The evolution of an ordinary tumbler, as just described, and as illustrated diagrammatically in Fig. 8, is typical of the whole process of hollow-glass blowing, but of course the number of operations, as well as the care and skill involved in each step, increases rapidly as the form of the vessel becomes more complex; in the highest class of work a very considerable element of artistic taste and judgment on the part of the operative also becomes essential, for, although the form of the object as well as the choice of colour and ornamentation are chosen by the designer, the blower has to translate the drawing of the designer into glass, and although his skill enables him to attain a considerable degree of fidelity in his rendering, many details remain at his own option, and the proper management of these is no small factor in the success of the whole work. In this connection mention should perhaps be made of the application of colour and other decorations to this kind of glass. A very considerable range of effects of this kind is now available to the glass-worker. In the first place the body of the glass used for the production of the articles in question may be coloured by the addition of suitable colouring materials to the molten glass or raw materials, as explained in Chapter XI., but this procedure has very obvious limitations; where the article is built up of glass from several gatherings--as, for example, is the case in an ordinary wine-glass, where the bowl, leg and foot are each made of separate gatherings--it is possible to use glass of different colours for these different parts, and this is commonly done in the production of wine glasses having ruby or green bowls and white legs and feet. A further modification in the application of colour is obtainable by taking up two or more gatherings on the same pipe and superposing a large gathering of white glass on a smaller one of coloured glass; this is analogous to the process of “flashing” sheet glass, described in Chapter X. and this process lends itself to a variety of manipulations resulting in the distribution of the coloured layer of glass in almost any desired manner over the object in hand. The principal objection to this process, however, lies in the fact that pots of molten glass of all the colours desired must be kept available to the blower at the same time, and this is not easily arranged for in any reasonably economical manner. For this reason, and also because the manipulations are simpler, coloured glass intended for application to blown glass-ware is generally used in the form of short rods previously prepared; these rods are suitably heated, and the coloured glass can then be applied to the article in hand at any desired place and in as small or large a quantity as required. If the two glasses thus brought into contact are properly related to one another as regards chemical composition and physical properties, they blend very readily