TRANSACTIONS OF THE SECTIONS. 37 



The Action of Sunligld on Colourless and Coloured Glass, 

 Bij Thomas Gaffield, of Boston. 



The author's experiments on this subject, of which some accounts have appeared 

 in American and European scientific journals, cover a period of nine years, and 

 embrace some eighty ditierent lands of glass, of English, French, German, Belgian, 

 and American manufacture, — of rough and polished plate, crown, and sheet window 

 glass, of Hint and crown optical glass,'of opal and ground glass, of coloured pot- 

 metal, flashed and stained glass of various colours, of glass ware, and glass in the 

 rough metal. They were carried on chiefly upon the window-sills and roof of the 

 author's house in iBoston, in a position exposed to the full force of the sun's raya 

 during the whole or gTcater portion of every day, only being protected by covers 

 in the event of snow-storms. 



The usual size of the glasses exposed is four by two inches, and several himdred 

 specimens show the efiect of sunlight in producing a change of colour by exposure 

 from one day in summer to several years. These changes in the colourless glasses 

 are from white to yellow, from green to yellowish green, from brownish yellow 

 to purple, from greenish white to bluish white, and from bluish white to a darker 

 blue. By the colours of colourless glass are meant those which are seen in looking 

 through the edges of the glass. They are not noticed in looking at the surface in 

 our windows, unless a white curtain furnishes a contrasting background. 



It is a curious fact that, while these various glasses before exposure can be sub- 

 mitted to great heat in a glass-stainer's kiln without any change, all the exposed 

 and changed specimens can be restored to their original colour by being placed in 

 the same kiln during a single Are. A second exposure to sunlight will reproduce 

 the same yellow and purple colours as before ; and this process of coloration by 

 light and decolorization by heat can be carried on indefinitely. 



During the last year, the author commenced an experiment with pot-metals, not 

 of the primary colours, but of the intermediate ones, which most nearly approach 

 tliose produced in colourless glass by sunlight exposure. In every specimen of the 

 brownish, j^ellowish, and rose or purple colours thus exposed, astonishing chano-ea 

 in colour or shade in a short time were observed. In some instances a few daj's 

 of exposure in the month of June of the present year sufficed to show the com- 

 mencement of the Sim's influence. These changes were from a coffee-colour to a 

 rose, from amber, yellowish, brownish, and purple to darker shades of the same 

 colours. 



Inasmuch as this class of pot-metal colours was used in the painted windows of 

 past ages, and as flashed and stained colours are subject to change in the colour- 

 less body of the glass, may not this series of experiments go far to solve many 

 interesting questions regarding the alleged superiority of the old cathedral glass ? 

 The fact of coloration or change of colour or shade by sunlight being established, 

 must we not transfer some of our praise for the old artists in glass to the wonderful 

 pencil of the brightest luminary of the heavens, which, during the centuries, has 

 noiselessly but unceasingly been at work, deepening and mellowing the colours 

 of all the windows of the venerable cathedrals of the world ? 



Exactly what this wonderful alchemy is, and what are the methods of its 

 operation, are questions on which various opinions may be given, but which only 

 a careful consideration and comparison of the observations and theories of many 

 different scientific men can accurately decide. Some have attributed it to the pre- 

 sence of oxide of ii-on, some to arsenic, and some to sulphur in the constituent 

 materials of the glass. Some think oxide of manganese (singular as it may seem 

 used as a decolorizer) to be the great colourist in this matter. The author thinks 

 that in many coloured and colourless glasses it plays a very important part in the 

 effects produced. But in some experiments made with glasses containing no man- 

 ganese, decided changes of colour from greenish to yellowish have been produced. 



Perhaps the question cannot be accurately solved until some glass-manufacturer 

 will make, with great care and for this special purpose, a series of specimens of 

 colourless and coloured glass, which shall be exposed for months and years to the 

 influence of sunlight. Knowing the exact constituents of each specimen, a good 

 foimdation could be laid for a thoroughly scientific investigation ot the subject. 



