The Methods of Glass Blowing and of Working Silica in the Oxy-Gas Flame / For the use of chemical and physical students

by W. A. Shenstone

Fig. 38.

[Fig. 38] represents the standard measure, and the ruler.

At AA are the millimetre divisions on the edges of the measure, the longer transverse lines at BB are placed at intervals of five millimetres and of centimetres. The ruler is in the form of a right-angled triangle; it is shown, by the dotted lines, in position on the standard metre measure at I; and again, with its under surface upwards, in the smaller figure at 2. It consists of a perfectly flat sheet of metal, about ten centimetres in length from C to C, sufficiently thick to be rigid, and has a ledge, DD in each figure, which is pressed against the side of the measure when using it, to ensure that the successive positions of the edge (LL) shall be parallel to each other. At GG are two small holes, into which fit small screws with fine points. These must be in a line parallel to the edge (LL), so that when the ruler is in position on the scale, the points of the two screws, which project slightly, shall fall into corresponding cuts on the divided scales (AA).

To graduate a strip of glass, or a glass tube (HH), the surface to be marked must first be coated with wax, which should be mixed with a little turpentine, and be applied to the surface of the glass, previously made warm and dry, by means of a fine brush, so as to completely cover it with a thin, closely-adherent, and evenly-distributed coat of wax, which must be allowed to cool.

Fix HH firmly on a table, and fix the standard measure by the side of HH. If the thickness of HH be about equal to, but not greater than that of the standard measure, this may be done by large drawing-pins. If, however, a large tube or thick sheet of glass is to be graduated, fix it in position by two strips of wood screwed to the table on each side of it. One of these wooden strips, on which the measure may be placed, may be about as broad as the standard measure, and of such thickness that when the measure lies upon it beside the tube to be graduated, the ruler, when moved along the measure, will move freely above the tube, but will not be elevated more than is necessary to secure free movement. The second strip of wood may be narrower, and of the same thickness as the broader piece on which the standard measure rests. In any case, let the standard measure and the object to be graduated be very firmly secured in their places. Bring the ruler into position at any desired part of the tube by placing the points of the screws (GG) in corresponding divisions of the scales (AA). With the style, which may be a needle mounted in a handle, make a scratch in the wax along the edge of the ruler at F, move the ruler so that the screws rest in the next divisions, and repeat the operation till the required number of lines has been ruled. Longer marks may be made at intervals of five and ten millimetres. Great care must be taken to hold the needle perpendicularly, and to press it steadily against the edge (LL) of the ruler in scratching the divisions.[19] The length of the lines marking the millimetre divisions should not be too long; about 1 mm. is a good length. If they are longer than this, the apparent distance between them is diminished, and it is less easy to read fractions of millimetres. Before removing the scale to etch the glass, carefully examine it to see that no mistakes have been made. If it is found that any lines have been omitted, or that long lines have been scratched in the place of short ones, remelt the wax by means of a heated wire, and make new marks. Finally, mark the numbers on the scale with a needle-point, or better, with a fine steel pen.

The marks on the wax should cut through it. When they are satisfactory, they may be etched by one of the following processes:—

(1.) By moistening some cotton wool, tied to a stick, with solution of hydrofluoric acid, and gently rubbing this over the scratched surface for a minute or so; then washing away the acid with water, and cleaning off the wax. This is the simplest method, but the marks made are generally transparent, and therefore not very easy to read. The simplicity of this method is a great recommendation, however.

(2.) Expose the tube to the fumes of hydrofluoric acid generated from a mixture of powdered fluor-spar and strong sulphuric acid, in a leaden trough. The marks produced in this way are usually opaque, and are therefore very visible, and easily read.