Copy for the half-tone process should be as nearly perfect as possible. Only the best photographs should be selected. Prints on semimat velox and glossy haloid papers are regarded as the best photographic copy for reproduction. Every part of the photograph or drawing should be absolutely clean. If any part that should be pure white becomes soiled or stained the defects will be reproduced. If a photograph needs retouching it should be retouched with great care and just sufficiently to correct defects and to bring out or strengthen the important details. In many photographs the skies may be "muddy" or uneven in tone, and this defect can be corrected by the use of an air brush, the only medium that will produce an almost even tone. As already stated, half-tone plates can be improvedby re-etching and tooling, but tooling tends to destroy the effects of nature and produces an artificial appearance in the print. One who is preparing wash drawings for reproduction by the half-tone process should remember that brush marks and other inequalities of tone will be reproduced with as much fidelity as other details. Such drawings should therefore be made two or three times larger than the engraved cut in order to subdue all unnatural effects and to soften the general tones.
Line drawings are not generally suitable copy for the half-tone process, but it is occasionally desirable to use that process instead of zinc etching for reproducing a line drawing that has been inexpertly prepared if the cost of redrawing would more than offset the difference in cost between zinc etching and the more expensive half-tone process. In reproducing a pen drawing by half tone the lines become softened and represent the details and shading only; but the pen drawing may be further developed by brush work. Examples of this type of reproduction are Plates V,A, VI,A, and XV, figure 10, and other illustrations in Survey Monograph 34.
Vignetting, which consists of a skillful grading off of the edges of a picture, as well as extensive tooling or hand engraving, is often employed for artistic effect but should be specified only for exceptional illustrations. The plates made for the Survey are either "square trimmed" or the ground tint is entirely omitted or routed away; they are not usually tooled or vignetted.
Half-tone cuts etched on copper cost 20 to 60 cents a square inch, the cost being varied according to a standard scale based on the ascertained cost of reproduction. Those that require a screen finer than 150 lines cost 25 per cent additional. The minimum charge for a single cut is $3.
Half tones etched on zinc (100-line screen or coarser) cost 25 per cent less than those etched on copper.
THREE-COLOR HALF-TONE PROCESS.
The three-color process is practically an adaptation of the half-tone process to color printing based on the theory that all colors or hues in nature can be reproduced by combinations of three colors of the spectrum—red, blue, and yellow. The process differs from the ordinary half-tone process particularly in the use of color filters in making the negatives and in the character of screens and diaphragms used. This process, like all others, is worked somewhat differently in different establishments. In what is called the indirect method, the one most commonly used, twelve photographic operations are necessary to produce one illustration, or the three plates or cuts from which one illustration is to be reproduced by printing. These twelveoperations produce three chromatic negatives, each representing one color; three transparencies or positives, made from the chromatic negatives; three half-tone negatives, made from the positives; and finally three contact prints, made on sensitized metal plates. In what is called the direct method the half-tone screen is placed in front of the photographic plate so that it becomes also a half-tone negative from which a print is made on a sensitized metal plate. Thus the photographic operations in the direct method are reduced to six, but the interference to the passage of light offered by the half-tone screen and by the prism used to reverse the image on the negative lengthens the time of exposure.
Unfortunately, no pigments have been found that can reproduce in purity the colors of the spectrum, and to this fact is due the failure of the process to reproduce exactly all the colors, tints, and shades of an original. When a drawing in black on white paper is photographed only the white paper affects the negative film. The transparent parts of the developed negative thus represent the black, and the opaque parts, which have been acted upon by light, represent the whits. Theoretically, when a chromatic negative is made for the yellow plate a purple-violet filter cuts out all the yellow and allows the red and blue rays to affect the plate; when a negative is made for the blue plate an orange filter similarly cuts out the blue and allows the yellow and red rays to affect the plate; and when a negative is made for the red plate a green filter cuts out the red and permits the blue and yellow rays to affect the plate. These color filters, which are usually made of transparent stained gelatin, are generally placed in front of the lens. When printing plates like those used in the half-tone process have been made from the three negatives and the plates have been inked with yellow, blue, and red ink, respectively, a combined impression from them will produce a close approximation of the subject photographed. The colored inks often used are light yellow, peacock or prussian blue, and bright, transparent crimson.
