Lateral centring of the print is facilitated by the use of a divided rule with the zero in the middle, the two sides being numbered sym metrically (V. Jobling, 1919). 2 As a general rule, the margins are relatively much larger for a small print than for one of very large size.
In multiple mounting, use is made alternately of bands of very unequal width, some being so narrow as to be merely lines. For example, the picture may be separated from the outer border (which is always the largest) by a line and band, or a line between two bands, or a band between two lines. Trials may be easily made on one of the corners of the picture, the successive mounts being applied so as to show the final appearance along two sides only of the picture_ 3 707. Various Methods of Fixing the Print on the Mount. For many years the fixing of photo graphs on to their mounts was done with starch paste or, sometimes, gum arabic. The incon venience of this method is that the print expands before being applied to its mount by the moisture of the paste, and thus becomes distorted in shape, since the expansion takes place entirely at right angles to the fibres of the paper (§ 613), and persists after drying. Moreover, the strain put upon the card by the print tends to curve the whole, the print then appearing on a concave mount, unless steps are taken to prevent this curling or to remedy it.
For mounting glazed or enamelled photo graphs (§§ 614 and 615) a narrow edging of thick rubber solution may be applied to the print, there being then no moisture to impair the brilliancy of the surface.
In course of time, dry-mounting came into use. In this process thin sheets of gutta-percha (manufactured for waterproof dressings) have been used, but the usual " tissue " is thin paper (" onion skin " or foreign post paper), impreg nated on both surfaces with a preparation of shellac. The sheet is placed between the print and the mount. In either case, pressure of the whole at a sufficiently high temperature melts the gutta-percha or the shellac, thus ensuring perfect adherence of the print, without distortion of the image or curling of the mount, and with the additional advantage that the print is isolated from the mount and its possible im purities by an impervious coating. Also that the print may at any time be detached by sub jecting it to a temperature slightly higher than that used for mounting.' More recently it has been the custom to attach prints on their mounts only by the upper edge, which gives the print no protection at all against being rubbed or torn. When this method
of mounting is adopted by professional photo graphers it is customary to use a mount whose horizontal dimension is twice the ordinary width, and which is folded over on the print (folder mount) so as to provide it with a protect ing wrapper.
708. Mountants. Adhesives for mounting photographs must not contain either acid (found in many liquid office pastes), which would deteriorate the print or alter its colour, nor hygroscopic substances (e.g. glycerine), which, by keeping the print slightly damp, renders it liable to be affected by atmospheric agencies.
These conditions are fulfilled by freshly prepared starch paste, which is specially suitable for thin papers, and by dextrine, which is thicker and more adhesive, and is suitable for thick papers.
In exceptional cases use is made of a solution of gum arabic, or of strong liquid glue (the kind usually sold in tubes for domestic purposes), or of rubber solution (mixture of parings of un vulcanized rubber sheet in rectified benzol) of the consistency generally used for pneumatic tyre repairs.
To prepare starch paste, about 2 OZ. (100 grm.) of rice starch are mixed with 2 oz. (I00 c.c.) of water (preferably in a mortar), making a thick cream, without lumps. This is poured slowly into about 5 oz. (250 c.c.) of boiling water, which is kept boiling while the mixture is stirred with a spatula or wooden spoon until it turns to a bluish translucent liquid, which thickens on cooling.' Whilst the mixture is still warm about Li gr. (0•2 grin.) of salicylic acid or of thymol may be added to it, to retard slightly the spontaneous liquefaction, which, otherwise, be gins in a few hours. The paste should be filtered while warm through a piece of fine linen, so as to remove lumps and any extraneous matter. 2 Dextrine paste is a thick, white, opaque mount ant, which keeps indefinitely and is much used in offices. It is made by bringing about 20 oz. (1,00o c.c.) of water to a temperature of about 176° F. (this temperature should be maintained throughout the operation) ; in this is mixed 10-12 OZ. (Soo-600 grm.) of white dextrine, adding a little at a time, and maintaining the temperature until a translucent liquid is ob tained ; to this is added about io gr. (1 grm.) of thymol. It is filtered through fine linen and put into pots, closed at first only by a piece of linen. At the end of a few days the mixture becomes thick paste ; the pots may then be corked. For use, the paste is thinned with a very little water, about as much as the brush will hold.