Given sufficient heat, glass can be made by this means alone, but the heat required is so great that it has only been done in recent years for special purposes by means of the electric furnace. Failing this, the sand must be induced to melt at a lower temperature by means of a flux, for which either potash or soda may be used, and to which lime or lead must be added, to enable the glass to resist moisture. (Theophilus, describing the process in his Treatise, certainly no later than the thirteenth century, recommends the use of beech twigs calcined in an earthen pot, whence the name "Pot-ash.") [Sidenote: Colouring.] Glass may be coloured or "stained" while in a molten condition by the admixture of various substances, mostly metals, gold, copper, manganese, and so on, the result depending on the temperature to which it is subjected and on the exact composition of the glass as well as on the colouring matter used. [Sidenote: Blowing.] Glass is made into vessels, as most people know, by "blowing." The workman takes a dab of molten glass on the end of a long metal pipe, and putting his mouth to the pipe blows the glass--soap-bubble fashion--into a hollow bulb. Then by a rapid and dexterous series of alternate and repeated heating, blowing, and spinning, and manipulation with tools, most fascinating to watch, he shapes it into the form required. If a flat sheet be required for window glass it may be run out flat when liquid, or blown as described above, and worked into a cylindrical form, split open, and unrolled. This is called "muff glass." But glass can also be formed, by rapidly spinning it while soft, into a large flat disc, called a "crown," and is then known as "crown glass." It was by these last two methods, the "muff" and the "crown," that all the material of the windows we have to consider in this book was made. [Sidenote: Cutting.] When cold, the sheet or disc of glass may be cut to the shape required, either, as in the old days, by running a hot iron slowly along the proposed line of fracture, in which case a crack will follow the iron, or by scratching it with a diamond and then bending it so as to break along the line of the scratch. The latter is a comparatively modern invention, and has in its turn been superseded by the use of a little steel wheel with a sharp edge. [Sidenote: Pliny's story.] Pliny gives a story of the invention of glass which, if false, is still so picturesque that I cannot resist quoting it here.