The process of blowing crown glass may be briefly described as that of first blowing an approximately spherical hollow ball, then opening this at one side and expanding the glass into a flat disc by the action of centrifugal forces produced by a rapid rotation of the glass in front of a large opening in a special heating furnace. The actual process involves, of course, the preliminary of gathering the proper quantity of glass, much in the manner already described in connection with sheet-glass manufacture. This gathering is then blown out into a hollow spherical vessel. This vessel is now attached to a subsidiary iron rod by means of a small gathering of hot glass, applied at the point opposite the pipe itself, the glass being thus, for a moment, attached to both the pipe and the “pontil” or “punty” (as the rod is called). The pipe is, however, detached by cracking off the neck of the original glass, which now remains attached to the pontil in the shape of an open bowl. This bowl is now re-heated very strongly in front of a special furnace, the open side of the bowl being presented to the fire. The pontil is meanwhile held in a horizontal position and rotated. As the glass softens the rotation spreads it out, until finally the entire mass of glass is formed into a simple flat disc spinning rapidly before the mouth of the furnace. This flat disc or “table” of crown glass is allowed to cool somewhat, is detached from the pontil by a sharp jerk, and is then annealed in a simple kiln in which the glass is stacked, sealed up, and allowed to cool naturally. It is obvious that by this process no very large sheets of glass can be produced; tables 4 ft. in diameter are already on the large side, and these can only be cut up into much smaller sheets on account of the lump of glass by which the table was originally attached to the pontil, and which remains fixed in the centre of the finished disc. For certain ornamental purposes, where an “antique” appearance is desired, these bullions are valued, but for practical purposes they interfere very seriously with the use of the glass. As a matter of fact, even several inches away from the central bullion itself, crown glass is generally marked with circular wavings, which render it readily recognisable in the windows of older buildings, but which decidedly detract from the perfection of the glass. On the other hand, crown glass is still valued for certain purposes, such as microscope slides and cover glasses, where entire freedom from surface markings, such as those found in sheet glass as a result of the flattening operations, is desirable. While, therefore, the process has merely an historical interest so far as ordinary sheet-glass purposes are concerned, it is still used in special cases. CHAPTER XI. COLOURED GLASSES.