is produced by the action of lichens." I am, how- 

 ever, open to correction if Mr. Heaton will give the 

 authority for his statement. 



NO. 2814, VOL. I 12] 



The only modification I can make with regard to M 

 the glass of the twelfth to the fiftfv'nth rontnri^ is ^ 

 that certain gla.ss of ^ 

 but is this not to s( 

 each century ? It is reassuring 

 " the glass used nowadays as a r 

 be desired " as regards durabilit>', \m 

 that certain stained glass of so r«:ent 

 second half of the nineteenth rfntm > -.i.w». ... 

 advancc'l stiti of corrosion. In tins case lich< ns 

 have apparently played no part. 



The <iuality of the gla^ is undoubtedly a factor of 

 great importance in ensuring its durabilitv. l^iit ii 

 cannot prevent the growth of lichens, a.s ! 



these plants find a suitable substratum on th- .1 



im altered surface of the glass. The application <ii a 

 lii|iiiil mastic to exclude the lichen spores is intri- ji (l 

 for those windows difficult of acc^s for 

 purposes. What can be the objection to r. 

 old glass and not on new '' The suggestion \i, not 

 my own ; it tmds fa\(jur with one who has more than 

 forty years' experience in the art of stained glass, 

 medieval and modem, and has the keenest apprecia- 

 tion of aesthetic value. 



It may be mentioned that the destructive effect 

 of lichens on their subslratum is remarkabl" ••-'- -t 

 on the marble statues at Versailles, — somt- 

 months ago it was derided to arrest the corrw.,..,-. ,., 

 cleaning the marhle and then treating it with a mastic. 



Does .Mr. Heaton use the word ' disease " in its 

 popular or pathological sense ? It i> to my mind as 

 wrongly used in connexion with the corrodetl glass 

 as it would be if applied to the weathering and 

 disintegration of rocks. 



Through the courte^\ of .Mr. J. .\. Knowles. of 

 York, I have had acct--^ t" Mr KnowieK's own work 

 and once more read Mr. ipers on 



the composition and dec. e no in- 



consistency between these papers and my article 

 in Natire of August 25. E. Mellor. 



University College, Reading. 

 September 15. 



Painted Pebbles from the North- East Coast 

 of Scotland. 



The statement that Azilian painted pebbles do 

 not occur further nortli tlian Basle was made by me 

 in a review appearing in Nature, August 25, p. 

 276. It has been challenged and the so-called 

 painted pebbles found by Sir F. Tress Barry on the 

 N.E. coast of Scotland recalled. These interesting 

 objects cannot, however, be referred to the Azilian 

 culture, and this for two reasons, namely : 



(i) They were found in connexion with and in the 

 precincts of Broch buildings, admittedly from their 

 archaeological and faunal content of much later date. 

 It has been suggested that the Broch had been con- 

 structed on an older Azilian settlement, but this idea 

 is vetoed by, 



(2) When the actual objects are seen and handled 

 it is found they in no respect resemble the .\zilian 

 painted pebbles. Prof. H. Breuil, of Paris — pre- 

 viously a partisan of the early age for these objects — 

 at once rejected the Azilian date on seeing the speci- 

 mens. I may add tliat I also came to the same 

 conclusion when I saw and handled the stones. 



However, it need not be added that the above in 

 no wav detracts from the interest of these queer 

 objects" from the Broch, and the problem of their 

 meaning and object still remains unsolved. 



M. C. B. 



