1907.] Conner catioti of Buildings and Frescoes. 597 



WEEKLY EVENING MEETING, 



Friday, April 12, 1907. 



Sir Ja:hes Crichton-Browne, M.D. LL.D. F.R.S., Treasurer and 

 Vice-President, in the Chair. 



Professor A. H. Church, M.A. D.Sc. F.R.S. M.R.I, 



Conservation of Historic Buildings and Frescoes. 

 [illustrated by experiments, etc.] 



The title of this discourse as announced is in one way too wide, in 

 another too narrow. For " Conservation of historic buildings " 

 should be substituted " Conservation of urban stone-work," and for 

 " frescoes," " wall-paintings." 



We have to consider the nature of the attack and of the defence 

 — the weapons and the armour — the damage and its repair. 



First amongst the destructive agents at work, I am bound to 

 place sulphuric acid. I own myself a thiophobist. Dr. S. Rideal has 

 given an estimate of the amount of sulphuric acid poured year by 

 year into the atmosphere of London by the coal burnt in the Metro- 

 polis. His minimum figure is half a million tons, his maximum 

 twice that amount. But before dealing with this branch of the 

 subject in some detail, let me enumerate the chief of the other 

 hostile agents. These are soot and tarry matters from coal-smcke ; 

 water, and an abnormal amount of carbon dioxide. Minor enemies, 

 such as hydrochloric acid, nitric acid, and ammonia salts, with 

 sulphuretted hydrogen and sulphurous acid, may be put on one side 

 for the nonce. These assist in doing damage —damage of a special 

 kind and to particular materials, such as metals and organic pig- 

 ments — but our attention ought to be focused on the chief mischief- 

 makers. Before dealing with these, I may observe that it is 

 fortunate that nitric and hydrochloric acids exist in but small pro- 

 portion in the atmosphere of towns, save in the immediate vicinity 

 of certain factories. I say " fortunate," for at present no efficient 

 chemical means of neutralising their bad effects upon stone-work 

 seems available. Of course, one may use mechanical methods of 

 protection, but a stone which has been attacked by nitric or by 

 hydrochloric acid does not allow of the entire conversion of its 

 soluble salts into insoluble, nor, in consequence, of its reconsolidation. 



Even now I cannot approach closely the story of our arch-enemy, 

 sulphuric acid, without previously saying a few words about soot 

 and tar, about water and carbon dioxide. The important role of 

 these last in the natural weatherinc^ of stone does not need discussion, 



