Soda-glass

The addition of a relatively small amount of such substances as soda, potash, or borax, all known as fluxes or fluxing materials, lowers the melting temperature of silica sand from over 1700°C to below 1000°C, a temperature feasible in ancient furnaces (see Textbox 28). 

Soda (composed of sodium carbonate) was acquired in antiquity either in the form of natron, or, when prepared, as soda ash. Natron is a natural mixture of sodium bicarbonate, sodium carbonate, lesser amounts of common salt, and sodium sulfate, and some organic matter. It occurs in a few places in the world, such as in dry lakebeds in desert regions, in Egypt and Siberia, for example. From these few sources, natron was traded and transported to many others in the ancient world, where it was used (von Lipmann 1937 Lucas 1932). 

Potash (composed of potassium oxide), also a flux, was mainly used as a glass modifier. It was generally introduced into the glass melt in the form of either pearl ash, composed of potassium carbonate, vegetable ash, one of the main constituents of which is potassium carbonate, or saltpeter, a mineral composed of potassium nitrate. 

Soda as well as potash have also been made, since early antiquity, by burning weeds until only their ash remains - thus known as either soda ash or potash ash. The ash may also contain as much as 5% of sodium or potassium carbonate. Kelp, a large seaweed of the order Laminaria, and barilla plants, of the genus Salsola, which grow on many seashores, have 

another fluxing material. Is a mineral (composed of hydrated borate of sodium), which has also been known since antiquity. There seems to be no evidence however that it was used in the past as a flux it served mainly as a mordant, for dyeing textiles. Until the eighteenth century borax was apparently procured only from a lake In Tibet, whence it was exported to the Near East and Europe. Sources of borax in Asia Minor and in Tuscany, italy, were discovered only In much later times. 

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The word glass commonly means the transparent substance obtained when white sand is fused with metal oxides or carbonates to give a mixture of silicates. Ordinary or soda-glass has the approximate composition NajO. CaO. 6Si02. (This is the composition obtained by analysis it does not represent the compounds present.) If sodium is replaced by potassium the melting point is... 

Fig. 5.11. (a) Atom packing in amorphous (glassy) silica, (b) How the addition of soda breaks up the bonding in amorphous, silica, giving soda glass. 

Taking the coefficient of cubical expansion of soda glass as about 0.000030 and of borosilicate glass about 0.000010 per 1 °C, Part A of Table 3.1 gives the correction to be added when the sign is +, or subtracted when the sign is —, to or from the capacity of a lOOOmL flask correct at 20 °C in order to obtain the capacity at other temperatures. 

Soda glass Borosilicate glass Soda glass Borosilicate glass... 

If the preference for hydrogen ion exchange shown by lime-soda glasses can be reduced, then other cations will become involved in the ion exchange process and the possibility of an electrode responsive to metallic ions such as sodium and potassium exists. The required effect can be achieved by the introduction of aluminium oxide into the glass, and as shown in Table 15.2, this approach has led to new glass electrodes of great importance to the analyst. 

The Ginger Snap, on ice in a soda glass, runs hot and cold on the palate. It s a fusion Southside—a limeade with a curry-like kick. 

Fig. 18. Plot of residual depth parameter as a function of the ratio H/E according to equation 10. A value of v = 0.41 is taken for PE. Data for die-drawn PE (O) and POM ( ) and for soda-glass (A) and hard-steel (A) are shown. H/E values for lamellar isotropic PE with a 200 A thick surface (ft) and isotropic chain-extended material (1 2 x 103 A) (f) are also indicated...

This type, made by the General Electric Co., is resistant to sodium vapour and is used in sodiiun vapour discharge lamps. It has a very high boric oxide content, a low softening temperature, and a low electrical resistance. It is used as an internal layer in soda glass tubing X.8. ... 

Tubes for sealed tube reactions, such as the Carius determination of halogens and sulphur, can be made from Pyrex, Monax or soda glass. The mechanical strength of the glasses is about the same, but a soda Carius tube is much more likely to crack as a result of thermal strain than a Pyrex or Monax one. The Carius tubes are usually made from tubing of approximately 20-25 mm diameter and 3 mm wall thickness—Pyrex extra heavy tubing of external diameter 22 mm has a wall thickness of 2-5-4 mm and can be used up to 600°C. 

It is advisable before making a piece of apparatus such as a filter piunp to prepare all the separate components, since the work, if left whilst a component is made, may crack on re-heating—in fact if soda glass is used it probably will do so. Hence for a filter pump of the type shown in Figure 53, V, the jet, the cup, the side arm and the tube for the jacket are first made separately. 

This pxunp (Figure 69) is due to H. P. Waran (1923) and is designed such that it may be constructed by the most average of experimentalists, Soda glass is satisfactory because all joints are subject to a minimiun, if not a zero, temperature gradient. Pyrex glass would, of course, be an advantage but is not a necessity. 

Joints, borosilicate to soda glass, 25 Joints in closed systems, 168 Joints, Pyrex to Phoenix, 25 Joints, tungsten-sealing to Xovar-seal-ing glasses, 24 Joints, Vitreosil, 175 Joints with capillary tube, 69 Junctions, 4 and 5 way, 68... 

Pids is a soda glass for sealing to platinum, and is made by Plowden b Thompson Ltd. The linear expansion coefficient (20-300°C) is pi K 10-. ... 

This method is not on the whole satisfactory with soda glass, the cold inner tube will probably shatter, and if a borosilicate glass is used to avoid this, the difficulty of getting the inner tube central still remains. A better method is the following the outer tube is... 

Extremely unpleasant cracks can be sealed in Pyrex glass. With soda glass, however, the sealing of cracks is a hazardous imdertaking, and the whole apparatus may be lost. Cracks can be sealed in Monax. 

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