seems to lie in the hands of the printers as much as in that of the ink manufacturer or paper-maker. Imitation art paper is largely used for catalogues and cheaper printings where clearness of detail is not so essential as in the best art papers. It is a very efficient substitute in many respects, but it lacks strength and durability. It is usually made by adding large quantities of china-clay to a mixture of wood pulp and esparto, and producing a heavy loaded paper in which only a small proportion of rosin size is used. The paper is made on the machine in the ordinary way, and treated as it passes on to the calenders of the machine by means of the water doctor. This is a device which causes the surface of the paper to come into contact with water just before it is drawn through the rolls of the calender. The somewhat bulky sheet wetted superficially in this manner is suddenly compressed by the pressure of the rolls and flattened down so that the surface has the appearance of a coating. The proportion of clay in an imitation art paper is often as great as 35 per cent., and this, coupled with the "soft sizing," is the cause of the somewhat limp feel and handle of this class of paper. It is, however, produced cheaply and answers its purpose. (Fig. 81.) Duplex coated papers may be produced by colouring a suitable coating-mixture with an aniline dye for one side, and colouring a second coating-mixture for the other side of the paper. The Printing Process. The production of a half-tone block from any photograph or picture is an interesting process, which may be described very briefly here. The picture is mounted on a board in front of a camera and brilliantly illuminated by means of two powerful arc lights placed on either side, and a photograph of the picture taken in the ordinary way except that a screen grating is put in front of the sensitive plate. The screen grating is an important feature of the process, and is prepared in the following manner: A sheet of plate glass of good quality is coated with an asphalt varnish. Lines are ruled diagonally at forty-five degrees across the varnished surface by an automatic ruling-machine, which is capable of ruling parallel lines up to any degree of fineness. The number of lines per inch depends on the work for which the screen is to be used. For coarse, rapid work such as newspaper printing the lines are 50 to an inch; for book illustrations, the lines are 170-200 to an inch. The lines are etched on the glass by means of hydrofluoric acid, which eats away the glass along the lines ruled by the machine. A white enamel is rubbed into the etched lines, baked in by heat, and the glass carefully polished. Two such glasses placed together with the lines crossing at right angles are cemented with Canada Balsam to form a screen grating. The effect produced by the interposition of such screen grating is to give a picture formed entirely of a very large number of small dots, shadows being represented by the closeness of the dots, and the lighter tones by dots of various sizes. The gradation of the light and shade depends greatly on the experience and skill of the operator in adjusting the screen at a correct distance. The screen negative is developed in the usual way and used for the production of a print on a copper plate. The plate is carefully washed and coated with a sensitive solution, made by mixing fish glue, albumen, bichromate of ammonia, and dilute chromic acid in certain proportions. The plate, brought into contact with the screen negative, is exposed to an arc light, for a period of six to eight minutes, and then thoroughly washed in order to remove all the portions of the coating not rendered insoluble by the light. The washed plate is then immersed in a bath of some aniline dye, usually methyl violet, which stains the picture and renders all the detail clear and distinct. When dry the plate is enamelled by the heat of a Bunsen flame, the colour of the picture changing from blue to grey, and finally to a deep brown tone. When this point is reached the plate is cooled down, and the coating resembles a hard enamel. The "etching" process requires a good deal of skill and technical experience, as well as a knowledge of artistic effect. The process is a simple one, consisting in the immersion of the plate, the back of which has been previously coated with black varrish, in a bath of weak nitric acid; the dots being protected by the enamel remain intact, while the spaces between are eaten out by the acid. From this plate a rough proof is taken, which affords the operator the opportunity of determining how much further the etching process must be taken, and the extent to which it may be necessary to etch some portion of the plate deeper than others in order to give good results in the final picture. The finished block is mounted on a slip of wood, and in that condition is ready for the printer. ક CHAPTER IX THE PHYSICAL QUALITIES OF PAPER Weight and substance-Thickness-Bulk-Strength-Elasticity-Resistance to crumpling and folding Sizing qualities-Absorptive capacity-Imperviousness-TransparencyColour and finish-Surface and other qualities. Weight and Substance. These qualities of paper are usually expressed in terms of the weight of a ream of 480, 500, or 516 sheets, as the case may be. For high-class papers the ream usually contains 480 sheets, while with common printings 500 or 516 sheets go to the ream. The methods for determining the weight of the ream are very simple. TJ.M & CO 454 (1) The sheet of paper is cut to a given size, and weighed on some form of special paper-scales which indicate without further calculation the weight of a ream, such as Leunig's scales. (Fig. 82.) Example.