The ordinary half-tone screen, which bears lines cut at an angle of 45° to the sides of the plate, is rectangular, but the screens used for three-color work are made circular in order that they may be turned in the camera to make the lines intersect at other angles, the angles being varied to avoid producing an undesirable pattern or a moire effect. Turning the screen also prevents the exact coincidence or superposition of the red, blue, and yellow dots, which would produce black. In other respects the screens do not differ essentially from those used in ordinary half-tone work.
As special experience is necessary in printing three-color plates the engraver generally delivers the printed illustrations to the purchaserinstead of the plates, which he furnishes for other kinds of relief printing.
The copy for this process may consist of anything in color, such as specimens, objects, paintings, or properly colored photographs. The process does not usually reproduce all the colors and tints of an original with equal exactness and is not used by the Survey for work that demands precise reproduction of color, but it is satisfactory for reproducing most colored drawings, colored photographs of specimens, or the specimens themselves if they show individual variations in color. As the process is entirely photomechanical it gives more scientific accuracy in detail than chromolithography, in which there is much hand work, and it is much less expensive. If the colors shown in proofs are not satisfactory they can be modified.
The four-color process, in which four color plates are used, gives a closer approximation of true color values than the three-color process, and at a comparatively small increase of cost. The additional color used is generally a neutral gray or black.
WAX ENGRAVING (THE CEROTYPE PROCESS).
The wax or cerotype process does not require finished drawings and is especially suitable for making text illustrations and small maps, although it may be used also for large work. For this process blue prints, pencil sketches, old prints, or rough copy of any kind may be submitted—that is, it is not necessary to furnish carefully prepared drawings in black ink, as it would be for photo-engraving, for the wax engraver will reproduce in proper form any illustration in which the copy and the instructions show what is wanted, just as an experienced draftsman will make a good drawing from the rough original furnished by an author. Full and clear instructions should always be given, however, as to the size of the cut wanted and what it is to show.
In this process a polished copper plate is coated with a film consisting of beeswax, a whitening medium, and other ingredients, and the coating, which varies in thickness according to the nature of the copy, is sensitized as in the ordinary photographic processes. The map or other design to be engraved is first photographed to publication size and a contact print is made on the wax coating from the negative. The lines and other parts of the photographed image are then traced or cut through the wax to the copper plate with steel tools and straightened or perfected by the engraver, but the lettering is set in printer's type, which is pressed into the wax until it also touches the metal plate. After the work of cutting through the wax has been completed the larger open spaces between the lines are "built up" by the addition of wax to give greater depth to the plate,so that the wax plate thus built up corresponds to an electrotype mold. The plate is then dusted with powdered graphite and suspended in a solution containing copper, where by electrolytic action a copper shell is formed over its surface. When this shell is sufficiently thick it is removed from the solution and reinforced on the back with metal, and proofs are taken from it. If the proofs are satisfactory the plate is blocked type-high.
Wax-engraved plates may be used for printing colored maps or diagrams, in which variations of tint are produced by various kinds of machine rulings. The effect of some of the colors thus produced is almost a "flat" tint, in which a pattern can be detected only by close scrutiny. Some color work is printed from a wax base plate in combination with half-tone color plates.
The price of a wax engraving depends entirely on the size of the cut, the amount of work involved, and the character of the original copy, but it should not exceed very much the cost of a carefully prepared pen drawing plus the cost of a zinc etching made from it. Cuts engraved by the wax process, like zinc and half-tone plates, are delivered to the purchaser. If colored work is ordered, however, the printed sheets, not the cuts, are delivered.
WOOD ENGRAVING.
Wood engraving was once the universal method of producing cuts for illustrations that were designed to be printed on an ordinary press. It is said to be the oldest of all methods of engraving illustrations. The engraving is made on a block of boxwood, a very dense, hard wood of a light-yellow color. The block is cut type-high across the grain, and the engraving surface is made perfectly smooth by nibbing it with pumice or other stone. When a cut is to be larger than 3 or 4 inches square the wood block is made up of pieces securely dovetailed or joined together to prevent splitting and warping. A woodcut is not used for printing but is electrotyped and the electrotype is used in the press.
