Identifier: londonencyclopae13curt (find matches)
Title: A London encyclopaedia, or universal dictionary of science, art, literature and practical mechanics : comprising a popular view of the present state of knowledge : illustrated by numerous engravings, a general atlas, and appropriate diagrams
Year: 1829 (1820s)
Authors: Curtis, Thomas, fl. 1820
Subjects: Encyclopedias and dictionaries Aeronautics
Publisher: London : Thomas Tegg
Contributing Library: University of California Libraries
Digitizing Sponsor: Internet Archive

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d copy from the glass instead ofconsulting the drawing. In placing the draw-ing upon the stone, it is very important to recol-lect that_ a space or margin, of at least half aninch, and more if possible, must be left betweenthe extremities of the drawing and the edge ofthe stone, and a still larger space left at the bot-tom than at the top of the orawing, in order tohave room for the writing, if any is ii.tended tobe placed under the subject; for, unless this beattended to, the drawing cannot be printed, andthe whole labor of the artist is lost. It happens,however, that some stones have small black veins,like hair lines, which run across them, as well asother little imperfections. The artist ought,therefore, always to take these defects (if exist-ing) into consideration, when he puts his tracingon the stone; for often the drawing may be soplaced that these black veins (which an: apt toprint) are lost in the dark tints of the foreground :or if there are ^^tached and separate subjects on
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L I T II O G R A P II Y. 49 tl.e stone, lie may place these so that t^ie veinsshall come between them, as in this case theyare of little consequence : or else, if tne stone islarge enough to allow it, there is no absolutenecessity to place the drawing in the middle ;but it may be made on one side so as to avoidthe veins, &c. The drawing, being finished on the stone,is sent to the lithographic printer, on whoseknowledge of his art the success of the impres-sions entirely depends. The first process is toetch the drawing, as it is called. This is doneby placing the stone obliquely on one edge overa trough, and pouring over it very dilute nitricacid. It is poured on the upper part of the stonje,and runs down all over the surface. The stoneis then turned, and placed on the opposite edge,and the etching water, being collected from thetrough, is again poured over it in the same man-ner. The degree of strength, which is little morethan one per cent, of acid, should be such as toproduce a

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