In performing this and all the other operations of glass blowing, the following points must be constantly kept in mind:—
(a.) That it is rarely safe to blow glass whilst it is still in the flame, except in certain special cases that will be mentioned subsequently. Therefore always remove apparatus from the flame before blowing.
(b.) That when heating glass tubes, unless it is specially desired to heat one portion only, the tube must be constantly rotated in the flame to ensure that it shall be uniformly heated, and to prevent the tube or mass of glass from assuming an irregular form.
(c.) Always blow gently at first, and slowly increase the force applied till you feel or see the glass giving way. It is a good plan to force the air forward in successive short blasts rather than in one continued stream.
Fig. 8.
(d.) When it is necessary to force air into tubes of fine bore, such as thermometer tubes, the mouth must not be used, for moisture is thereby introduced into the tube, which it is very difficult to remove again in many cases. All tubes of very small bore should be blown with the aid of an india-rubber blowing-bottle, such as are used for spray-producers, Galton’s whistles, etc. The tube to be blown must be securely fixed to the neck of the bottle, which is then held in one hand, and air is forced from it into the tube as it is required. These bottles are frequently of service to the glass-blower—e.g., when tubes of very fine bore have to be united, it is necessary to maintain an internal pressure slightly exceeding that of the air throughout the operation, in order to prevent the viscous glass from running together and closing the tube. An india-rubber blowing-ball is very convenient for this purpose.
To seal the end of a glass tube ([Fig. 8]), adjust the flame so that it will heat a zone of glass about as broad as the diameter of the tube to be sealed (see A, [Fig. 8]). Hold the tube on each side of the point where it is to be sealed in the manner described in the description of bending glass tubes ([p. 28]). Bring the tube gradually into the flame, and heat it with constant rotation, till the glass softens (for lead glass the oxidising flame must be used, as has been already explained).[6] When the glass begins to thicken, gently pull asunder the two ends, taking care not to pull out the softened glass too much, but to allow the sides to fall together, as shown at A. When this has occurred, heat the glass at the narrow part till it melts, and pull asunder the two ends. The closed end should present the appearance shown at D. If the glass be drawn out too quickly its thickness will be unduly reduced, and it will present the appearance shown at B. In that case apply a pointed flame at b, and repeat the previous operation so as to contract the tube as at c, taking care not to allow the glass to become much increased nor decreased in thickness.
If a considerable mass of glass be left at d, it may be removed by heating it to redness, touching it with the pointed end of a cold glass tube, to which it will adhere, and by which it may be pulled away.