a66 Bleaching of Imprej^om ft an Engravings, 



the hirtory of the art. But their fcarcity renders them flill more Valuable. Moftofthofe 

 which arc ftiil extant are defaced by negligence, during the time of their remaining fufpend- 

 ed a^aiaft walls cxpofed to fmolte, vapor, and the excrements of infeiSs, Colle£lurs of 

 prints have not, however, (hewn the fame partiality as antiquarians for the patina; but on the 

 contrary they have fought and praiitilcd a method of clearing prints from thefc impurities. 



This method conTdls in hmple wafhing with clear water, or a ley made of the afhes of 

 vine ftallcs or reeds, and laftly by a long expofure to the dew. Aqua fortis is alfo ufed for 

 the fame pnrpofe, but with a degree of rifque at lead equal to its advantages. The ley 

 difn>!ves not only the impurities but likewife the oil of the printing ink, and either difcharges 

 it totally, or leav s a cloudy appearance. The aqua fortis afts on the vegetable fibre, of 

 which the paper itfelf is compofed, and produces a dark colour, which cannot be removed 

 by means of this liquid, but by an adlion which would confiderably injure the paper itfelf. 



The difcovery of Prieftley, of the fluid erroneoufly named by him, but fuice known by 

 the name of oxygen ; and the information we have obtained from Scheelc, of the effecis of its 

 combination with muriatic acid, have led Berthollet to the ufeful application of its properties 

 to the a£t of bleaching cloths, Chaptal to that of bleaching prints and books, and Giobert to 

 the art of painting. But the method of making this preparation is too inconvenient for a 

 mere amateur and colledior of prints, and the oxygenated muriatic acid is not yet to be pur- 

 chafed ready prepared in Italy. It may not, therefore, be unacceptable to defcribe an eafy 

 method of effeding this purpofe without the difficulties of chemical procefTes, and within 

 the ability of any perfon to perform. 



It is known that oxygen is abundantly contained in the combinations called metallic calceSy 

 though in a ftate of inaftivity ; and it is equally well afcertained, that thefe fubftances have 

 a very flrong af.raftion for it. On the other hand it is a faft, that fome of the metallic 

 calces of very moderate price are capable of eafily yielding the whole or the greateft pro- 

 portion of this conftituent part. Manganefe is not very well adapted for this purpofe ; but 

 minium is much better. Nothing more is required to be done, but to provide a certain 

 quantity of the common muriatic acid, for example, three ounces, in a glafs bottle, with a 

 ground-ftopper, of fuch a capacity that it may be only half full. Half an ounce of minium 

 mufl then be added ; immediately after which the ftopper is to be put in, and the bottle fet 

 in a colJ and dark place. The heat, which foon becomes perceptible, fhews the beginning 

 of the new combination. The minium abandons the greateft part of its oxygen with which 

 the flu d remains impregnated, at the fame time that it acquires a fine golden yellow, and 

 emits the deteftable fmell of oxygenated muriatic acid. It contains a fmail portion of 

 muriate of leud ; bu this is not at all noxious in the fubfequent procefs. It is alfo necelTary 

 to be obferved, that the boitie muft be ftrongj and the floppcr not too firmly fixed, other- 

 wife the aftive eLflic vapor might burfi it. The method of ufing this prepared acid is as 

 follows : 



Provide a fufHciently large plate of glafs, upon which one or more prints may be feparate- 

 )y fpread out. Near the ed es 'ct there be raifed a border of foft white wax half an inch 

 high, adhering well to the glafs and flat at top. In this kind of trough the print is to be 

 placed in a bath of fre'fh urine, or water containing a fmall quantity of ox gall, and kept in 

 this fituation for three or fuur hours. The fluid is then to be decanted off, and pure warm 

 water poured on, which mull be changed every three or four hours uatii it pafTes iimpid and 



clear. 