-A sheet measuring 20" by 30" placed in the pan of the Leunig's scales indicates the weight of a ream as follows: or 120 ozs., that is 7 lbs. (3) If the piece of paper is not full size, then it is necessary to cut a piece to some convenient size, and calculate the weight of a full-sized sheet and subsequently the weight of the ream. A specially sensitive form of paper-scales has been constructed by Leunig, which indicates the weight of a ream of demy (171" by 221") by means of a piece of the paper cut to a standard size of 4 inches square. Example.-A sheet measuring 4" by 4" when placed on Leunig's demy scales may indicate a weight per ream of 56 lbs. (4) A slip of paper cut to any convenient size can be weighed on a fairly sensitive balance, and the weight of the ream calculated. Example.-An odd piece of paper cut to the size 5 inches by 10, weighs 70 grains. From this it follows that a sheet measuring 20 inches by 30 would weigh 840 grains, and a ream 480 times the weight of a single full-sized sheet, that is 57 lbs. (5) In some cases the piece of paper available is exceedingly small, but the method of determining the substance is exactly the same, the only precaution necessary being the employment of a very sen sitive balance. If the weighing of a small piece is not carried out carefully a serious error will be introduced. Example.-A piece of paper cut to a size measuring 1 inch square weighs grain. Hence, weight of a ream of 480 sheets double crown (20 inches by 30), is: 1 × 20 × 30 × 480 grains or 144,000 grains, that is 20-6 lbs. Since 7000 grains equal 1 lb. the weight of the ream is 20.6 lbs., that is to say, just over 20 lbs. The necessity for using a sensitive balance is obvious from the fact that the weight of the small piece is multiplied by 288,000. A special form of Leunig's scale sensitive to pieces 2 inches by 1 may also be used for this purpose. (Fig. 83.) Fig. 83. Sensitive Paper-scales for small Samples. Equivalent Weights. The equivalent weights of paper are readily determined by the simple rules of proportion, although it is more customary to ascertain the weight of paper in any standard size for some other standard size by reference to published tables. For unusual dimensions the equivalent weight must be calculated. Example.-A sheet 17" by 25" weighs 20 lbs. per ream (480 sheets). What is the equivalent weight for a sheet 51" by 20"? Since 17 x 25 x 480 square inches Then 51 x 20 x 480 square inches or equivalent weight The "Rule of Three" can be applied to all questions relating to the weight of paper. Equivalent weights are in direct proportion to the areas of the single sheets, and this fact can be expressed in the shape of a formula capable of general application. Let W represent the weight of a ream of paper. A w a Then the relation is area of a sheet in the ream. weight of another ream of paper. area of a sheet in the second ream. W A In its extended form A a = (L x B) and a = (b) where L, B, and 7, b, are the lengths and breadths of the respective sheets. Various problems can be solved by means of this simple formula, of which several examples are appended. Example. What is the equivalent weight of a ream 34" by 45" weighing 40 lbs. for a paper 68′′ by 54"′′. In this case W Example. A ream of paper 36" by 29" weighing 14 lbs., being damaged, is cut down to 36" by 27." What is the weight afterwards? A ream of sheets 36" by 29" weighs 14 lbs. A ream of sheets 36" by 27" weighs = 56 lbs. Example. A reel of paper is 80 inches wide, the paper having a substance of 20" by 30" The paper is re-reeled and slit into two smaller reels 50 inches and 30 inches respectively. What are the relative weights of the reams in the reels? = = On the full-sized reel, before slitting, a complete sheet measures 80′′ × 30′′. w = a = w 50 × 30. Hence w = 140 lbs. (for a ream of 50′′ × 30′′). On the second reel, the sheet measures 30" × 30." w 30 × 30. W A 56 20 × 30 = 84 lbs. (for a ream of 30" x 30"). 56 lbs. In all the reels the paper has the substance 20" x 30" Example. A ream of paper 194 inches by 26 weighs 44 lbs. equivalent weight of another ream is 35 lbs., the sheet the width of the sheet? Thickness and Bulk.-(1) The bulk of a paper may be expressed in terms of the thickness of a single sheet or the thickness of a ream. If the ream of paper is available its thickness may be determined at once by direct measurement with a rule, or by means of ordinary callipers. (2) The thickness of a single sheet of paper may be measured by means of a micrometer gauge, the construction of which may be seen from Fig. 84. With the cheaper forms of this instrument it is difficult to obtain very accurate or uniform readings on account of the absence of any method for regulating the pressure exerted on the sheet of paper. This error can be avoided by the use of a gauge fitted with a small friction coupling which slides round on the micrometer screw directly the pressure reaches a fixed maximum, so that any further movement of the screw is prevented. |