154' ^^ COPAL VARNISHES. 



Particular de- The copal niu(l be grofsly powdered and put into a conve* 

 ^"^r^c^sfof ct^l"'®"*^ S'^^* veflel; the fpirit of turpentine and ammonia muft 

 in fp. of turpcn-be well fliaken together, and the mixture poured upon the co- 

 ^ by ammo- j^i. ^^^ ^^ff^] clofed, and To much heat applied as Hiall, as 

 ^^ fpeedily as poffible, make the fpirit boil fo ilowly that the bub- 



bles may be counted as they rife to the furface; if this is IkiU 

 fuUy performed, the folution is feen to take place in the follow- 

 ing manner; the mixture has a milky appearance when firll 

 applied to the copal; the upper furface firfl becomes clear, the 

 cloud gradually fubtides, and attaches itfelf to the copal, which 

 fwclls and becomes opaque ; a portion oF it in a diflblved ftate 

 gradually exudes with the air bubbles, and diffufes itfelf in the 

 fpirit, and this continues till it has taken up as much as can be 

 The fuccefs fufpended in it ; no more is then diflblved: It is on the (kilful 



depends on the jnanagement of thefe circumftances that the fuccefs of the 



management ot o 



thccircum- procefs depends; for, if lefsheat than the degree required be 



ftan^cs. given, the folution flops ; if more, the bits of copal inftead 



of being opaque and adhering together at the bottom of the 

 veflel, refume their tranfparency, and are agitated feparately 

 by boiling of the fpirit ; and when either of thefe accidents 

 happened, I could never afterwards diflblve the fame portion 

 of copal either in the fame or any frefti portion of fpirits. If 

 I might venture to explain thefe phenomena, I fhould fay, that 

 the ammonia gradually quits the fpirit of turpentine, and unites 

 with the copal, and reduces it to fuch a ftate, that with the 

 afliflance of the peculiar degree of heat that I have defcribed, 

 the fpirit of turpentine is enabled to diffolve fo much as will 

 form a varnifh ; but, if either more or lefs heat is applied, the 

 copal cannot be diflfolved, and then the operation is abortive, 



Excellence of Whether this explanation be falisfaflory or not is of fmall 



* *^"**» confequence, the exiftence of pidlures in my pofleffion, which 

 wc^re varniflied with this folution many years ago, and ftill re- 

 main more perfeA than they would have been if covered with 

 any other varnifh, is fufficient to prove that all Mr, Tingry has 

 faid on this fubjed has not the fmalleft foundation in truth. 



Hint which pro. I had examined all the books which I had accefs to, without 



duced the dif- getting any information refpe61ing the folution of copal for the 

 purpofe of making varnifh, till in Lewis's tranflation of Neu- 

 man's works I found the following. Vol. II. p. 27. " Solutions 

 of copal have been greatly efteemed as varnifties, and the me- 

 thod of making the folution kept a fecret'in particular hands: 



Junker 