Originally the smoothed surface of the wood block was coated with prepared chalk or Chinese whits, and on this coating a finished drawing was made with a brush and pencil by an illustrator. According to more recent practice the surface of the wood is covered with a sensitized coating, on which the drawing or design to be engraved is photographed. The engraver then, with various kinds of gravers and other tools, cuts out the parts of the picture that are to be represented by white paper and leaves the lines, dots, and black areas as a printing surface, thus translating the shades and tints of the picture into a system of lines and dots which exactly duplicate, in effect, the details and tones of the original design. In order to produce aline effect of an area in which the tone is intermediate between whits and black the engraver must space his lines so that one-half the area will remain as printing surface and the other half as white spaces, and he must give character and direction to his lines, so that, if he is skillful, he can reproduce not only the delicate tones but the texture and details of the original picture. Many wood engravers became noted for their artistic rendering of magazine illustrations, of famous paintings, and of other works of art.
The Survey began to abandon this method of engraving in 1884, when the Sixth Annual Report was in press, substituting for it the cheaper photomechanical processes, zinc etching and half-tone engraving, and entirely abandoned its use in 1892.
Many good examples of wood engraving may be found in the early monographs and annual reports of the Geological Survey. Monograph 2 contains numerous examples.
PHOTOGELATIN PROCESSES.
Bichromatized gelatin is used in several photomechanical processes of reproducing illustrations, but in the photogelatin processes the gelatin not only receives the image by exposure to light through a negative but becomes a printing surface on a plate from which prints are made somewhat as in lithography. The several photogelatin processes are much the same as the original collotype process and are best known by the names collotype, heliotype, albertype, artotype, and the German name lichtdruck.
In working these processes a thick plate of glass, after certain preliminary treatment, is coated with sensitized gelatin. The plate is then placed in a drying room or oven having a temperature of 120° F., baked until it is thoroughly dry, and allowed to cool gradually. The subject to be reproduced is then photographed in the usual manner, and unless a prism or mirror box has been used the negative is stripped and reversed in order to make the print reproduce the original in proper position. From the negative a contact print is made on the gelatin-coated plate, the parts or molecules of gelatin being hardened in proportion to the amount of light that affects them. After the contact print has been made the gelatin plate is thoroughly washed in cold water, in order to dissolve and wash out the bichromate and stop any further action of light on the plate, and is then thoroughly dried. Before prints are made from the gelatin-coated plate water is flowed on it and penetrates different parts of the gelatin according to their hardness. The darkest parts of the picture will correspond to the hardest and densest parts of the gelatin, which will not absorb water; the lighter parts will take up more water. The surface water is then removed with a rubber straight edgeand an absorbent roller and the plate is ready for inking. The ink, being greasy, has no affinity for water, and when it is rolled over the plate it adheres only to the dry parts of the gelatin, and in the press is carried to the paper in all the lights and shades of the illustration. The plate is kept moist in printing.
The paper used for printing from photogelatin plates must be free from chemicals that will affect the gelatin. A nearly pure rag paper is generally used.
The photogelatin process is well adapted to the reproduction of paleontologic drawings, wash drawings, photographs, photomicrographs, works of art, old manuscripts—in fact, any kind of subject in which the reproduction of delicate lights and shades is essential. If properly manipulated it has distinct advantages over the half-tone process in that it can reproduce details and light and shade without showing the effect of a screen and without the use of coated paper. Excellent reproductions by the heliotype process are also made in color by first printing the design in a neutral tone and superposing appropriate transparent colors on this print, somewhat as in chromolithography, so that the colors softly blend with the shaded groundwork.
Reproductions made by the photogelatin process are more expensive than those made by the half-tone process, for the prints are generally made on better paper and are printed with greater care. They give no screen effect and are perhaps unrivaled by prints obtained by any other process except photogravure, in which the image is printed from a metal plate that has been sensitized, exposed under a reversed negative, and etched.
Changes can not be made on photogelatin plates except by making over the corrected parts. All retouching must be done on the originals or on the negatives made from then.
LITHOGRAPHY.
ORIGINAL PROCESS.
The general term "lithography" is sometimes used to indicate not only the original process so named, said to have been invented by Senefelder, but chromolithography, photolithography, and engraving on stone, as well as engraving on copper as a means of supplying matter to be transferred to and printed from a lithographic stone.
