The Calotype cannot be compared with the Collodion process for sensitiveness and delicacy of detail, but it possesses advantages for tourists and those who do not wish to be encumbered with large glass plates. The principal difficulty appears to be in obtaining a uniformly good paper, many samples giving a speckled appearance in the black parts of the Negative.
The Waxed Paper process of Le Grey.—This is a useful modification of the Talbotype introduced by M. Le Grey. The paper is waxed before iodizing, by which, without involving any additional operation, a very fine surface layer of Iodide of Silver can be obtained. The Waxed Paper Process is well adapted for tourists, from its extreme simplicity and the length of time which the film may be kept in a sensitive condition.
Both English and foreign papers are employed: but the former take the wax with difficulty. Mr. Crookes, who has devoted his attention to this process, gives clear directions for waxing paper; it is essential that pure white wax should be obtained direct from the bleachers, since the flat cakes sold in the shops are commonly adulterated. The temperature must also be carefully kept below that point at which decomposition of the wax takes place; the use of too hot an iron being a common source of failure (see 'Photographic Journal,' vol. ii. p. 231).
The sheets of paper, having been properly waxed, are soaked for two hours in a solution containing Iodide and Bromide of Potassium, with, enough free Iodine to tinge the liquid of a port-wine colour. The greasy nature of wax impedes the entry of liquids, and hence a long immersion is required. The iodizing formulæ of the French Photographers have been encumbered by the addition of a variety of substances which appear to introduce complications without giving proportional advantage, and Mr. Townshend has done the art a service by proving that the Iodide and Bromide of Potassium, with free Iodine, are sufficient. This latter ingredient was first used by Mr. Crookes; it seems to add to the clearness and sharpness of the Negatives; and as the papers are coloured by the Iodine, air-bubbles cannot escape detection. The process of exciting with Nitrate of Silver is also rendered more certain by the employment of free Iodine, the action of the Bath being continued until the purple colour gives place to the characteristic yellow tint of the Iodide of Silver.
Waxed Paper is rendered sensitive by immersion in a Bath of Nitrate of Silver containing Acetic Acid; the quantity of which latter ingredient should be increased when the papers are to be long kept. As the excess of Nitrate is subsequently removed, the solution may be used weaker than in the Calotype or Collodion process.
After exciting, the papers are washed with water to reduce the amount of free Nitrate of Silver to a minimum. This lessens the sensitiveness, but greatly increases the keeping qualities, and the paper will often remain good for ten days or longer.
It is a very important point, in operating with Waxed Paper, to keep the developing dishes clean. The development is conducted by immersion in a Bath of Gallic Acid containing Acetic Acid and Nitrate of Silver; and being retarded by the superficial coating of wax, there is always a tendency to an irregular reduction of Silver upon the white portions of the Negative. When the developer becomes brown and discoloured, this is almost sure to happen; and it is well known to chemists that the length of time during which Gallic Acid and Nitrate of Silver may remain mixed without decomposing, is much lessened by using vessels which are dirty from having been before employed for a similar purpose. The black deposit of Silver exercises a catalytic (καταλυσις, decomposition by contact) action upon the freshly-mixed portion, and hastens its discoloration.
The Waxed Paper process is exceedingly simple and inexpensive,—very suitable for tourists, as requiring but little experience, and a minimum of apparatus. It is however slow and tedious in all its stages, the sensitive papers frequently taking an exposure of twenty minutes in the Camera, and the development extending over an hour or an hour and a half. Several Negatives however may be developed at the same time; and as the removal of the free Nitrate of Silver gives the process a great advantage during hot weather, it will in all probability continue to be extensively followed. The prints which have been sent to the Exhibition of the Photographic Society, show that waxed paper in the hands of a skilful operator may be made to delineate architectural subjects with great fidelity, and also to give the details of foliage and landscape Photography with distinctness.
The Albumen process upon Glass.—The process with Albumen originated in a desire to obtain a more even surface layer of Iodide of Silver than the coarse structure of the tissue of paper will allow. It is conducted with simple Albumen, or "white of eggs," diluted with a convenient quantity of water. In this glutinous liquid Iodide of Potassium is dissolved; and the solution, having been thoroughly shaken, is set aside, the upper portion being drawn off for use, in the same manner as in the preparation of Albuminized paper for printing.
The glasses are coated with the Iodized Albumen, and are then placed horizontally in a box to dry. This part of the process is considered the most troublesome, the moist Albumen easily attracting particles of dust, and being apt to blister and separate from the glass. If an even layer of the dried and Iodized material can be obtained, the chief difficulty of the process has been overcome.