C—3a

56

Paper-machine. The experimental machine is practically a duplicate of the commercial Fourdrinier machine, consisting of two presses, nine driers, and a stack of calenders. The sheet obtainable is of 12 in. trim. The paper is wound on a reel, and can be obtained calendered or uncalendered. The paper taken from the reel is cut into sizes suitable for both records and paper tests. (See Plate 14.) Rod Mill. The experimental rod mill is 3 ft, in diameter, 5 ft. long (inside measure), arid rubber-lined. The charge of steel rods consists of fifty-two rods varying from 1 i in. to 2| in. in diameter, 4 ft. 10 in. long, and weighs 3,720 lb. The beating action results from tumbling of the rods by the rotation of the mill. The mill is driven by als horse-power motor at a speed of 28 r.p.m. The mill in these experiments was operated in batch runs. Commercially, however, the mill is operated continually. (See Plate 13.) Yield Determinations. In the case in which the pulp is screened and run over the wet machine, the screenings are collected, dried in the oven, and weighed. The screened pulp is thickened and run into a sheet on the wet, machine, the total quantity being weighed. Samples of the sheet are obtained throughout the wet-machine run and these combined (average) samples oven-dried and weighed. The yield may be calculated from these data, •In the case of semi-chemical or very raw pulps the pulp is washed thoroughly and drained, then pressed in the hydraulic press. It is opened up in a shredder, weighed, and sampled for moisture. The oven-dry weight and yield can be calculated from moisture-determination figures. APPENDIX IV.- COLOUR-ANALYSIS BY MEANS OF THE IVES TINT-PHOTOMETER. General. The Ives tint-photometer is used for a numerical evaluation of the colour of pulps and papers. This instrument, and its use for this purpose, has been described in detail in the technical papers of the Technical Association of the Pulp and Paper Industry, page 123, Series X, 1927. It measures the amount of light of certain wave-lengths as reflected from the sample by comparison with a standard white surface. Readings are taken only in the red, green, and purple-blue portions of the spectrum, as these three colours are sufficient to characterize the colour of the sample. The results are definite and reproducible, and may be interpreted in terms of ordinary visual impressions. Apparatus. The Ives photometer consists essentially of a source of white light, a standard white surface, a clamp for holding the sample for analysis, an optical system for comparing the intensity of light diffusely reflected from standard and sample, and three colour-filters. The light-source is a standard daylight lamp. The standard white is a block of magnesia. The sample, which must be of the same size and shape as the standard surface, is held in a special clamp which ensures a smooth, level surface, and in the same plane as the standard. It must be of sufficient thickness to prevent any transmission of light from the sample-holder through the sample. The colour-filters, which have been carefully standardized, transmit pure light of the following wave-lengths (in millimicrons) : 638 (red), 535 (green), and 475 (purple-blue). light from the two surfaces enters the instrument through slits fitted with sliding shutters. The shutter admitting reflected light from the standard is operated by a lever, which works over a scale -graduated in 100 units ; the other is operated by a micrometer screw which is set permanently. The field viewed in the eyepiece is divided, half being illuminated by light from the standard, the other half by light from the sample. Operation. The instrument is set in adjustment by matching two standard surfaces and regulating the setscrew of the object-slit so that the match is perfect when the standard slit is open to JOO on the scale, A sample and the standard are then placed in their proper positions, and the lever moved until the intensity of illumination is the same for both halves of the field. The scale-reading then gives the parts of light reflected by the sample in terms of the standard white. Readings arc made with each of the three colour-filters. Three or more readings are made with each filter, depending upon the degree of accuracy desired.- The setting is a matter of matching intensity only, not tint, and the method is purely objective, thus avoiding to a considerable extent errors due to individual differences in colour sense. Interpretation of Results. Conversion of the results of an analysis into terms of visual impressions will be briefly explained and illustrated. It is based upon the theory of colour and the special construction of the instrument. The three filters transmit light of the so-called primary additive colours. When light of these three colours is mixed in proper proportions, white light results. Since the readings are referred to a white surface as a standard, equal parts of red (Rj, green (G), and purple-blue (B) as measured would give white light. Furthermore, equal parts of two primary colours give secondary colours, and these may be still further combined, if necessary. Thus red and purple-blue give magenta ; green and purple-blue, peacock-blue ; red and green, yellow ; yellow and red, orange. These are all the combinations