Senefelder discovered that limestone will absorb either grease or Water, and that neither one will penetrate a part of the surface previously affected by the other. He found that if a design is drawn on limestone with a greasy crayon and the stone afterward properly prepared with a solution of nitric acid and gum, greasy ink willadhere only to the parts that are covered with the crayon, and that the stone will give off an impression of the design.
Lithographic stone is described as a fine, compact, homogeneous limestone, which may be either a pure carbonate of lime or dolomitic—that is, it may contain magnesium. Although limestone is one of the most common rocks, limestone of a quality suitable for use in lithography is found at only a few localities.[11]There are two general classes of lithographic stone, known to the trade as "blue" or hard stone and "yellow" or soft stone. The blue stone is adapted for engraving and to the better grade of fine-line printing; the yellow stone is rated as somewhat inferior.
[11]Kubel, S. J., Lithographic stone: U. S. Geol. Survey Mineral Resources, 1900, pp. 869-873, 1901.
[11]Kubel, S. J., Lithographic stone: U. S. Geol. Survey Mineral Resources, 1900, pp. 869-873, 1901.
In the original process, which may here be termed plain lithography, two methods are employed in putting on stone the design to be reproduced. In one the subject or picture to be reproduced is drawn on the printing stone either with a lithographic crayon or with a pen dipped in lithographic ink or "tusche," which is oily or fatty, like the crayon. In the other method the drawing is made on transfer paper and transferred to the stone. In drawing on stone it is necessary to reverse the design, so that all lettering must be drawn backward. In doing this the artist often uses a mirror to aid him. If the drawing is made on transfer paper the design and the lettering are copied as in the original—not reversed.
Before a drawing is made on stone a stone of the quality suited to the particular design in hand is selected. The stone is then ground and polished, and if the drawing is to be made with crayon it is "grained" according to the special requirements of the subject. If the drawing is to be made with a pen and is to consist of "line work" the stone is polished. The first step is to obtain on the stone an outline or "faint" of the design. There are several ways to do this. By one method a tracing of the design is made, a sheet of thin paper covered with red chalk is laid face downward on the stone, the tracing is laid face downward over it, and the design is again traced in red-chalk lines on the stone. The method described is simple, but there are others that are more complicated and that are particularly applicable to the reproduction of photographs and other illustrations. Crayon work is often used in combination with pen and ink, stipple, and brush work. This method of drawing on stone is used also for preparing color stones in the process of chromolithography, in which there are many added details of manipulation. After the drawing has been made on the stone or transferred to it the stone is "gummed"—that is, it is covered with a solution of gum arabic and nitric acid—and dried. The stone is then dampened with water and carefully rolled with lithographic ink, which adheresto the pen or crayon work and is repelled elsewhere. It is then "rubbed" over with powdered rosin and talcum, which adheres to the ink and further protects the drawing from the effects of the etching fluid, which is next to be applied to the stone. This fluid consists of a 10 per cent solution of gum arabic to which 2 to 7 per cent of nitric acid has been added, the degree of acidity being varied according to the subject and the hardness of the stone. The fluid is applied with a brush or sponge and is left on the stone just long enough to decompose slightly the carbonate of lime on its surface and, after washing, to leave the design or drawing in very slight relief. The stone is again gummed and dried, and the design is "washed out" or brought out by removing the surface gum with a wet sponge and applying to the stone a rag sprinkled with turpentine and charged with printing ink. These operations wash away the tusche and the crayon that have been decomposed by the acid and expose the design faintly in white at first, but it gradually grows darker as it becomes charged with printing ink from the rag. The stone is next "rolled up" or inked. The slightly moistened surface repels the ink and the design takes it up, so that when the stone is run through the press the design is carried to the paper.
Lithographic prints from stones prepared in this way are made on a flat-bed press. The stone is carried forward to print and on its return is dampened and inked, an operation slower than that of rotary printing.
Corrections and changes are made on the stone by carefully scraping or polishing away the parts to be corrected and making the changes with a crayon or pen, but the design can not ordinarily be corrected twice in the same place, as the scraping or polishing removes a part of the surface of the stone and thus lessens the pressure at that place, and the impression there may be imperfect or may completely fail.
