Just before using, sufficient quantity of one of the mixtures given is prepared in a measur ing glass for the size of the sheet of paper to be coated. For a sheet of paper 24 X i8, about two ounces of the solution will be required. It is applied to the surface of the paper by means of a pad of cot ton wool encloSed in a piece of soft flannel. The paper must be kept flat upon a level surface. A simple method is to streteh a long piece across a table, the ends overlapping, and having attached to them American clips tied to a kitchen poker or other suitable weight, as shown in Fig. 000. This will keep it taut, even as it expands with the application of the wet solution. The sensitizing solution is poured poured on and immediately spread over with the dabber, until the coating is as even as possible. Drying the care must be exercised in this, as much of the subsequent success is dependent upon the operation.
Directly the sheet is coated it is hung up by its corners, until all the moisture on its sur face has entirely disappeared. It is then immediately dried before a fire or stove. When perfectly dry the lemon color of its surface will have changed to an orange-yellow. It should be dried perfectly without scorching, as this would produce fogged prints. It is of the utmost importance that not less than five, nor more than ten minutes should elapse between the coating and the drying operations. If it becomes dried too soon the image will probably wash away in the developer, and if not dried quickly enough the picture will be flat and sunken in. In very warm weather the surface moisture will some times disappear in less than five minutes. In this case the walls and floor must be sprinkled with water or the paper placed in a dampened cupboard.
Keeping the thoroughly dry the sheets of sensitive paper, and also the prints, are preserved in chloride of calcium boxes. Figs. 351 and 352 will serve to show the construction.
A is the compartment containing the paper. The cover B is divided into two parts, the lower part F which is placed on the the top of the box consists of a cylinder G, perforated with holes, which slides into the box A, and serves to hold. some lumps of dry calcium chloride wrapped up in calico or double fold of muslin. The upper part C serves to close up the top of the cylinder G. An elastic band EE slips over the slots, and keeps them perfectly air tight. The calcium must be examined from time to time, if moist it must be changed for a fresh supply.* To secure the most brilliant results the sensitized paper before, during, and after exposure, must be kept dry as possible. The effect of damp is a want of vigor, muddy tones, and impaired purity of the whites.
The following instructions for platinotype printing are given by Platino type Co.:— the paper in the printing frame beneath the negative, and between it and the pad insert a sheet of thin vulcanized indiarubber, as it is of the first importance that the pads in contact with the paper be quite dry. The correct exposure (about one-third of that required with silver printing) is ascertained by inspection of the paper in a rather weak white light in the usual manner. A little experience will enable the exposure to be determined very ac curately. The sensitized surface before exposure to light is of a lemon-yellow color. During exposure the parts affected by light become of a pale grayish-brown color, and sometimes of an orange tint under those parts of the negative which present clear glass or nearly so. As a general rule all parts of the picture except the highest lights should be visible when the exposure is complete. When examining the prints in the printing-frame care should be taken not to expose them unduly to light ; for the degra dation of the whites of the paper due to slight action of light is not visible until after development. Damp paper gives a less visible image than dry paper, hence it may easily be over-exposed. Remove prints to a cal cium tube as soon as exposure is complete, unless they are to be at once developed.
should be conducted in a feeble white light, similar to that used when cutting up the paper, or by gas light. It may take place immediately after the print is exposed, or at the end of the day's printing.
The developer is made by dissolving i lb. of oxalate of potash in 54 ounces of water. Used hot water for making the solution, of which a large quantity may be made up ; it will keep indefinitely. It is well to have at hand some unused solution, since, in the event of inferior prints being made, a new bath may at once be tried.
The solution is conveniently contained in a flat-bottomed dish of enameled iron, heated by a small spirit lamp or Bunsen burner for the smallest dish, or for the larger dishes a paraffin stove. Troughs for large prints are fitted with a tube gas-burner.
The developing tray recommended by Pizzighelli and Hiibl is shown in fig. 353. A is an enameled iron vessel containing the oxalate solution : a its cover of zinc plate ; B is another hol low vessel, with a double wall of zinc plate, which acts as a water bath ; C is a gas or spirit lamp. The vessel B is filled with hot water through a little hole let into the upper side, and hot solution of oxalate is then poured into the tray A ; it can readily be kept at the required temperature by means of the lamp underneath.