BACKINGS, PLATE A backing is a coating upon the plain (glass) side of a dry plate in order to prevent reflections progress of development and the density of the image. One of the commercial backings dis solves hi the developer without affecting the working. Perhaps the best of the home-made backings is : Crystal caramel powder . . i oz.
Water . . . . . 7 „ Methylated spirit . „ The ingredients need to be mixed well together. Another formula is :— Caramel . . . . . i oz.
Powdered burnt sienna . •I 1) Office gum . . . . I „ For isochromatic plates lampblack should be used instead of the sienna. J. S. Teape recommends the following :— Caramel . . . oz.
Saturated solution of gum traga canth . . . . . I oz.
Powdered burnt sienna . . 2 „ Methylated spirit . . . 2 „ The spirit is added after the ingredients have been well mixed. The above are liquid back ings, which are in optical contact with the glass.
Instead of caramel, it is possible to use various dyes and pigments, such as sienna, lampblack, etc. The following are typical formula I. Powdered burnt sienna. . oz.
Powdered gum arabic . . „ Glycerine . . . . I „ Water - . 5 ,, 2. Essence of cloves . . 6 parts Turpentine . . . . 7 „ Lampblack . . . . q.s.
to form a paste that may easily be distributed on the back of the dry plate.
3. Methylated spirit . . . I° oz.
Soap . . . . . zoo grs.
Erythrosin . . . . I drm.
• Scrape the soap, and allow it to digest in the spirit for a week, shaking at intervals ; filter, and add the dyes. This gives a good yellow backing solution, which dries at once.
Paper Backings.—Backing sheets or papers are not so effective as liquid backings. Red or black paper is cut to size, one side smeared with glycer ine, and pressed into close contact with the glass side of the plate. Special backing pads, which Dissolve by heating, and apply when warm to pieces of stout paper or calico, which require to be squeegeed on to the backs of the plates, and may be used over and over again if smeared occasionally with glycerine.
Comparisons.—The following table shows the effectiveness of the various backings, the test subject being severe :— Halation. Unbacked plate . . . Very bad. Plate backed with— Caramel and water . . Very slight.
Bitumen . . . Very slight.
Sienna and water . . Very slight.
Black paper and water . Bad.
Black paper and glycerine . Not so bad. Shoe blacking . . . Slight.
Red and black currant jam Slight. Canada balsam and lampblack Nil.
Caramel and sienna . . Nil.
Films and lantern-slide plates are sometimes backed, but they do not show halation so badly as dry plates, and therefore the simplest backing will serve, if needed at all. (See also " Hala tion.") Applying Backings.—The work of backing plates must be done in the dark-room, and varies somewhat with the nature of the material used. It is not absolutely necessary to wait until the backing is dry, although it is desirable to do so ; but if a plate is used with the hacking still wet it should be covered with paper, preferably the oiled kind in which sensitive paper is sometimes wrapped. The backing must be applied evenly
are preferable to the above, may be made by coating strong paper with the following mix ture : Gelatine • . . . . I oz.
Water . .
Glycerine . • . . I „ Indian ink, sufficient to colour.
and not in streaks, and should cover every part of the plain glass side of the plate. The simplest method for small plates is carefully to put two plates film to film, and to clip them together at both ends as at A. The backing may then be applied by means of cotton wool or soft flannel (brushing causes streaks), and the plates then.
hung up or laid aside to dry. In this way, and with ordinary care, the backing mixture is kept from the films.
For very messy backings the worker may prefer to use a holder as at B. In the centre of a piece of flat wood D, stout enough to prevent warping, is glued a piece of black velvet or other soft-surfaced material of the size of the dry plate. A piece of millboard or of thin wood E, of the same thickness as the dry plate and the same size as to, to which it is hinged, has at its centre a space V' of the size of the plate, so that when the flap z is placed over the base D the velvet is seen through the hole. The plate is placed film side downwards upon the velvet, and E is brought into position over D, the back of the plate coming through F. The backing is spread over the plate, any superfluity going on to the card. The flap is then lifted up, and the plate removed.
When using slow-drying backings, a dark box is convenient, and this can be made with a strong cardboard box i in. deeper than the longest side of the plates to be coated ; the corners and joints should be covered inside with black paper so as to make the box light-tight. Place corrugated paper G or strips of cardboard along the bottom, as in illustration C, to prevent the plates from slipping. The first plate leans against the end of the box and supports the next one, a piece of red or black paper being interposed. The lid is made light-tight in the following manner : Under the edge of the lid H glue a strip j, and on j glue another strip K, covering over the edge of the lid of the box by an inch or more. When the lid is put on, the box will be quite light-tight, because of the light trap formed by j and K.
In process work, it is found that backing the plate gives greater freedom from grain between the lines or dots, and is quite worth the extra trouble. A good backing for this purpose is Lichtenstein's caramel with a little water and lampblack added.
BAG (See " Camera Bag," " Changing Bag," etc.) (Fr.) A hot-water bath which in its simplest form is that of a jar in a saucepan of water, the gelatine, etc., to be dissolved or melted, being