The higher grades of quality in glass, which thus require increased refinement in the raw materials, also demand increased refinement in the furnaces and appliances employed in their melting. The tank-furnace, which holds the field in bottle manufacture, is scarcely met with in the production of the better qualities of hollow glass-ware; medicine bottles and other articles of moderate quality might be produced in tanks, but the quantity of glass required for such purposes is seldom large enough to justify such large plant. For the best qualities of colourless glass-ware, however, the tank-furnace could not be used on account of the fact that both as regards colour and freedom from defects, the product of a tank-furnace is never equal to the best product of pot-furnaces. For flint-glass, indeed, _covered_ pots or crucibles must be used in order to adequately protect the molten glass from the reducing action of the furnace gases and from contamination by dust. The materials of which the pots are constructed are also chosen with a view to avoiding all risk of introducing colouring or otherwise injurious impurities from that source. In all processes for the production of hollow glass-ware, the glass or “metal” is taken from the pot by the process of gathering which has already been described; where blown articles are to be produced, as distinct from pressed goods, the initial stage is always the formation of a small hollow globe or bulb at the end of the glass-blower’s pipe. The subsequent manipulations depend upon the nature of the article to be produced. The article may either be made entirely by hand work, or rather “chair” work, as it is usually called, or the manipulations may be facilitated and the product cheapened--while its character is, of course, also modified--by the aid of moulds, which are used to bring the object to its proper shape and to impress upon it certain decorative mouldings or markings. As we have already seen, ordinary bottles are now always blown with the aid of moulds, and the same applies to medicine bottles, lamp chimneys, and the bulbs for electric light; in connection with lamp-chimneys it should be noted that they are blown in moulds in the form of cylindrical bottles with a flat bottom and a domed top, the ends being subsequently cut off. Many of the cheaper varieties of tumblers and glasses are also blown in moulds, but they can be, and sometimes are, produced by hand, and as their manufacture is typical of that of all hand-blown hollow ware, we shall now describe it in some detail as an example of this class of work.