The presence of ground wood is easily determined by a drop of either strong nitric acid, which turns the paper brown, or a drop of phloroglucine, which gives a reddish-brown tint from contact with ground wood. Aniline sulphate produces a yellow tinge. The presence of starch may be ascertained by using a dilute solution of Iodine which leaves a black stain in contact with starch. Note.--For more technical information see “Paper Technology” by R. W. Sindall. CHAPTER TEN PRESSROOM DIFFICULTIES Technical difficulties with paper in the pressroom arise from many sources. They may be conveniently classified into three groups: Difficulties for which the manufacturer is responsible; difficulties for which the printer is responsible, and difficulties due to atmospheric and other natural conditions not entirely within human control. Let us consider some of the first group. UNIFORMITY.--Probably the most frequent source of trouble is lack of uniformity, either in weight, thickness or finish. This is chargeable to carelessness on the part of the paper-machine tender. A run of paper which varies in weight will naturally vary in thickness, and, obviously, this could account for uneven color in presswork. These variations would not necessarily be accompanied by a variation in finish. To make paper uniform in all three respects necessitates, firstly, uniform consistency of the pulp--or “stuff,” as it is technically called--at the point where it flows onto the machine. A uniform volume of stuff and uniform speed of the machine are also demanded. The speed of the machine and the volume of stuff are quite readily controlled, but as the amount of water used by the beater-man in preparing the stuff is usually judged by the appearance of the pulp in the beater, there are always such variations as are peculiar to this human factor. The difficulties of the machine-tender may often be traced to the beater-man, not only on account of the amount of water in the mixture, but also because of the irregularity in the length of fiber from one beaterful to another. Assuming that the stuff is right and the formation on the machine is good, the pressing of the paper next demands close attention. It is obvious that any unevenness of pressure will result in the water being expelled unevenly from the web of paper, with a consequent variation in thickness. In this case there would also be a damp streak in that part of the web where the pressing was too light. The result is that such paper can not be dried evenly all the way across the machine because this damp streak will still have an excess of moisture after the adjacent areas of the web have become properly dried.