Solution of gum arabic is prepared with the pale gum, roughly broken up and hung in a small muslin bag, in its own weight of warm water, with the addition of about o.i per cent of thymol. When completely dissolved it may be filtered if necessary.
For mounting by the edges, or by the upper edge only, liquid adhesives in tubes may be used, thinned, as required, with a little water, this glue being run round the print (Fig. 186) by means of a small oilcan (N. E. Nilson, 1925).
709. Full Mounting. Mounting is done with a fairly stiff hog-hair brush, of the flat, " fish-tail " type. The brush, once charged with the neces sary amount of mountant, should not be put back into the paste, nor should it be left so that the hairs pick up dust or soil the work table. In the handle, as near as possible to the metal binding, may be screwed small wooden pegs, the end of the handle being weighted with lead, so that, when the brush is laid down, the hairs are kept out of contact with anything.
The prints to be mounted are soaked for a few seconds in water, and piled face downwards on a glass (obviously this method is not suitable for glazed (stripped) prints, which must be mounted dry by means of an edging of stiff mountant). The top print is then coated with mountant. The print is then lifted off and at once laid in its place on the mount;' it is then covered with a piece of hard paper, and contact is ensured by pressure from the centre outwards with a linen pad or a rubber roller squeegee. 2 To keep the mount flat during the drying of the print, the mounted photographs are piled, face down (placing the first on a piece of blotting paper or very clean ordinary paper), with a weight on top. Also, while drying, they may
be curved in the contrary direction by slightly springing them between pairs of wooden strips nailed to a board, the distance between the strips being a little less than the width or height of the For mounting on canvas, the canvas should first be stretched on a stretcher frame (similar to those used for oil paintings). The print is laid face down on a glass slab and mountant applied to the back. The canvas is then laid on, firm contact ensured, the glass removed, and the print and canvas are left to dry, the canvas not being taken off the frame until completely dry.
710. Burnishing. Burnishing was largely practised in the days of albumen papers, both for flattening prints which had curled through wet mounting and for slightly increasing the gloss of the print. It is now almost abandoned, and, in any case, it can be used only on glossy or smooth papers.
In carrying out the operation, the print is hot-rolled under pressure by means of a heated cylinder, its surface having been soaped with a woollen wad, rubbed over a cake of very dry soap (or moistened with an alcoholic solution of soap), or waxed with a flannel rag rubbed over a piece of wax (in which is incorporated, by melting, a little mastic). The rolling must be done without jerking or stopping, otherwise marks will be left on the print. The operation is generally repeated two or three times, pressure being increased each time.
711. Dry-mounting. Dry-mounting, by means of adhesives with a shellac base, is suitable for all papers the gelatine coating of which has been hardened by alum, provided both print