This form of lithography is seldom used for Survey illustrations but was formerly much used and is well adapted to the reproduction of drawings of fossils, particularly of remains of dinosaurs and other types of large extinct animals. Examples may be seen in Monographs 8 and 10 and in other early reports of the Geological Survey. The drawings for these illustrations were made directly on stone.
A drawing made on one stone may be transferred in duplicate or in any desired number to another stone, or to a properly grained sheet of zinc and aluminum, from which impressions may be printed on a lithographic press. Both these metals are also used for lithographic printing on rotary presses, the zinc or aluminum plate being bent and secured around a cylinder which rotates continuously in one direction. As one impression is made at each revolution of thecylinder the printing is rapid; but the best printing from a metal plate is inferior to the best printing from a lithographic stone.
PHOTOLITHOGRAPHY.
Photolithography, like other lithographic processes, has been improved greatly during the last few years—not particularly in results but in methods—by the introduction of metal plates, the rubber blanket offset, the Ben Day films, and many mechanical and chemical devices, so that a brief description of it will not explain the process except in a most general way. As photolithography is a direct process and is relatively cheap it is the one most used for reproducing large maps and other line drawings that have been carefully prepared. Zinc and aluminum plates are now much used in photolithography, for a direct contact photographic print can be made on them, they can be printed flat or bent for use on a rotary press, and they can be stored for future use more economically than stones.
There are two somewhat distinct methods of producing photolithographs. In both the ordinary photographic methods are used, but it is often necessary to "cut" or trace parts of the negative in order to open up lines and other features that are not sharp or well defined, so that the negative will print them sharp and clear. If the copy to be reproduced shows three colors, three negatives are made, one for each color, and the parts to be shown by each are preserved by "opaquing" or painting out all other parts. By the older method the negative thus perfected is placed in a printing frame in contact, under pressure, with sensitized transfer paper and is exposed to light. The printing frame is then carried to the dark room and the paper is removed from the frame and its surface covered with transfer ink. The paper is then laid face upward on water and soaked for several minutes, after which it is placed in the same position upon a slab of stone or metal and thoroughly washed with water. This washing removes the ink and the sensitive film from the parts that were unaffected by the action of light (the parts corresponding to the white paper in the design), but the ink still adheres to the lines of the design in the precise sharpness and clearness of the negative. The design is now ready to be transferred to the printing stone or zinc plate. The sheet is again slightly dampened between moist blotters and laid face downward in its correct position on a prepared stone or zinc plate, which is then pulled through a press under heavy pressure. The paper is then removed from the stone or plate, to which it has carried the design. From this point the gumming, etching, and other operations are practically the same as those used in ordinary lithography.
The bichromate-gelatin transfer process described above has been replaced in the Survey by a more satisfactory one, which insures absolute scale and reproduces the finest line drawings perfectly without thickening the lines or without distortion. In this process, which is known as the planographic process, a photographic negative of the "copy" is placed in a vacuum printing frame in contact with a zinc or aluminum plate that has been sensitized with a bichromate-albumen solution and exposed in front of an arc lamp. After proper exposure the plate is removed from the frame, inked over, and placed under water. The parts not hardened by the action of light (the unexposed parts) are then rubbed away with cotton, and the plate is chemically etched, gummed over, and dried. The plate is then ready to be printed from in a lithographic press. If a large map is to be reproduced it is photographed in parts, and contact prints are made on zinc plates. From these plates transfers are pulled and the parts are assembled and laid down in proper position on a stone or an aluminum plate, which is then prepared for printing.
A drawing that is to be reproduced by photolithography should be made on pure-white paper in lines, dots, or black masses with black waterproof ink. It should be one and one-half to two or three times the size of the finished print.
Photolithography is particularly adapted to the reproduction of maps, plans, and other large drawings. Within certain limitations, lines may be changed and details may be added after proofs have been submitted. The process is ordinarily used for reproducing illustrations in one color (black), but it is used also for printing in more than one color, generally over a black outline base, each color being printed from a separate stone, as in chromolithography.
OFFSET PRINTING.
In the offset process the design is "offset" from a lithographic plate or stone to a rubber blanket on a cylinder, from which it is printed. By thus obtaining an impression from an elastic surface the finest details can be printed on rough, uncoated paper, which can not be used in other processes, which can be folded without danger of breaking, and which is more durable than coated paper. PlatesII,III,IV,VII, andVIIIin this pamphlet were printed by this process.
