CHAPTER IV.

HOW TO KEEP THE SENSITIZING BATH IN ORDER.

Experience tells us, however strong we may make the bath solution to coagulate the albumen on the paper, that a certain amount of organic matter will always be carried into it. At first this is not apparent, since it remains colourless in the solution; but after a time, after floating a few sheets of paper, the organic silver compound gradually decomposes, and the solution becomes of a brown or red tint, and if paper were floated on it in this condition there would be a dark surface and uneven sensitizing. It is, therefore, necessary to indicate the various means that may be employed to get rid of this impurity. The earliest, if not one of the best, is by the addition of white China clay, which is known in commerce as kaolin. A teaspoonful is placed in the bottle containing the solution, and well shaken up; the organic matter adheres to it, and precipitates to the bottom, and the liquid can be filtered through filter-paper or washed cotton-wool, when it will be found decolourized. Another mode of getting the liquid out of the bottle is to syphon it off by any syphon arrangement, and this prevents a waste in the solution from the absorption of the filtering medium. The accompanying arrangement (fig. 4) will be found useful for the purpose, and can be applied to other solutions where decantation is necessary. A is a wide-mouthed bottle holding the solution. B is a cork fitting the mouth, in which two holes have been bored to fit the two tubes, D and C, which are bent to the form shown. When the kaolin has subsided to the bottom, air is forced by the mouth into the bottle through C, the liquid rises over the bend of the tube D, and syphons off to the level of the bottom of the tube inserted into the liquid, provided the end of D, outside the bottle, comes below it.

To bend a tube, a common gas flame is superior to a Bunsen burner. The tube is placed in the bright part of the flame in the position shown; by this means a good length of it gets heated, and a gentle bend is made without choking the bore, which would be the case were a point of a flame used.

Another method of purifying the solution is by adding a few drops of hydrochloric (muriatic) acid to it. Chloride of silver is formed, and when well shaken up, carries down with it most of the organic matter, but leaves the bath acid from the formation of nitric acid.[13] This must be neutralized unless a little silver carbonate is left at the bottom of the bottle as described at page 20. A camphor solution may also be added for the same purpose. Make a saturated solution of camphor in spirits of wine, and add a couple of drachms to the solution, and shake well up. The camphor will collect the albumen, and it can be filtered out. In case the first dose does not decolourize it, another one must be added.

Another plan is to add potassium permanganate (permanganate of potash) to it, till such time as the solution takes a faint permanent rose tint. The theory is that the organic matter is oxidized by the oxygen liberated from the permanganate, and falls to the bottom. It is not strictly true, however, and the solution will never be as free from organic matter as when the other methods are employed.

The final and best method is to add a small quantity of sodium carbonate (say 5 grains), and expose it to daylight. When the organic matter becomes oxidized at the expense of the silver nitrate, the metallic silver with the oxidized organic matter will fall to the bottom. This plan answers admirably when time is no object, but in dull weather the action is slow. When once the precipitation fairly commences it goes on quickly, and if a little freshly precipitated metallic silver be left at bottom of the bottle the action is much more rapid. This is a wrinkle worth remembering in all photographic operations where precipitation is resorted to.

We have hitherto supposed that the only contamination of the bath is organic matter, but it must be borne in mind that each sheet of paper floated on the solution transfers a certain amount of nitrate of the alkali[14] with which the albumen is salted.

It will thus be seen that in an old bath there will be no need to add the soluble nitrates given in page 17, since they will be already formed. When they are in excess the best plan is to precipitate the silver by some means,[15] but we select one which is easy of application, since it requires no watching. Evaporate the solution to half its bulk, and slightly acidify it with nitric acid (10 drops to the pint of solution will suffice); throw some ordinary granulated zinc into the jar or bottle containing it; the silver will now be rapidly thrown down in the metallic state, and in the course of two or three hours the action will be complete. Next carefully pour off all the fluid as close as possible to the residue. Pick out all the lumps of zinc, and add a little dilute hydrochloric acid to dissolve up all the small particles of zinc which may be amongst the precipitated silver. Filter the solution away, and wash the residue once or twice with water. Take out the filter paper, and dry it before a fire, or in an oven, and then detach the silver, and transfer it to a small crucible, which place, with its contents, over a Bunsen burner or spirit lamp flame till it is red hot. The heat will destroy all organic matter, leaving a residue of carbonous matter behind, which, after subsequent operations, will be eliminated by filtration. Next cover the silver with nitric acid,[16] and in an evaporating dish slightly warm it over a spirit lamp or Bunsen burner. Red fumes will appear, and when all action has ceased, more acid must be added till such a time that very nearly (but not quite) all the silver is dissolved up. Then evaporate off all the fluid and allow it to cool, when water can be added to such an extent that it is _over strength_ for the bath. Now measure the whole bulk of the solution in a glass measure, and test by the argentometer for strength. An argentometer is, in reality, an instrument for taking the specific gravity of a liquid. It is as shown in the figure. A B is a glass tube, inside of which is a graduated scale showing grains; C is a hollow glass cylinder, which has a little glass ball filled with mercury. When immersed in water, the instrument sinks till the scale reads 0--that is, A B is deeply immersed. When any soluble salt is dissolved in the water, the stem rises further. If the soluble salt be silver nitrate, the scale is made to read grains per ounce. It is then evident, if the bath contains any other soluble salt besides the nitrate of silver, the readings will be untrustworthy. Supposing you have a total quantity of 10-1/4 ounces of solution, and the argentometer tells you it is of a strength of 105 grains to the ounce, you must make a small calculation to see how much water you must add. In 10-1/4 ounces of solution there will be 10-1/4 × 105 or 1076-1/4 grains of silver nitrate. If you want to make the bath 40 grains to the ounce, you must divide this quantity by 40, which is very nearly 27. The original amount of fluid (10-1/4 ounces), when deducted from this number of (27) ounces, will give you the amount (16-3/4 ounces) of water that is to be added to give you a bath of the required strength. When the water is added, the solution should be filtered from the carbonaceous matter, and the bath, after neutralizing with sodium carbonate, will be ready for use.