In a form of design which is carried out with pieces of coloured glass cut to the necessary shapes and held together by strips of grooved lead, which is soldered into position, this structural lead-work presents considerable difficulty. It follows the lines of the composition wherever possible, but when the shape of the glass makes another course desirable, it does not hesitate to go across a space, and in that case, being frankly used for structural reasons, it must not clash with those lines that help to define form. In short, design in this, perhaps more than in other arts, must conform to the dictation of the material. Thus it is important that the pieces of glass should be designed of cutable shape without too small re-entering angles, and the limits of bending in the lead must also be recognized. Its passing across objects is vindicated by structural necessity, and by that alone, and narrow places are leaded across, not only because of any difficulty that there may be in the cutting, but because the glass would probably break there when being fired in a kiln, or when under the strains that are set up in a window by wind pressure. The tendency of outline to lose itself in the darker of the colours that it divides has already been referred to, and is very notable in this connexion. When therefore the objects are light on dark, the lead-work will sink into the background, and although it may leave small space for the glass, it is sometimes surprising how efficiently that little lights up and expresses the colour. If the charges are inconveniently small for the leading, resort is had to what is called flash glass, which consists of two layers, of which one only is coloured, and is made by dipping a piece of molten white into a coloured glass, when the mass is about to be blown into the bulb which, shaped and expanded, ultimately becomes a sheet of glass. The desired shapes are pierced through the coloured layer of the composite sheet by means of grinding, or by etching with hydrofluoric acid, and are left white, or stained yellow with a solution of silver, as the case may be. In addition to the lead-work, which defines the general forms with more or less accuracy, details are depicted by means of a brown enamel colour, which serves also as a general shadow tint, being painted on the glass, and then fired. The brown enamel is also used for the diapering which is so especially valuable in glass decoration, and for this purpose it may express the design in lines drawn with the colour or, being applied as a broad wash; the diaper pattern may be scratched out of it with a point. As a general rule over-painting should be used as sparingly as possible for the purpose of defining or emphasizing form. The silver stain can be similarly put on, either in lines or in washes, these materials, either alone or in combination, serving to express surface decorations of all degrees of elaboration.