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.
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 The latter is a comparatively modern invention, and has in its turn been superseded by the use of a little steel wheel with a sharp edge.
Pliny gives a story of the invention of glass which, if false, is still so picturesque that I cannot resist quoting it here.
A certain merchant-ship touched on the coast of Syria, and the crew landed near the mouth of the river Belus, on a beach of fine white sand which, Pliny says, was still in his day of great repute for glass-making. The ship's cargo consisted of natron,--a natural alkaline crystal which was much used in ancient times for washing,[1]--and the crew having lighted a fire on the sand used lumps of it from the cargo to prop up their kettle. What was their surprise to find afterwards a stream of molten glass running down from their camp-fire. In this case the natron acted as a flux and enabled the sand to melt in the heat of the camp-fire, which, however, must have been a very large and hot one.
Now, whether this story is true or not, it cannot have been the beginning of more than a local industry, for the art of glass-making was known in Egypt from very early times indeed.