Millboard is the name given to 'board' made of paper material, and varying in thickness from th to th of an inch. It is of a gray colour, as the various kinds of waste substances—viz. old ropes, old sacking, scraps of paper and of cardboard—from which it is usually made are not bleached either separately or when mixed and reduced to a pulp, as in the manufacture of white paper (see PAPER). The best millboard, such as that employed for binding ledgers, is made on moulds by hand; but by far the larger quantity of millboard put on the market is machine made. In the machine commonly used a revolving wire-cloth cylinder dips into a cistern containing the pulp, and takes on a layer of it about the thickness of stout brown paper. This pulpy layer is by pressure taken continuously off the wire cylinder by a felt blanket passing over a wood roller. The felt carries the single layer, in the wet state, to a pair of rollers 7 feet away from the wire cylinder. Here the layer is wound upon the upper or wooden roller till the required thickness is made up. The lower or iron roller presses by means of a lever and weights against the upper one with sufficient force to consolidate the layers of pulp. The hollow cylinder of millboard on the wood roller is then cut longitudinally with a knife, and opened out into a flat sheet. It is afterwards dried by steam heat or otherwise, calendered by chilled iron rolls, and cut to size by strong circular cutters.
Millboard is used for bookbinding and for making boxes, but has been largely superseded by Strawboard (q.v.). It is still employed for jointing flanged pipes and other engineering work; but for this purpose asbestos millboard (see ASBESTOS) is now preferred. Millboard or thick cardboard made from straw or wood is used for many miscellaneous purposes. Mounting board consists of several layers of paper pasted together. A few years ago a mill on a large scale was in operation near London for the manufacture of millboard from stable manure, but the process was not a success commercially.