CHROMOLITHOGRAPHY.
The chromolithographic process, by which illustrations are printed in color from stone, is used in Survey publications principally for reproducing geologic maps, but it is sometimes used for reproducing colored drawings of specimens.
There are several kinds of color printing from stones. One produces a picture by superimposing colors that combine and overlap without definite outlines and thus reproduce the softly blended colors of the original. Another reproduces the original by printing colors within definite outlines on a "base" which has been previously printed in black. The first kind is used by the Survey for reproducing colored drawings of specimens. The second is followed in reproducing geologic maps.
As each color must be printed from a separate stone and properly fitted with respect to the others a tracing from the original is made of the precise outlines of each color; or, if the design is to be reduced, a tracing is made over a properly reduced photographic print. This tracing can be made on specially prepared tracing paper or on a sheet of transparent gelatin or celluloid, which is laid over the copy and on which all the outlines and overlaps of the various colors are scratched with a steel point. The scratches thus made on the celluloid are filled with red chalk or like substance, and rubbed in with cotton, and by reversing the sheet and rubbing it the chalk lines are deposited on as many stones as are needed to produce the colors of the original design, each stone bearing all the outlines of the design. Sometimes all the outlines are engraved on what is called a key stone and an impression from it is laid down on each of the color stones. The parts on each stone that are to have one color are then inked in or engraved, and at the same time guide marks are indicated, so that in the composite print from the stones each color will fit its proper place. This fitting is called "register" and is an important part of printing, for each stone must be adjusted to a nicety while on the press in order to make each impression fit the others exactly. The process was originally manipulated entirely by hand, but photography has now replaced much of the handwork and has given rise to several methods by which the same kinds of subjects are reproduced in radically different ways. Tints are sometimes produced by the half-tone and other screens and by machine ruling, and printer's type is used almost exclusively for titles and other matter that was formerly engraved or drawn on the stone.
In reproducing a geologic map the base may be engraved on stone or on copper or it may be photo-lithographed. By either process the map may be transferred to the printing stone. The color stones for geologic maps are prepared by hand, but the geologic patterns, which are printed in colors, are engraved separately on plates, from which impressions are pulled when needed and transferred to their proper places on the printing stones in the shapes required according to the "key" design. The lighter, more transparent colors are generally printed first, and often twelve or more colors and many distinctive patterns are used to produce a geologic map. When proofsof such a map are pulled each stone must be taken up and carefully adjusted on the press, so that the work of proving maps that are printed from a considerable number of color stones is laborious and expensive. It is therefore customary to approve first combined proofs conditionally—that is, subject to the corrections and changes indicated on the proofs—and to hold the lithographer responsible for any failure to make the corrections.
This process is the most expensive one used for reproducing illustrations. Changes may be indicated on proofs, but changes can not be made on a stone twice in the same place without danger of affecting the printing or making it necessary to retransfer the parts affected. All changes are expensive because a slight modification at one point may involve corresponding changes on a number of stones, each of which must be taken up, corrected, and proved to insure the exact coincidence of the parts affected. It is often less expensive to retransfer the entire job than to make extensive changes on the original stones.
ENGRAVING ON STONE AND ON COPPER.
Engraving on stone is distinctly lithographic, but engraving on copper is sometimes included among lithographic processes because the work produced by it is usually printed from stone and thus becomes lithographic. In other respects engraving on copper is not a lithographic process. Roughly prepared maps and any rough line copy that is accurate in statement and clear as to intent are appropriate for both methods of engraving, but drawings that are expertly prepared are more suitable for reproduction by photolithography. In engraving on stone the lines of a design are scratched on the blackened surface of a stone with a steel-pointed tool; in engraving on copper the lines are cut with a graver on a sheet or plate of copper, the matter to be engraved being first shown on the plate by what is called the photo-tracing process, which was devised in the Geological Survey. There is, however, no great or essential difference in the printed results of the two processes, but most lithographers employ only stone engravers.
A stone on which a design is to be engraved is ground and polished according to the kind of work to be engraved, is coated with a thin solution of gum arable and allowed to dry, and is then washed until the superficial gum is removed while the surface pores remain filled. As the lines made by the engraver must be visible the stone is blackened with a pigment composed of lampblack and gum or is covered evenly with red chalk or Venetian red. It is then ready to receive the design to be engraved.
