August 25, 1923] 



NA TURE 



299 



Lichens and their Action on the Glass and Leadings of Church Windows. 



By Dr. Ethel Mellor, University College, Reading. 



Fig. I. (X36.) 



{a) Opaque discs with beginnings of pits ; 

 (/') pit bordered by opaque glass. 



THE gradual deterioration and destruction of the 

 stained glass of church windows is a subject 

 of general and scientific interest. It will, therefore, 

 probably be admitted that the technical and practical 

 knowledge of the stained glass artist should be 

 reinforced by the theoretical and laboratory studies 

 of the scientific worker. One of the several possible 

 lines of research was approached nearly three years 

 ago at the Sorbonne under the direction of the late 



Prof. Matruchot 

 and afterwards of 

 the late Prof. 

 Bonnier. 



The deteriorated 

 glass is scaly and 

 iridescent, or pitted 

 and opaque. Both 

 surfaces are at- 

 tacked, and though 

 the outer shows 

 the greater altera- 

 tion, it is on the 

 inner that the ac- 

 tion is sometimes 

 first apparent. The 

 opacity may extend 

 over the whole sur- 

 face, but more fre- 

 quently appears as 

 scattered disc-like 

 points; these grad- 

 ually get bigger and 

 frequently run together. Meanwhile, the glass at 

 the centre of the original opaque discs disappears 

 and so arise the beginnings of the pits, each bordered 

 by opaque glass and later lined by iridescent scales 

 visible under the binocular lens. As the alteration 

 of the glass continues, the pits increase in diameter 

 and often unite, forming channels of diverse outline 

 and length (Fig. i). The maximum breadth measured 

 was 5 mm., and depth 1-9 mm. Two pits on opposite 

 surfaces will sometimes increase in depth until the 

 separating wall disappears and a perforation of 

 the glass results. Microscopical examination of tlic 

 opaque glass shows markings and surfaces similar 

 to geographical contours, and the contortions and 

 cleavages of rocks. 



In some cases there is no opacity, and there are 

 no pits. The surface is iridescent and may appear 

 slightly irregular over more or less extended areas. 

 The alteration here takes the form of scaling in thin 

 horizontal plates shown under the microscope to 

 consist of several superposed layers variously cleft 

 and resembling a crazy-tiled garden path. 



Unstained and stained glass are similarly deterior 

 ated, but certain colours show more susceptibility li > 

 alteration than others. Purple, green, blue, red, 

 amber, and particularly amethyst glasses, are all 

 deeply corroded, while grey tones are less so, and 

 the golden-yellow glass resulting from the vitrifica- 

 tion of the silver salts is more or less immune. This 

 immunity is well illustrated in the reproduction of 

 a fragment of fifteenth-century glass (Fig. 2) ; the 

 surface of the grey and colourless portions is corroded 

 and opaque, and outlines clearly the golden-yellow 

 border and leaf. 



There is little reason to believe that the glass of 

 any century is the more frequently or gravely 

 attacked. The glass of the twelfth to the fifteenth 

 centuries is more refractive than that used later 

 and shows a slower rate of alteration, but the cumula- 



tive destructive effects are great. The extent of 

 the corrosion does not depend upon age — portions 

 of fourteenth-century glass are still unaltered ; 

 specimens of nineteenth-century glass are sometimes 

 badly pitted. The corrosion is of considerable im- 

 portance aesthetically, but, though it continue until 

 perforation occurs, it does not affect the actual 

 duration of the window ; this depends upon the 

 leadings. 



The oldest leadings are heavy and have well 

 resisted chemical change ; the lighter lead used 

 since the fifteenth century is much more liable to 

 conversion into carbonate of lead, friable and un- 

 stable. The transformation is often completed in 

 less than fifty years. This is a matter of supreme 

 importance, for the leadings constitute the skeleton 

 of the window and their appearance remains re- 

 assuring after the chemical change has taken place. 

 They are, however, no longer solid, and the crucial 

 moment arrives when an external condition, such 

 as a gust of wind, causes them to disaggregate and 

 allow the glass to fall. It is in this way that so many 

 of the marvellous windows of the last few centuries 

 have perished. This destructive process has been 

 studied and pointed out repeatedly during the last 

 thirty years by M. Felix Gaudin of Paris, a well- 

 known peinte-verrier ; it cannot be emphasised too 

 much that it is through the leadings and not through 

 the glass that historic windows are often lost. 



The alteration of the lead is purely chemical ; that 

 of the glass is due to two causes, chemical and 

 mechanical. Strange though it may seem, the 

 windows serve as a substratum for lichens. These 

 plants retain water between their tissues and the glass 

 by capillarity ; they also find favourable conditions 

 for growth in proximity to the leadings, which check 

 the drainage where they approach the horizontal 

 plane, and when loose hold water. The amount of 

 carbon dioxide normally dissolved in water is con- 

 siderably increased by that evolved by the lichens 



I'll,. .1. (Nalui-.il M,c.) 



A, Golden-yellow glass : (a) scaly glass ; (/') opaque glass. li, Grey and 

 colourless glass : (c) opaque surface ; {li) pit. 



during the process of respiration, and the chemical 

 change of glass and lead is thus accelerated. Evidence 

 of this augmentation of the chemical action is given 

 by the opacity and squamosity of the glass closely 

 following the track of the lichen. 



The mechanical action of the lichens accounts for 

 the disappearance of the opaque glass, and the con- 

 sequent formation of pits and channels. The minute 

 fissures in the opaque glass are penetrated by the 

 lichen hyphae, which by their varying states of 



NO. 2808, VOL. 112] 



