The pasteboard used for fireworks is made up from several sheets of good grade paper for the inner portion, and white for the outer, fastened together with flour paste. It should be sufficiently thin to be rolled easily in forming the case. Three thicknesses of this suffice for small rockets, up to and comprising those of 18 lines in diameter. Five sheets are necessary for larger rockets, and eight sheets for the pots a aigrettes. Large brushes of hog's bristles are used in pasting. Two hundred sheets are thus prepared, and then placed in a press between two smooth pieces of board. If a press be lacking, the boards may be held down by heavy weights. After the pasteboard sheets have remained in the press for six hours they are taken off to dry. Each sheet is pierced in two of the corners, and they are then hung on a line equipped with brass hooks, which are caught into the holes. After thoroughly drying, the sheets are returned to the press, in order to correct any warping.
The paste used in constructing the sheets, and also in the subsequent molding, is made from wheat flour. The flour is first thoroughly soaked in water, after which it is placed on the fire, and boiled until it loses all trace of the flour smell. It is finally passed through a screen of horsehair, in which it is worked in order to reduce the lumps, and to remove anything that might cause unevenness on the surface of the pasteboard.
Father d'Incarville brought from China precise information as to the method of making paste. The Chinese, for the sake of obviating the danger of accidents by fire, introduced both clay and common salt into the paste used for making cases. The effect of these constituents is to lessen the possi-: bility of the case taking fire. Experiments along this line have been made in France, and the value of the method has been proved. But it has been found that alum is even better suited to the purpose than is common salt, since it does not attract humidity, as salt does, and it is equally incombustible. It is, therefore, recommended that a handful of powdered alum should be added to a pound of flour before boiling. When the paste is taken off the fire there should be mixed with it an almost equal quantity of clay, reduced to the same consistency by the use of water.