152 



PHOTOGRAPHY 



discovery that niaile what i g.-m-ially known ax 

 autotype or carlion printing a practical working 

 procea. The method ale described U called 

 tin- single transfer process, mnl produces a print 

 reversed as to right and left. The double transfer 

 ppM-ews merely differs in the adoption of a tern 

 porary support of o|>al gloss, zinc, or pajier coated 

 with a suitable preparation (the inotit convenient 

 being Sawyer's tlcxible supi>ort), which holds tin- 

 print till developed, and mat which the print is 

 then transferred to its tinal support it is then 

 non-reversed. When practicable it is usual to take 

 a reversed negative, and thus avoid the double 

 transfer. 



Powder Procets. By what is called the powder 

 process prints are produced on paper in plumbago, 

 or any other impalpable powder insoluble in water. 

 It luts been a good deal used on the Continent. A 

 slightly sticky or 'tacky' preparation of sugar, 

 gum, glycerine, and potassium bichromate, when 

 exposed to light, loses its tackiness in proportion 

 to the intensity of the light acting on it. A 

 glass plate coated with this preparation will there- 

 fore, when exposed under a negative, represent 

 the picture, so to speak, by different degrees of 

 tackiness. In this state ft fine powder sprinkled 

 over it will adhere in pro|>ortion to the stickiness 

 of the surface. When the rapwfluoM powder is 

 removed, and the film coated with tough collodion, 

 it can be detached and, if required, put on any 

 support such as pa|T, but the soluble portion of 

 the gum, &c. is previously removed by washing. 



Pnotoprnphic Knamels on Gliiss and Porcelain. 

 If the image as described in the preceding para- 

 graph lie developed by suitable metallic oxides 

 such as the underglaze colours of the porcelain 

 painter and the resulting image coated with col- 

 lodion be washed, the film can be transferred to 

 glass, china, or enamelled metal, and after firing 

 in a suitable kiln it will become vitrified, and will 

 !M> its permanent as a burnt in painting on the 

 same substances. Some of the very finest vitro 

 enamels are, however, produced by a 'substitu- 

 tion ' proves*, in which a collodion trans|>arency is 

 toned to saturation with platinum, or indium, all 

 traces of silver being carefully eliminated. This 

 modified lilni is now transferred to an enamel 

 tablet, and burnt in its in the previous method. 



liiiizn/ !//>. A new colouring matter or dye of 

 a primrose tint was discovered by Arthur C. (Jreen 

 in IHS7. who mimed il ' primuline.' This dye 

 affixes itself verv tenaciously to cotton fibre, -u 

 that by merely iioiling the fabric in it* ai|iieons 

 solution a |K*rm:inent yellow colour results; and 

 tbix yellow luuds acts as a mordant, permitting, 

 when acted on by appropriate developing agents, 

 the building up of mi immense variety of 'ingrain 

 colours ' that admit of wide practical use. 



Ity passing the yellow cloth through a bath of 

 a'-etic acid and nitrate of -ml a the material is said 

 to be 'dia/.otised,' and then is of a brighter yellow- 

 colour that is extremely fugitive under the influ- 

 ence of light. This constituted for a time an 

 insuperable objection to its use as a dye, but 

 suggested photographic jMissibilities. Experiments 

 proved that, if a material containing dia/.otised 



pri line be excised to light under a design, those 



pails acted on by light s|>ccdilv decomposed, while 

 I lie pails prot.-etcd remained unaltered; the latter 

 on treatment with a phenol or amine produce 

 many |x>riimnent com|x>iiiid colours, and the former 

 remain unchanged. rpon this is founded the 

 diazot\|>c process, by means of which every kind 

 of fabiic, cotton, muslin. velveteen, wool." linen, 

 silk, iVe. , its well as colloid films and paper, are 

 dyed by the influence of light. The process ;. 

 exceedingly simple, very cheap, and gives a /(//> 

 print, the most opaque objecto or parts of a design 



coming out darkest, the reverse l>ciiig the case in 

 ordinary silver printing. 



