The next stage in the manufacture of a sheet of glass is the flattening and annealing process. For this purpose the split cylinders are taken to a special kiln, generally known as a “lear,” or “lehr,” where they are first of all raised to a dull red-heat; they are then lifted, one at a time, on to a smooth stone or slab placed in a chamber of the kiln where the heat is great enough to soften the glass. Here the cylinder is laid down with the split edges upwards, and by means of a wooden tool the glass is slowly spread out, being finally rubbed down into perfect contact with the slab or “lagre.” From the flattening slab, the sheet as it now is passes into the annealing kiln, which communicates with the flattening chamber. This consists, similarly to other continuous annealing kilns already described in connection with other varieties of glass, of a long tunnel, heated to the temperature of the flattening kiln at one end and nearly cold at the other. The sheets are moved down this tunnel at a uniform slow rate by the action of a system of grids which, at intervals, lift the sheets from the bottom of the kiln, move them forward by a short distance, and again deposit them on the bottom, the grids themselves returning to their former position by a retrograde movement made below the level of the kiln-bottom, and therefore not affecting the glass. On leaving the annealing kiln the sheets of glass are sometimes covered with a white deposit arising from the products of combustion in the kiln and their interaction with the glass itself. This deposit can be removed by simple mechanical rubbing, but it is usual to dip the glass into a weak acid bath, which dissolves the white film and leaves the glass clear and bright, ready for use. From the annealing kiln the finished sheets of glass are taken to the sorting room, where they are examined in a good light against a black background, and are sorted according to their quality for different purposes. The defects which are found in sheet-glass are of a very varied nature, as would be anticipated from the long and complicated process of manufacture which the material undergoes in the course of its transformation from the raw materials into the finished sheet of glass. A full enumeration of all possible defects, with their technical names, need not be given here, but a description of the more important and frequent ones will be useful. The defects may be conveniently grouped according to the stage of the process from which they originate.