If the design is a map which is to show culture, streams, and surface contours, and each of these sets of features is to be printed in aseparate color, impressions of the work to be engraved must be placed on three stones. One method of doing this is to make a scratch tracing of the original drawing on a sheet of transparent gelatin or celluloid in the manner employed in chromolithography, except that a dry pigment, generally chrome-yellow, is used to fill the scratch lines instead of red chalk or Venetian red. From this tracing a "faint" or imprint of all the details of the three separate features of the map is made on each of the three stones, and the engraver then cuts on each stone only the lines and other features, including ample register marks, that are to be printed in one color, the imprint made from the tracing making it possible to engrave each set of features in its exact position relative to the other two. By another method the matter to be engraved is photographed directly on the stone.
The engraving is done with a steel needle inserted in a small wooden cylinder, an instrument resembling an ordinary lead pencil. The size and shape of the needles used are varied according to the requirements of the matter to be engraved. With this instrument the lines and lettering are lightly scratched into the stone through the dark coating and show as light lines. The points of some of the needles are fine; those of others are V-shaped; and some have spoon-shaped points, for use in thickening lines and shading letters. All features are engraved in reverse.
After the engraving is completed the stones are prepared for printing by wiping off all the superficial color and filling the engraved lines with a greasy ink—generally a thin printing ink—which is rubbed into the lines with a soft rag. Impressions are then pulled on transfer paper and transferred to three printing stones for use in printing the three colors, the register marks enabling the pressman to fit each color exactly in its proper place.
In all lithographic processes the titles and other marginal lettering can be and usually are transferred from type impressions to the printing stones. It is therefore unnecessary to letter such matter carefully on an original drawing that is made for lithographic reproduction, for appropriate faces of type will give better printed results than hand lettering.
Corrections can not be made on a stone or copper engraving as readily as on a drawing. If a stone engraver makes an error or if a change is required after his engraving is finished, the parts to be corrected must be scraped off and a new ground laid before the correction can be made. Sometimes he will engrave the parts corrected on another part of the original stone and transfer it to the printing stone. Corrections are made on copper plates by "hammering up" the plate from beneath, polishing off a new surface, and reengraving the part to be corrected.
APPENDIX.
The matter given in this appendix is much used in making geologic maps and other illustrations. The Greek alphabet and the groups of signs presented are given chiefly to show the correct formation of each letter and sign.
MISCELLANEOUS TABLES.
Length of 1° of longitude measured along given parallels from the Equator to the poles.[From U. S. Coast and Geodetic Survey Report for 1884, Appendix 6.]
Length of 1° of latitude measured along a meridian at given parallels.[Parallel given is in center of the degree whose length is stated.]
Metric system and equivalents.
[The units of linear measure most commonly used are millimeters (mm.), centimeters (cm.),decimeters (dm.), meters (m.), and kilometers (km.), 1 m. = 10 dm.; 1 dm. = 10 cm.;1 cm. = 10 mm.; 1 km. = 1,000 meters = 0.62137 mile; 1 m. = 39.37 inches = 3.280833 feet.]
The "vara," used in Texas, is equivalent to 331/3inches and is computed as representing 2.78 feet.
Geologic eras, periods, systems, epochs, and series.
Chemical elements and symbols.
Greek alphabet.
Roman numerals.
Mathematical signs.
NAMES OF ROCKS.
The following list was prepared in the geologic branch for the use of geologic draftsmen to enable them to select appropriate symbols for rocks that may be referred to in preliminary drawings by name only. For sedimentary rocks dots and circles, parallel lines, and broken or dotted lines are used; for metamorphic rocks short dashes arranged without definite patterns; and for igneous rocks patterns composed of short dashes, triangles, rhombs, crosses, and cross lines. All these patterns are shown inPlate III.
Sedimentary material.
[Including residual, detrital, eolian, glacial, organic, and chemically precipitated material.]
Metamorphic material.
Igneous material.
Igneous material.—Continued.
INDEX.
[A] [B] [C] [D] [E] [F] [G] [H] [I] [J] [L][M] [N] [O] [P] [R] [S] [T] [V] [W] [Z]