MrCrecn, in conjuiiciion with his partners Messrs 

 Craw and llevan, published the process at a meet 

 ing of the Itritish Association at Leeds in 1H90. 



MKCHANK AI. I'IMXTIM;. II'-W////;-//////,, ,,<! 

 Xtunnnti/i>r I'nirtstt*. It has long lieen known that 

 if a leaf, a bit of lace, or any similai -object w.i- placed 

 on a sheet of soft metal, ami considerable pn-ssun- 

 applied, the impression of leaf or lace was sunk into 



tl etal. From this metal plate prints weie taken 



as from an engraved plate, ami the process was 

 called Nature printing (q.v.). If we laid an ordinary 

 autotype print instead of a leaf on a Hat piece of 

 iron, covering it at the same lime with a smooth 

 piece of sheet-lead, ami then put them under sufli- 

 cient piesMirc. the result would IK' an imperfect 

 Woodhurytype mould in the soft lead. The metal 

 reierse would Iw faulty, liecausc in this cae the 

 gelatine film is too thin to give enough of relief. 

 In order to obtain a proix-r mould a layer of M 

 tiseil gelatine, considerably thicker than that used 

 for an autotype print, is ex|ised under a negative. 

 It is developed as in the autotvpe process, and 

 presents the image in considerable relief. The 

 print is then covered with the lead, and they are 

 pressed together in a hydraulic press, which pro- 

 duces a reverse or mould of the picture in the soft 

 metal without injuring the gelatine relief. 



The production of ordinary Wixidburytype print* 

 is a purely mechanical operation, the chemical 

 action of light not being called into play ; they 

 exhibit true gradation of tint, and in that respect 

 Wood bury type is the only perfect photo -mechanical 

 printing process known. The mould is placed in 

 a printing-press of a peculiar but simple construe 

 tion, ami a warm solution of pigmented gelatine 

 forms, so to speak, the printing-ink. This is 



poured on the mould, and a thin, hard, strongly 

 sized paper placed on the top of it. The lid of 



the press has hcncai h it a perfectly Hat glass plale. 

 ,.',% . iti it ii 



which Is now brought down on the mould ami 

 the lid firmly locked by a catch. The pressure 

 causes all the superfluous gelatine to exude, whilst 

 that in the mould adheres to the paper. In a 

 short time the gelatine sets, when the plate is 

 raised and the print withdrawn. It has now only 

 to be placed in a solution of alum, which renders 

 the gelatine forming the picture insoluble. 



The Slannotype (or printing from a surface of 

 tin) hits lieen called a simplified Woodhiirytypc 

 pioeess. Mr Wood bury, to whom it also is due, thus 

 describes it: 'A positive is first made from the 

 negative preferably by the carbon process. r'rom 

 this carbon or other transparency a negative is 

 made also in carlion ; but in this case the lissue 

 possesses much more body and much leas colour, 

 so as to obtain a certain amount of relief. This 

 (gelatine) relief negative is then coated with a 



thin india ruhlx-r varnish. A pi f tinfoil is 



laid over it, and the whole passed through a pair 

 of india-rilhlier rollers a species of mangle, in 

 fact. We have now a printing mould ready for 

 placing in the press and printing from in gelatinous 

 ink.' This process does not always give the Ipeauti- 

 ful results Obtainable by the original Woodbury 



type method, but remarkably good results are 



secured |,y it. 



Ill '',,,/if/ii- mill l'/intr,ty/,c I'mratsw.-- lioth of 

 these arc photo -mechanical methods, in \\ hich the 

 gelatine relief is itself used to print from in some 

 form of printing press, instead of being covered 

 with tinfoil as in the stannotype process. I.itho 

 graphic ink is used. The film or layer of gelatine 

 forming the printing surface requires to \w specially 

 and carefully piepared. This process, under the 

 name of Collotype, is much used for book illustra 

 ; tion and advertising purpose*. 



